A Sound Foundation Through Early Amplification 1998
Contents
1. instrument e Can be uncomfortable to wear for long time periods e Do not easily adjust to any head size resulting in an unstable fit Hearing Loss The degree and configuration of a child s hearing loss are primary factors in the preselection of FM sys tems Children with minimal or unilateral hearing loss as well as children with normal hearing and attention prob lems only recently have been considered candidates for FM system use Output saturation levels are of major concern with this population and reduced output and gain are required to prevent the possibility of noise induced threshold shifts A nonoccluding method of cou pling the FM to the child s ear is usually selected This may include lightweight type headphones earbuds or open earmolds Personal FM systems are most often used but BTE FM systems with adjustable gain and out put also may be appropriate For children with hearing loss in the mild to moderate range many options are available Children with milder hearing loss may find a nonoccluding coupling prefer able Personal FM systems coupled to hearing instru ments or self contained FM systems including BTE FM s can be used Children with severe to profound hearing loss can also use a personal FM system coupled to their hearing instruments Coupling choices may be more restricted depending on the severity and configuration of a child s hearing loss For example neckloop coupling is not appropri2. 1995 Seewald 1988 Seewald 1994 Seewald et al 1997 Seewald et al 1993 is a prescriptive approach that is recommended because it was designed specifically for the pediatric population and provides measures of aided speech audibility In actuality any prescriptive approach can be used because the principle of matching the FM response to a target hearing instrument response can be applied to all When a child has an appropriate hearing instrument to use as a target for the FM response the following steps of evaluation are needed First When matching the frequency response of the FM to a target hearing instrument response the hearing instrument is first set to target values typically in gain with a 65 to 70 dB input level figure 19 The response of the hearing instrument is then measured in dB SPL using the same input levels to allow comparisons among the different input levels to the FM and environmental micro phones The saturation response of the hearing instru ment at use and full on volumes is also confirmed In a 2cc coupler saturation response can be measured at use and full on volumes but real ear saturation response measures are not recommended at full on volumes and are best preset in a 2cc coupler second After making measurements with the target hearing instrument the output of the FM microphone is measured with an input level appropriate for the micro phone being used The response of the FM system should be equ
3. do not seem to be following the lesson One of those stu dents is Mary who is drawing pictures rather than looking at the teacher Mary who has a moderate hearing loss and wears binaural hearing instruments has been placed near the front of the class for preferential seating In addition she was placed with her back to the window so that outside light would not make it difficult for her to see the teachers face Today that seating arrangement places her next to the overhead projector and near the open windows Another student Tommy sits in the sec ond row of the group of students and toward the back of the class Sometimes Tommy who has unresolved 87 chronic otitis media appears to function well in the class room Today is not one of those days and Tommy is mak ing a paper airplane out of his math paper Finally there is Bobby He has a mild to moderate hearing loss and functions well with hearing instruments However today like most days he did not bring those instruments to school Thus despite preferential seating Bobby rarely seems to know what is going on in class and has signifi cant difficulty with most new material In addition he is often tired and restless by the afternoon All of the other students who have normal hearing are attentive and fol lowing instructions except Jenny who has been sent to the principal s office Mr Goodteacher is concerned about the three stu dents who have hearing d
4. talkers in a relatively contained area e g around a table in a small group discussion In most models the trans mitter plugs into a microphone base that is placed on the table Sound is directed to the microphone from around the table More recently small conference microphones have been introduced that can easily be carried by the user Several manufacturers have introduced small hand held microphones figure 7 for use with FM systems r yl r Figure 5 Collar style FM microphone photo courtesy of Audio Enhancement Figure 6 Conference style FM microphones photo courtesy of Audio Enhancement These microphones are designed to be used by the indi vidual wearing the FM receiver Instead of the talker wearing the transmitter the listener holds the transmitter and points it toward the talker Adjustable directional omnidirectional microphones allow the user to focus in on a talker in the presence of background noise Each of the microphones has its own benefits and limitations and these are summarized in table 3 In addi tion the microphone features that are selected may affect the microphone s use and performance For example many of the microphones may be either directional or omnidirectional Directional microphones have openings around the sides and are less sensitive to sounds coming into the side openings than into the top of the micro phone When worn correctly they can be helpful in noisy F
5. arrangement it is located on the receiver Self contained receivers also may be coupled to personal hearing instru ments with direct audio input neckloops or directly to the ear via lightweight headphones or earbuds They also may be coupled to bone conduction transducers or to the speech processor of a cochlear implant The receiver s antenna is located in the cord connecting the receiver to the transducer or hearing instrument A recent addition to the category of self contained receivers is the combination BTE hearing instrument FM receiver figure 10 Available from several manufactur ers these devices have internal controls for adjusting the response of the system A small antenna extends from the back of the receiver Personal receivers figure 11 originally were designed to be worn in conjunction with personal hearing instruments As such they do not have internal adjust ments allowing the hearing instrument to perform fre quency shaping and output limiting They may be coupled to hearing instruments using direct audio input or a neck loop They also may be coupled directly to the ear via Figure 9 Self contained FM receiver photo courtesy of Telex Communications Inc Figure 10 Behind the ear BTE FM receiver hearing instrument photo courtesy of Telex Communications Inc headphones earbuds to the processor of a cochlear implant or to a bone conduction transducer A recent addition to the category of per
6. hearing instru ment alone hearing instrument or EM plus FM and FM alone Priority should be given to testing in noise Proper placement of the FM microphone during sound field test ing is critical The recommended protocol is monitored live voice testing with the tester wearing the FM micro phone and the child seated in the sound field as seen in figure 14 Because the radiation pattern of loudspeakers and the assembly of the various loudspeaker elements for different frequency regions affect both the spectrum shape and the input levels to an FM microphone placed within inches of the loudspeaker this microphone place ment method is strongly discouraged ASHA in press Lewis 1992 In figure 14 input levels to the hearing instrument microphone or EM are 55 dB HL approximately 65 to 70 dB SPL with a background noise level of 50 dB HL resulting in an SNR of 5 dB Word recognition testing is completed first with the hearing instruments alone Testing is repeated in the FM only mode of operation When testing in noise with the FM microphone the noise level should remain at 50 dB HL The monitored live voice input to the FM microphone is estimated at 80 to 85 dB SPL based on speech input to a chest level microphone or 90 to 95 dB SPL for a boom microphone resulting in an improved SNR for speech in noise with the FM sys tem Finally testing is completed in the FM plus EM mode The true SNR advantage may be reduced with the FM system
7. is to main tain at least a 10 dB difference between the FM and EM signal when both are active In actual use the precedence circuit will provide a further reduction of the EM signal beyond what is measured electro acoustically tain approximately a 10 dB difference between the FM and EM signal when both are active For some degrees of hearing loss it may be neces sary to reduce the EM gain below targets Set to targets using Rationale and Procedures for FM System Fitting 103 EM Settings a 65 70 dB SPL speech weighted SW signal Use targets to set the maximum out put using a 90 dB SPL swept pure tone SPT signal SeET 10 Targets USING 65 70 dB SW signal Use targets to set the maximum output using a 90 dB SPT signal Set to within 0 to 5 65 70 dB SW signal Use targets to set the maximum output using a 90 dB SPT signal Set to targets 0 to 10 dB using an 80 dB SW signal Document maximum output with a 90 dB SPT signal Set to targets 0 to 10 dB with 80 dB SW ignat Docume maximum output with a 90 dB SPT signal Set to targets 0 to 10 dB with 80 dB SW signal Document a 90 dB SPT signal The amount to which the FM can be set above targets may be limited by the degree of hearing loss maxi Comments mum output of the system AGC in the teacher s mic and the method of output limi tation used e g peak clipping vs compression The amount to which the F
8. which the compression circuit is activated varies from one manufacturer to another however a majority of sys tems use approximately 75 dB SPL At least one manu facturer has a variable control in the transmitter so that the activation point or knee can be adjusted It must be remembered that a single transmitter may be used in conjunction with multiple receivers In such situations the kneepoint should be appropriate for all individuals who will be using FM receivers in a classroom A compression knee of 75 dB SPL means that the FM signals will almost always be in compression during typical conditions This will not be the case for signals in the hearing instrument or environmental microphone EM portion of the FM system The effects of compres sion must be taken into account when setting the FM sys tem and will be discussed in a later section Receiver Options FM receivers can be divided into two categories self contained and personal receivers Self contained receivers figure 9 originally were designed to be worn in place of hearing instruments and as such have internal controls that allow adjustment of gain tone SSPL and the relationship between FM and EM signals Traditionally the receivers have been body style units worn on the chest or at the waist They may be coupled to the users ears with behind the ear BTE or button transducers In the first arrangement the EM is located on the BTE transducer and in the second
9. with learning disabilities or central auditory pro cessing disorders Flexer Millin and Brown 1990 non native English speakers Crandell and Smaldino 1996 and children with a history of conductive hearing loss Gravel and Wallace 1992 Rosenfeld Madell and McMahon 1996 Classroom Acoustics One strategy to improve the listening conditions in classrooms is through the use of modifications to reduce extraneous noise from all sources outside the school inside the school but outside the classroom and within the classroom Examples include barriers to isolate the school from traffic noise locating high noise areas of the school e g cafeterias or music rooms away from regu lar classrooms and the use of absorptive materials with in classrooms For further information on classroom acoustics and acoustic modifications the reader is referred to anumber of excellent resources Berg 1987 1993 1997 Crandell and Smaldino 1995 Gegg Rosenberg 1995 Another strategy to improve listening conditions in classrooms and other learning environments is through the use of amplification systems This latter strategy is the focus of this chapter Amplification Options for the Classroom Hearing Instruments The most common assistive devices considered for individuals with hearing loss are personal hearing instru ments As an amplification option for the classroom hear ing instruments have a number of benefits First using the studen
10. years In 1992 the Federal Communications Commission FCC approved the addition of eight narrow band and 10 wide band channels in the 72 to 76 MHz range used by indi vidual FM systems FCC 1992 These new channels have alleviated problems with FM interference because Rationale and Procedures for FM System Fitting 93 Figure 11 Personal FM receiver photo courtesy of Phonak Inc they are located in a region where power usage is limited to one watt As a result there is less likelihood that other more powerful FM transmissions will interfere with the operation of individual FM systems In 1996 the FCC approved the addition of new FM transmission channels in the 216 to 217 MHz range Although not yet used by all FM manufacturers these additional channels will help to reduce FM interference as well as enable manufacturers to reduce the size of FM receivers FCC 1996 An important issue in FM system use is the relation ship between the FM and EM portions of the system when both are active simultaneously Hawkins 1984 reported an SNR advantage over hearing instruments of 11 8 to 15 1 dB when an FM system was used in the FM only condition however when the EMs were activated the advantage dropped to 1 8 to 3 9 dB Despite the obvi Figure 12 FM receiver in an audio boot photo courtesy of Phonak Inc 94 x A Sound Foundation Through Early Amplification ous advantages of using an FM system in the FM only mo
11. CHAPTER EIGHT One Size Does Not Fit All Rationale and Procedures for FM System Fitting Dawna E Lewis and Leisha R Eiten Introduction Welcome to Adventures in Audiology This week s installment is entitled The Nightmare Classroom As we begin our adventure we find ourselves in Mr Good teacher s second grade class A glance around the room reveals 20 students seated at desks arranged in groups of four and angled toward the front of the room Two hard walls are covered with student artwork while a chalk board covers the third It is an early fall day when the tem peratures in this part of the country are still quite warm Since this school does not have airconditioning the win dows which cover the fourth wall are open and portable fans are running to keep the temperatures at a reason able level Outside we hear the sounds of lawnmowers from neighboring yards and acity crew working on road repairs in front of the school Today Mr Goodteacher is using one of the school s overhead projectors as he introduces new math facts to the class We can hear the projector s fan as soon the equipment is turned on In addition we hear a constant humming which we soon realize comes from the fluo rescent lights overhead As Mr Goodteacher surveys the classroom he sees the usual levels of activity as students wiggle in their seats open desk tops tap pencils and crinkle papers In addition he notes several students who
12. M can be set above targets may be limited by the degree Of Nearing toss Max mum output of the system AGC in the teacher s mic and the method of output limi tation used e g peak clipping vs compression The amount to which the FM can be set above targets may be limited by the degree mum output of the system AGC in the teacher s mic and the method of output limi tation used e g peak clipping vs compression ability to achieve all three goals for a particular child depends upon the severity of the child s hearing loss the particular FM system available and the educational pri orities for the child When all goals cannot be met com promise is necessary Audibility of the teacher and maintenance of a signal to noise advantage for the teacher s voice remain priorities for an FM system When an FM system is functioning in the FM only mode of operation maintaining the advantage of the teacher s voice is achieved with little difficulty but meet ing the goals of monitoring self and hearing other voices is impossible Because all three listening goals are impor tant FM systems are generally used in the FM plus EM mode of operation However this may reduce the SNR advantage available from the FM microphone As previ ously discussed FM systems use a variety of strategies to maintain the FM microphone advantage when the sys tem is operating in the FM plus EM mode Some sys tems incorporate an FM precedence ci
13. Switzerland Phonak AG Moodie K S Seewald R C and Sinclair S T 1994 Procedure for predicting real ear hearing aid perfor mance in young children American Journal of Audiology 3 1 23 31 Nabelek A 1980 Effects of room acoustics on speech per ception through hearing aids by normal hearing and hearing impaired listeners In G Studebaker and I Hochberg eds Acoustical factors affecting hearing aid performance pp 25 46 Baltimore Md University Park Press Nabelek A and Robinson P 1982 Monaural and binaural speech perception in reverberation for listeners of vari ous ages Journal of the Acoustical Society of America 71 1242 1248 Neuman A and Hochberg 1983 Children s perception of speech in reverberation Journal of the Acoustical Society of America 73 2145 2149 Nozza R J Rossman R N F and Bond L C 1991 Infant adult differences in unmasked threshold for the dis crimination of consonant vowel syllable pairs Audiology 30 102 112 108 A Sound Foundation Through Early Amplification Nozza R J Rossman R N F Bond L C and Miller S L 1990 Infant speech sound discrimination in noise Journal of the Acoustical Society of America 87 339 350 Olsen W 1988 Classroom acoustics for hearing impaired children In F Bess ed Hearing impairment in child hood pp 266 277 Parkton Md York Press Inc Rosenfeld R M Madell J R and McMahon A 1996 Auditory function in norm
14. al hearing children with mid dle ear effusion In D Lim C Bluestone M Casselbrant J Klein and P Ogra eds Recent advances in otitis media Proceedings of the sixth inter national symposium pp 354 356 Toronto B C Decker Ross M and Giolas T 1971 Three classroom listening conditions on speech intelligibility American Annals of the Deaf 116 580 584 Seewald R C 1988 The desired sensation level approach for children Selection and verification Hearing Instruments 39 18 22 Seewald R C 1994 Fitting children with the DSL method Hearing Journal 47 10 48 51 Seewald R C Cornelisse L E Ramji K V Sinclair S T Moodie K S and Jamieson D G 1997 DSL v 4 1 for Windows A software implementation of the desired sensation level DSL i o method for fitting linear gain and wide dynamic range compression hearing instru ments London Ontario Hearing Health Care Research Unit Seewald R and Moodie K 1992 Electroacoustic consid erations In M Ross ed FM auditory training sys tems Characteristics selection and use pp 75 102 Timonium Md York Press Inc Seewald R C Ramji K V Sinclair S T Moodie K S and Jamieson D G 1993 DSL 3 1 user s manual London Ontario University of Western Ontario Stuart A Durieux Smith A and Stenstrom R 1990 Critical differences in aided sound field thresholds in children Journal of Speech and Hearing Research 33 612 615 Thibod
15. al to or slightly higher than the hearing instrument response figure 20 If the FM system will be used in the FM plus EM mode of operation then the response of the FM microphone should also be evaluat ed in the setting to determine if the FM microphone response is affected at this setting Third The response of the EM is also compared to the target hearing instrument response and the response of the FM portion of the system figure 21 When the hearing instrument acts as the EM e g a personal FM it should be evaluated in the mode of operation to determine if any changes in the response occur at this setting The EM in a self contained FM may be at ear or chest level and should be evaluated in both the EM only and EM plus FM modes of operation Performance at input levels corresponding to the child s own voice as well as close and far voices should be evaluated to deter mine if all audibility goals are achieved figure 22 Fourth The saturation response of the FM system at use and full on volumes is evaluated and compared to the target response of the hearing instrument to ensure that both the FM and the EM signal will be comfortable and safe in all modes of operation figure 23 Again real ear saturation response measures are not recommended at full on volumes If a personal hearing instrument is not available for comparison or if the child s hearing instrument is not pro viding appropriate audibility of s
16. ance Objective methods that can be used to evaluate FM performance are probe microphone mea sures 2cc coupler evaluation and estimated real ear performance using individually measured or average real ear to coupler difference RECD values Moodie Seewald and Sinclair 1994 added to 2cc coupler results Each assessment method has benefits as well as limita tions Functional Gain Measures Limitations Functional gain testing has long been used for assessing both hearing instrument and FM per formance however functional gain measures of perfor mance with FM systems have distinct limitations A major limitation of functional gain measures of FM system performance results from differences in input to the FM microphone during testing versus during actual use Because functional gain testing is a threshold mea sure the input levels to the FM microphone will be rela tively low During typical use the input to the FM microphone will be significantly higher because of its close proximity to the talker s mouth Since most FM systems incorporate input compression into their microphone design functional gain results for low input levels may sig nificantly overestimate the amount of gain available for a talker s voice during typical use of the FM system Lewis 1997 Lewis et al 1991 Seewald and Moodie 1992 This makes functional gain values difficult to interpret due to the nonlinear function of the FM microphone Additionally sin
17. and Gibler A M 1986 Auditory performance of children with unilateral sensorineural hearing loss Ear and Hearing 7 20 26 Boney S and Bess F 1974 Noise and reverbation effects in minimal bilateral sensorineural hearing loss Presented at American Speech Language Hearing Association Convention San Francisco Boothroyd A 1981 Group hearing aids In F Bess B Freeman and J Sinclair eds Amplification in educa tion pp 123 138 Washington D C Alexander Graham Bell Association for the Deaf Cornelisse L E Seewald R C and Jamieson D G 1995 The input output formula A theoretical approach to the fitting of personal amplification devices Journal of the Acoustical Society of America 97 1854 1864 Crandell C 1993 Noise effects on children with minimal sensorineural hearing loss Ear and Hearing 14 210 217 Crandell C and Bess F 1987 Developmental changes in speech recognition in noise and reverberation Asha 29 170 Crandell C McQuain J and Bess F 1987 Speech recognition of articulation disordered children in noise and reverberation Asha 29 170 Crandell C and Smaldino J 1995 Acoustical modifica tions within schools In C Crandell J Smaldino and C Flexer eds Sound field FM amplification Theory and practical applications pp 69 82 San Diego Singular Publishing Group Inc Crandell C Smaldino J and Flexer C 1995 Sound field FM amplification Theory and practical ap
18. ate for children with severe to profound hearing loss because of reductions in the low frequency amplifi cation of the hearing instrument with this coupling Self contained FM systems are preferable for students with profound hearing loss if adequate amplification cannot be achieved without feedback using their personal instru ments In such cases the remote microphone of the self contained FM system may reduce feedback and provide better audibility of soeech than an ear level hearing instru ment Children with unusual audiometric configurations such as rising or trough shaped losses may require a personal FM system coupled to a hearing instrument that has been programmed or modified for their specific needs A personal FM system takes advantage of the hearing instrument s frequency response to keep the sig nal consistent between the FM and hearing instrument alone Self contained FM systems may not have the flex ibility to fit unusual configurations of hearing loss Personal Hearing Instruments A personal FM system coupled to a child s hearing instruments requires that the child have hearing instru ments that are appropriate for the hearing loss and avail able consistently during the school day A personal FM system may not be the best choice for a child who often comes to school without the personal hearing instru ments or who has instruments that are chronically non functional lost or in need of repair This is also true f
19. azimuths relative to a hearing instrument test box loudspeaker As this fig ure shows there is considerable variation in the signal as the directional microphone is moved but very little varia tion as the omnidirectional microphone is moved For lis teners with significant hearing loss these differences will affect the audibility of the FM signal if the person wearing the microphone moves his or her head relative to the microphone or does not wear the microphone in the appropriate position Another feature of FM microphone transmitters that affects how they are set and how they function is the 92 x A Sound Foundation Through Early Amplification DIRECTIONAL MIC OUTPUT dB SPL OMNIDIRECTIONAL MIC 250 1000 a000 FREQUENCY Hz Figure 8 Difference in output at the FM receiver for a directional upper panel and an omnidirectional lower panel FM microphone in four different positions in a hearing instrument test box From Lewis 1994b 1994 American Journal of Audiology 3 70 83 presence of a compression circuit in the transmitter microphone A benefit of compression in the microphone transmitter is that the talkers voice will remain fairly constant across a range of input levels Thus if the teacher is talking to Johnny in the front row and then raises his voice to talk to Carol in the back row of the class the level received by the individual wearing the FM receiver will remain virtually unchanged The level at
20. blems the audiologist should investi gate what if any noise reduction strategies are already available in the classroom It is important to remember that the child s teacher is often the key to acceptance and use of the FM system Adequate in service training for the teacher and the class is an important part of a suc cessful FM fitting Several helpful sources are available to assist the audiologist to provide adequate in service train ing DeConde Johnson et al 1997 English 1996 Merely sending an FM system into a classroom with little warn ing and no training is a recipe for failure The teacher and audiologist should work as a team communicating about how the system functions how to troubleshoot the device and how the child is benefiting from its use The teacher s flexibility and acceptance of assistive technolo gy and his or her comfort with required listening checks troubleshooting and daily use need to be considered A child with an FM system will need to be placed with a teacher who is willing and able to handle the technologt cal aspects involved Individual Characteristics 98 A Sound Foundation Through Early Amplification Information about individual characteristics of the child will also help the audiologist select the most appro priate FM system and coupling option Age and size of the user must be considered when choosing the type of FM receiver A BTE FM may be too large for small chit dren or for children wit
21. ce functional gain is a threshold measure information about the maximum output of the FM system is not avail able and harmonic distortion cannot be assessed Test retest variability and the statistical significance of threshold differences also must be considered when using functional gain measures Hawkins et al 1987 and Stuart Durieux Smith and Stenstrom 1990 found that threshold differences would have to be greater than 10 to 15 dB to be considered statistically significant even for older children and adults Humes and Kirn 1990 reported test retest variability for aided results ranging from 5 to 8 dB Functional gain testing also requires extended periods of cooperation from a child and poorer frequency resolution available compared to probe micro phone measures even when complete octave and inte roctave results are obtained For children with sharply sloping or rising hearing loss configurations functional gain results in regions of normal hearing are spurious and may be affected by the noise floor of the test room or noise from the amplification system underestimating the actual output in the child s ear Word Recognition Testing Despite the limitations of functional gain measures word recognition testing in the sound field can be used to assess the FM user s ability to perceive words in quiet and noise with both the personal hearing instruments and the FM system Ideally mea surements are made for these conditions
22. de this mode of operation is rarely chosen because it limits the listener s ability to monitor his or her own voice and to hear others who are not wearing the FM micro phone e g classmates Several FM manufacturers have attempted to deal with the SNR problem when FM systems operate in the FM plus EM modes using FM precedence With FM precedence the output in both the FM and the EM modes is set to the same target level ensuring that the signal will be audible and comfortable when either mode is used alone When both FM and EM modes are active simultaneously the output level of the EM portion of the system varies depending on whether there is input to the FM microphone For example when the teacher is not talking no input to the FM microphone the EM output is at target When the teacher is talking input to the FM microphone the EM output drops by a predetermined amount often 5 to 15 dB Theoretically this type of circuit should be beneficial because it would be assumed that when the teacher is talking the student does not need to hear sounds via the EM When the teacher is not talking however there is no reduction of the EM signal Research with these circuits is still needed to determine if predicted benefit occurs in typical listening conditions Throughout a given day FM system users may require access to multiple transmission reception chan nels This might occur as students move from a class room using one channel to ano
23. e to monitor own voice and hear classmates e Ear canal resonance will not be lost e May be more cosmetically appealing may be recommending the purchase of a new FM sys tem that the child will be using for several years The pro cess of preselection takes into account knowledge about the child the child s personal amplification and the child s learning environment to assist with these decisions Limitations e Loss of binaural advantage environmental microphones at chest level e Frequency response may differ from that of personal hearing instrument e Loss of high frequency information e Weight of transducer in very small ears may affect fit e Feedback may be more of a problem e More likelihood of breakage e FM only and FM plus environmental microphone option may not be available e Separate cords and boots must be in stock for each student s FM system e Dependent on function of personal hearing instrument e Amplification varies with orientation and distance e Amplification limited by strength of telecoil e Frequency response may differ from that of HA alone e Affected by electromagnetic interference e FM only and FM plus environmental micro phone options may not be available e Dependent on function of personal hearing instrument e Often produces higher harmonic distortion and internal noise e Changes in HA output may occur with move ment of silhouette e Dependent on function of personal hearing
24. easurements are being made in SPL rather than gain the input signal that is chosen affects the output that is measured Developing Targets Goals When fitting an FM system for a child the following three goals are important to keep in mind 1 the FM sys tem should allow the child to hear the teacher or primary talker at a level that is consistently audible above back ground noise 2 the child should be able to monitor his or her own voice and 3 the child should be able to hear the voices of talkers not wearing the FM microphone The Table 6 Input levels to FM and EM microphones Reprinted by per mission of Phonak AG FM Microphone Chest level 80 85 dB SPL Headworn 90 95 dB SPL Conference 60 65 dB SPL Environmental Microphone Own voice at ear level 75 80 dB SPL Own voice at chest level 80 85 dB SPL Own voice at waist level Not recommended Other raised voices at approximately 1 meter 70 dB SPL Other voices at average conversation level at approximately 1 meter 65 dB SPL Other softer voices 60 dB SPL Table 7 Setting output in FM and EM modes of operation for different FM systems FM Settings Systems with auto matic fixed EM reduction in FM EM mode Systems with FM precedence Systems without FM precedence or automatic EM reduction in FM EM mode Goal The goal is to main tain approximately a 10 dB difference between the FM and EM signal when both are active The goal
25. eau L 1992 Physical components and features of FM transmission systems In M Ross ed FM audito ry training systems Characteristics selection and use pp 45 73 Timonium Md York Press Inc
26. ecei f AR an Audio Boot e Cosmetically appealing e Receiver at ear level may be more practical e Absence of low battery and no FM lights e Practical as full time amplification e Can be used with a variety of different HA models Use of FM Systems Outside Educational Settings Listening challenges experienced by persons with hearing loss are not limited to the classroom Communi cation and learning take place in a variety of environ ments some presenting more obstacles to understanding than others In relatively quiet environments where talkers are in close proximity hearing instruments alone are often sufficient However as noise and distance between talk ers increase the need for a remote microphone becomes more noticeable On playgrounds during sports or other extracurricular activities and even when riding in the car persons with hearing loss may experience significant communication difficulties Communication problems posed by noise and distance are present across the life e Shorter transmission range than body style systems e Currently only available for use with HA s from two manufacturers e Durability unknown due to recent introduction to marketplace span For infants and young children it is especially impor tant to provide a consistently audible signal during critical language learning years As an individual with hearing loss gets older many work and social situations also pose communicati
27. ed and adjusted As hearing health care professionals gain expertise in these areas they are ensuring communica tion access for individuals with hearing loss throughout all areas of life SSPL of Fill in FM ond EM Modes va Hearing Inetrumant SOUND PRESSURE LEVEL dB SPL 250 8000 10K FREQUENCY Hz Figure 23 Saturation response of hearing instrument solid line and FM system in FM dashed line and EM dotted line modes of operation Lewis et al 1998 Reprinted by permission of Phonak AG FM systems for children Rationale selection and verification Strategies 106 A Sound Foundation Through Early Amplification Conclusion Welcome to our next installment of Adventures in Audiology entitled From Here to Audibility When we left Hometown Elementary School Mr Goodteacher and Ms N Charge were discussing what could be done to helo three children in Mr Goodteacher s classroom Enter Audie Graham audiologist who has been called in for consultation After soending the afternoon observing Mr Goodteacher s class and furiously taking notes Audie begins to attack the problem Classroom modifications are developed to improve the acoustic environment A committee is established to evaluate noise and reverber ation in the school and to develop an acoustical control plan Berg 1987 This may include carpeting on the floor sound absorbing materials on classroom walls barriers outside the school and red
28. est box This eliminates the steps of deac tivating the reference microphone and moving the listen er away from the loudspeaker figure 16 No matter what real ear system is used it is important to review the man ufacturer s instructions for the appropriate test setup to evaluate both FM and EM function 2cc Coupler Measures Benefits and Limitations Another choice in objective FM evaluation is 2cc coupler measurement 2cc coupler measures have many advantages They allow evaluation of FM performance at input levels comparable to typical speech input levels at the FM and environmental micro phones The 2cc measures provide a more complete fre quency response assess harmonic distortion and can evaluate maximum output In addition 2cc measurement a z allows the audiologist to assess an FM system without the FM user being present Not having the FM user present when testing the FM system requires that one of three requirements be met 1 2cc coupler values at use settings are available from a hearing instrument that has already demonstrated sat isfactory audibility and maximum output 2 average ear values and correction factors are used to verify the desired response of the FM in the person s ears or 3 the person s individually measured RECD values are available to predict the response of the FM in the ears Real ear responses may be predicted using RECD mea sures by adding the values to the FM response obtained in t
29. functioning in the FM plus EM mode In the FM plus EM mode the level of the talker at the hearing Rationale and Procedures for FM System Fitting 99 instrument or EM remains at 55 dB HL noise is also received at the EM at 50 dB HL and the FM microphone receives input from the talker at 80 to 95 dB SPL Probe Microphone Measures Benefits and Limitations The use of probe micro phone measures resolves some of the difficulties associ ated with functional gain testing Probe microphone testing is an efficient verification method It requires less cooperation from the child so reliability is enhanced Individual ear information is available more quickly because less time is required to obtain all measures Most probe microphone systems delineate the frequency response more completely and any irregularities in the response can be observed Most important probe micro phone systems can evaluate the amplification received by the FM user at input levels comparable to actual speech inputs to the FM and hearing instrument micro phones The probe microphone system will also allow verification of maximum output in the child s ears Probe microphone measurement does have limita tions First and most practically it requires access to a probe microphone system Even when a probe micro phone system is available testing may not be possible if a child s ear canals have drainage or too much ear wax present or if the child will not allow place
30. h very small ears Likewise the weight of button receivers may make them a less ideal alternative than a BIE environmental microphone for some young children using self contained FM systems For junior high and high school FM users cosmetics and peer acceptance become overriding concerns and less visible coupling with a neckloop or wireless receiver boots may determine whether or not a student will use the FM system For students involved in sports the right choice of receiver coupling and FM microphone may enable them to use their FM system in all activities For example choosing a BTE FM or wireless receiver boot and a boom microphone may allow the child to hear a teacher or coach even when the listening environment is extremely noisy Children with fine motor difficulties may benefit from a body worn receiver with larger more accessible controls If a child has poor head control an ear level EM may not be ideal and a chest level EM may provide better audibil ity without feedback The type of FM chosen for children diagnosed with attention problems may depend on the distractibility of the child when wearing the device If a child is distracted by the receiver headphones or cords of a system worn on the chest it may be placed on the back Choose this placement only if the system has ear level environmental microphones A smaller BTE FM receiver or wireless FM boot coupled to a personal hearing instru ment also could be selected In ca
31. hand to answer Mr Goodteacher s last ques tion to the class Mary and Bobby are also paying close attention and Bobby has lost that look of fatigue we noticed on our first visit Outside the school Audie Graham loads an audiometer portable test box probe microphone system and in service materials into the van and heads off into the sunset She will return on another day to continue efforts to ensure that all students in all classrooms can hear and be heard Now Mr Goodteacher can turn his attention to Jenny who once again is in the principal s office But that s another story References Allen P and Wightman F 1994 Psychometric functions for children s detection of tones in noise Journal of Speech and Hearing Research 37 205 215 ASHA 2000 Guidelines for fitting and monitoring FM sys tems The ASHA Leader 5 20 Audioscan In preparation Fitting and verification of FM systems using the Audioscan RM500 Hearing Aid Analyzer Real Ear Measurement System Dorchester Audioscan Berg F 1987 Facilitating classroom listening A handbook for teachers of normal and hard of hearing students Boston College Hill Press Berg F 1993 Acoustics and sound systems in schools San Diego Singular Publishing Group Inc Berg F 1997 Optimum listening and learning environ ments In W McCracken and S Laoide Kemp eds Audiology in education pp 348 384 London Whurr Publishers Ltd Bess F H Tharpe A M
32. he 2cc coupler The reader is referred to Moodie Seewald and Sinclair 1994 for complete information on RECD measurement and its uses with 2cc coupler mea sures Moodie Sinclair Fisk and Seewald 2000 Setup When assessing the performance of an FM system using 2cc coupler measures the FM microphone is placed in the test box and the FM receiver is attached to the 2cc coupler and measurement microphone The FM transmitter and receiver should be placed at least 2 feet apart for proper signal transmission This means that the 2cc coupler will be placed outside the test box when evaluating the FM microphone as can be seen in figure 17 Most coupling options DAI button receivers BTE receivers and personal hearing aids can be placed on a soft surface such as a piece of foam for testing When using neckloop coupling the FM system must be evalu ated with the personal hearing instrument in the correct position and orientation to the neckloop to represent actu al use conditions This is accomplished by testing the sys tem with the neckloop and receiver on the child and the hearing instrument attached to the 2cc coupler held at Praba Lin Pi mass Pal bia d Breccia at ae eve om Figure 15 Example of setup for probe microphone testing of FM systems from Frye Electronics 1993 used with permission Rationale and Procedures for FM System Fitting 101 PROBE MODULE FM RECEIVER Figure 16 Exam
33. he listener figure 1 As a classroom amplification option individual FM sys tems have benefits and limitations listed in table 2 Microphone Transmitter Options There are a vari ety of microphone transmitter options available for use with FM systems Lavalier microphones figure 2 house the microphone and transmitter in a single case that is worn on the chest approximately 6 to 8 inches from the talkers mouth An antenna usually hangs down from the bottom of the transmitter case Lapel microphones figure 3 are separate from the transmitter The transmitter usu ally is worn at the waist or in a pocket A microphone is plugged into the transmitter clipped to the talker s lapel and the cord of the lapel microphone serves as the anten na Other microphones that can plug into a separate Figure 3 Lapel style FM microphone photo courtesy of Phonak Inc 90 A Sound Foundation Through Early Amplification Figure 4 Headworn style FM microphone photo courtesy of transmitter include headworn microphones figure 4 which are worn over the top of the head or over the ears and collar microphones figure 5 which are worn around the neck In these last two selections the microphone is located much closer to the talker s mouth usually within 1 to 2 inches Another option less commonly used in classrooms is the conference microphone figure 6 This microphone is designed for use in situations where there will be multiple
34. iable EM level the FM response would be set first Because of the compression utput in Fid and EM Modes cf Operation SOUND PRESSURE LEVEL 4B SPL 2509 1000 8000 FREQUENCY Hz Figure 22 Output of EM with two different input levels solid line 65 dB SPL dotted line 75 dB SPL compared to output of FM with 80 dB SPL input level dashed line Lewis et al 1998 Reprinted by permission of Phonak AG FM systems for children Rationale selection and verification strategies Rationale and Procedures for FM System Fitting 105 8 E p SOUND PRESSURE LEVEL dB SPL 8 8 ot 1000 8000 FREQUENCY iHz Figure 21 Comparison of output of hearing instrument dotted line with environmental microphone EM solid line and FM micro phone dashed line modes of operation for an FM system in the transmitter microphone maximum output mea sures of the EM may need to be made first since output may be higher in the EM mode than in the FM mode It is critical to assess the FM system in the configurations in which it will be worn If the system is used in FM plus EM mode then both the FM and the EM must be set and evaluated in that mode of operation FM systems provide proven benefit to children with hearing and attending problems FM system choices con tinue to increase as new technology becomes available But in order for a child to receive maximum benefit from an FM system it should be properly selected fitt
35. ifficulties and during lunch he talks with Ms N Charge the school principal Together they decide that something must be done soon before the school year progresses and the students fall too far behind But what can they do Can this classroom be saved Tune in next time as we meet Audie Graham pediatric audiologist in our next installment From Here to Audibility Establishing Candidacy for Classroom Amplification Research has shown that children with bilateral sen sorineural hearing loss experience difficulties un derstanding speech in degraded listening environments Finitzo Hieber 1981 Finitzo Hieber and Tillman 1978 Gengel 1971 Nabelek 1980 Olsen 1988 Ross and Giolas 1971 More recently individuals with conductive minimal and unilateral hearing loss also havebeen shown to experience difficulties Bess Tharpe and Gibler 1986 Boney and Bess 1974 Crandell 1993 There are groups of children with normal hearing who also have been 88 A Sound Foundation Through Early Amplification reported to experience difficulties understanding speech in the presence of distance noise and or reverberation These groups include young children Allen and Wightman 1994 Crandell and Bess 1987 Elliott 1979 Finitzo Hieber and Tillman 1978 Nabelek and Robinson 1982 Neuman and Hochberg 1983 Nozza et al 1990 children with articulation or language problems Crandell McQuain and Bess 1987 Elliott 1982 children diag nosed
36. ignal reaching a hearing instrument microphone in the classroom will be degraded often by a significant amount Even if a hearing instrument were able to repro duce signals at the microphone perfectly the student would hear a perfectly reproduced imperfect signal Remote Microphone Table 1 Practical issues related to new hearing instrument tech nology as classroom amplification Wide Dynamic Range Compression e Increased potential for feedback e Circuit noise in regions of normal hearing e Output tied to volume control e Limited power for individuals with severe to profound hearing loss e Limited to ability to amplify speech from a distance Multiple Memories e Remote control Who is in charge amp Who monitors e Selecting appropriate memories for classroom use Multiple Microphones e Remote control e Appropriate use in and out of the classroom Understanding communication rules Lecture Class discussion e Decreased localization ability when in directional microphone mode Tranumitter and Fire Recelver Options Figure 1 Coupling options for use with FM systems From Lewis 1998 Reprinted with permission 1998 Children with hearing impairment Contemporary trends pp 277 295 Vanderbilt Bill Wilkerson Center Press One of the best ways to reduce the effects of dis tance noise and reverberation is to place a remote micro phone near the sound source usually the talker s mouth Many clas
37. igure 7 Hand held FM microphone photo courtesy of Phonak Inc Rationale and Procedures for FM System Fitting 91 Table 3 Benefits and limitations of FM system microphone options Reprinted by permission of Phonak AG Microphone Style Features Advantages Disadvantages Lavalier e Microphone and transmitter e Single case may simplify use e Weight of device around the Lapel e in one case Antenna extends from case Microphone usually directional Worn around the neck Microphone and transmitter separate Antenna in cord of lapel microphone Headworn Conference Hand held Microphone clips to lapel or may hang around neck Transmitter usually clips to belt or waistband Microphone may be direction al or omnidirectional Microphone and transmitter separate Antenna in microphone cord Worn on headband or glasses Microphone usually directional Microphone and transmitter separate Antenna in base of micro phone or in cord Microphone placed in single location usually table top Microphone is omnidirectional Newest models allow selection of different levels of directionality User holds microphone and directs it toward talker No need for waistband or belt to attach to transmitter e May be more comfortable than other options for some users e May be easier to troubleshoot because of separate components e Headworn microphone improves SNR e May be easier to troubleshoot because of sepa
38. ld s personal hearing instrument The output of the FM signal in the child s ear would be excessive if the FM and EM hearing instrument response were compared and set using gain because the input to the FM micro Figure 17 Example for 2cc coupler testing of FM systems From Audioscan in preparation Used with permission 102 A Sound Foundation Through Early Amplification Figure 18 Setup for 2cc coupler testing of an FM system coupled to a personal hearing aid using a neckloop From Frye Electronics 1993 Used with permission phone is higher Lewis et al 1991 Seewald and Moodie 1992 Input Levels Recommended input levels depend on the type of microphone its location and the situation being simulat ed A range of levels is displayed in table 6 When assessing the FM microphone 80 to 85 dB SPL inputs represent chest level lapel or lavalier microphones placed 6 to 8 inches away from the talker s mouth Inputs of 90 to 95 dB SPL are more representative of headworn microphones with microphone placement only 1 to 2 inches from the mouth A conference microphone would have a significantly lower input level 60 to 65 dB SPL depending on the distance between the talkers and the microphone For the EM or hearing instrument microphone the input level will vary depending on the speech input of interest For example 70 dB SPL is widely considered to represent raised conversational speech from a distance of approxima
39. lepin FM system photo courtesy of Phonic Ear Inc Rationale and Procedures for FM System Fitting 95 Table 4 Benefits and limitations of receiver options Receiver Body Style Self Contained Receiver Personal Receiver Instrument FM Receiver Benefits e Can receive FM signal when hearing instruments are not operational e May be less complicated to set when a hearing instrument is not involved e FM only and FM plus environmental microphone options available on most units e Often smaller and more lightweight than self contained units e Frequency response may be more simi lar to that of hearing instrument alone e May be less complicated to set depend ing on hearing instrument coupling e Cosmetically appealing e Receiver at ear level may be more practical e Practical as full time amplification Limitations e Frequency response not as flexible e Frequency response may differ from that of the personal hearing instrument e Traditional systems not cosmetically appealing e Frequency response may differ from that of personal hearing instrument e FM only and FM plus environmental microphone options may not be available e Traditional systems may not be cosmet ically appealing e Absence of low battery and no FM lights e Large ear level receiver may not fit well on some ears e Shorter transmission range than body style systems e Receiver antenna may break or become loose FNHR
40. ment of the probe in the ear For young children cooperation may be an issue because multiple measures are needed to assess both FM and environmental microphone function When a child has vibrotactile rather than auditory hearing responses probe microphone measures do not accu rately reflect whether speech audibility has been Figure 14 Suggested test arrangement for speech perception assessment with an FM system using monitored live voice Reprinted by permission of authors and American Speech Language Hearing Association 2000 The ASHA Leader 5 20 100 A Sound Foundation Through Early Amplification achieved As with functional gain measures harmonic distortion cannot be assessed with a probe microphone system Setup When evaluating FM s using a probe micro phone system care must be taken in positioning both the system s measurement and reference microphones as well as the remote FM microphone With one probe microphone test system the reference microphone is first leveled in the correct position relative to the output loud speaker The FM microphone is then substituted for the reference microphone which is deactivated and the per son is moved as far away from the loudspeaker as pos sible The FM microphone will then receive the signal from the loudspeaker at a known level figure 15 With another test system the audiologist can perform real ear testing on FM systems with the FM microphone placed directly in the t
41. oeech the audiologist will need to generate targets for the FM system response directly from a prescriptive fitting procedure This is easi ly accomplished using DSL because target responses can be specified in either gain or SPL Some test systems SOUND PRESSURE LEVEL idB SPL g9 1000 8000 FREQUENCY Hz Figure 19 Output of personal hearing instrument with 65 dB SPL input solid line and 90 dB SPL input dashed line signals Output of Fh ve Hearing Inetrument SOUND PRESSURE LEVEL dB SPL 250 8000 10009 FREQUENCY iHz Figure 20 Output of hearing instrument with 65 dB SPL input solid line compared to output of FM system with 80 dB SPL input dot ted line allow the FM response to be directly compared to DSL targets using simulated or actual probe microphone mea sures Once targets have been generated steps 2 through 4 are performed The most efficient order of testing will vary with the FM system being tested and with the coupling option being used When assessing a personal FM system or when using DAI or neckloop coupling the hearing instru ment is the EM for the system and will be tested first to determine targets for the FM response With a self con tained FM system the order will depend on whether the system has an EM or an FM level control With a variable FM level the most common configuration the EM response is set first and the FM set in relationship to it Alternately for a system with a var
42. on dilemmas FM systems provide improved communication in a variety of situations Advances in FM technology make these systems a practical solution for many individuals with hearing loss Preselection Issues With such a variety of transmitter receiver and cou pling choices available how does an audiologist select a system that will work best for a particular child In some instances there are few choices and the audiologist must determine if the system available from the school district is appropriate In other situations the audiologist 96 A Sound Foundation Through Early Amplification Table 5 Benefits and limitations of FM system coupling options Reprinted by permission of Phonak AG Coupling Options Button Transducers BTE Transducers Direct Audio Input Neckloops Nonoccluding Headphones Silhouette Benefits e Decreased feedback and potential for higher output e Environmental microphones at ear level e Frequency shaping possible with use of special tone hooks e Frequency response may be more similar to that of personal hearing instrument than with button transducers e Desired frequency response easily obtained e Amplification not affected by orientation and distance e Cosmetically appealing e Less chance of breakage e Desired frequency response may be more easily obtained e Cosmetically appealing e For hearing instruments with weak telecoils provides a stronger signal e Child abl
43. or the child whose hearing instruments are not providing Rationale and Procedures for FM System Fitting gt 97 adequate gain and output to make speech audible In these cases use of a self contained FM assures consis tent FM use and adequate audibility of speech The coupling option chosen may also depend on the child s personal hearing instruments Direct audio input DAI and or telecoil are not available in every model of hearing instrument The strength of the telecoil varies across manufacturers and models In addition it is impor tant to know if a particular hearing instrument model allows both FM only and FM plus EM modes of opera tion Educational Environment Obtaining information about the child s educational environment is critical when preselecting an FM system The size and organization of the child s classroom can be either a detriment or a benefit to overall audibility and can guide the audiologist in the selection process An impor tant issue to examine with the child and the child s teach er is how often he or she encounters difficult listening situations Is the classroom often noisy Are there multi ple activities going on simultaneously How difficult is it to hear in music class or physical education class Answers to these and other questions can help the audiologist nar row down microphone and receiver coupling choices Especially for children with minimal and unilateral hearing loss or attention pro
44. ple of setup for probe microphone testing of FM systems From Audioscan in preparation Used with permission the ear figure 18 If the child is not available for testing the FM receiver can be worn by the tester although results may vary due to differences in distance between the neckloop and telecoil The EM of a self contained FM system must also be evaluated electroacoustically Some FM manufacturers recommend that the receiver be oriented on its side in the test chamber when testing the EM For some test sys tems this will require that the test box be releveled with the lid open to allow this orientation Hearing instruments act as the EM when used with personal FM systems and also should be evaluated in the 2cc coupler Measurement Protocol RE700A COUPLER MIC wbo BTE COUPLER aan HEARING FM RECEIVER FM TRANSMITTER AID Once you have chosen the FM system decided on the best coupling option and selected the assessment method what inputs do you use to assess the system Input stimuli and levels are chosen to represent typical speech inputs to the FM and environmental micro phones The actual input levels will vary depending on both the FM and the EM microphone locations Because of the variation in input level to the different microphones it is important to make all measures in SPL rather than gain in order to accurately compare the response of the FM microphone to the environmental microphone and or the chi
45. plica tions San Diego Singular Publishing Group Inc Crandell C and Smaldino J 1996 Speech perception in noise by children for whom English is a second lan guage American Journal of Audiology 5 3 47 51 DeConde Johnson C Benson P V and Seaton J B 1997 Educational audiology handbook San Diego Singular Publishing Group Inc Elliott L L 1979 Performance of children aged 9 to 17 years on a test of speech intelligibility in noise using sentence material with controlled word predictability Journal of the Acoustical Society of America 66 3 651 653 Elliott L 1982 Effects of noise on perception of speech by children and certain handicapped individuals Sound and Vibration December 9 14 English K 1996 FM A proposed protocol for fitting FMs in school settings Educational Audiology Association Newsletter 12 1 8 9 Federal Communications Commission 1992 Additional Frequencies for Auditory Devices for the Hearing Impaired Amendment of Part 15 92 163 ET Docket No 91 150 April 7 Federal Communications Commission 1996 Amendment of the Commission s Rules Concerning Low Power Radio and Automated Maritime Telecommunications System Operations in the 216 217 MHz Band FCC 96 315 ET Docket No 95 96 July 25 Finitzo Hieber T 1981 Classroom acoustics In R Roeser and M Downs eds Auditory disorders in school chil dren The law identification remediation pp 250 262 New York Thieme Strat
46. rate components e No need to pass microphone e Able to pick up numerous talkers from single micro phone location e Portable e Can point rather than microphone e Directional capabilities attenuate unwanted sounds from sides and back pass neck may be uncomfortable e Single case for all components may complicate troubleshooting e Poor placement or head move ment may degrade signal e Poor placement or head move ment may degrade signal e Fit may be uncomfortable e Some benefit lost if worn dif ferently than recommended e Poor placement may degrade signal e Fit may be uncomfortable e Some benefit lost if worn dif ferently than recommended Amplify other unwanted sounds in the room e Distance of microphone from mouth of talker greater reducing SNR advantage e Listener responsible for transmitter e User or caregiver must un derstand appropriate use e Distance of microphone from mouth of talker greater reducing SNR advantage situations because they will pick up the talker s voice bet ter than surrounding noise However when not worn cor rectly directional microphones are less forgiving resulting in less intensity at the microphone than if an omnidirectional microphone was being worn Lewis 1994b Thibodeau 1992 Figure 8 from Lewis 1994b illustrates changes in response across frequencies for a directional upper panel and omnidirectional lower panel lapel microphone at four different
47. rcuit which automatically reduces the level of the environmental microphone when an input signal is detected in the FM microphone Other systems have an automatic fixed 104 A Sound Foundation Through Early Amplification reduction of the EM level whenever the system is func tioning in the mode For FM systems that have neither FM precedence nor automatic EM reduction the rela tionship of the FM and EM levels may be manually adjustable with an FM or EM level or volume control With all types of circuits the goal is to maintain approximately a 10 dB difference between the FM and EM signal when both are active The extent to which the 10 dB difference can be achieved will depend on degree of hearing loss and output limits of both FM and hearing instruments Table 7 gives recommendations for setting both the EM and the FM levels for different tyoes of FM systems Step By Step Developing electroacoustic targets for the FM sys tem should be consistent with other amplification goals Ideally all amplification that the child uses should provide audibility of the teacher self and others Amplified signals should also be comfortable and free from distortion If a child s personal hearing instruments have been shown to meet prescriptive targets for audibility then the response of the FM can be matched to the response of the hearing instruments For children the desired sensation level DSL approach Cornelisse Seewald and Jamieson
48. rders 49 409 418 Hawkins D B Walden B E Montgomery A and Prosek R A 1987 Description and validation of an LDL proce dure designed to select SSPL90 Ear and Hearing 8 162 169 Humes L E and Kirn E U 1990 The reliability of func tional gain Journal of Speech and Hearing Disorders 55 193 197 Leavitt R and Flexer C 1991 Speech degredation as measured by the rapid speech transmission index RASTI Ear and Hearing 12 115 118 Lewis D 1992 FM systems Letter to the Editor Ear and Hearing 13 290 293 Lewis D 1994a Assistive devices for classroom listening American Journal of Audiology 3 58 69 Lewis D 1994b Assistive devices for classroom listening FM systems American Journal of Audiology 3 70 83 Lewis D 1997 Selection and evaluation of classroom amplification In W McCracken and S Laoide Kemp eds Audiology in education pp 323 347 Manchester University of Manchester Lewis D 1998 Classroom amplification In F Bess ed Children with hearing impairment Contemporary trends pp 277 295 Nashville Tenn Vanderbilt Bill Wilkerson Center Press Lewis D Feigin J Karasek A and Stelmachowicz P 1991 Evaluation and assessment of FM systems Ear and Hearing 12 268 280 Lewis D Eiten L Hoover B and Stelmachowicz P 1998 FM systems for children Rationale selection and verification strategies Booklet accompanying video of same name in Phonak Video focus series
49. room settings the teacher may not always be the primary talker Computers TVs and VCRs are increasingly common teaching tools Distance reverber ation and background noise interference can also affect the audibility of signals from these devices The use of FM systems in conjunction with auxiliary devices expands the child s access to a variety of learning envi ronments Computers TV VCRs and tape and CD players can be connected to an FM system in a variety of ways The FM microphone transmitter may be placed close to the loudspeaker output of any of these devices This might be a good choice if an auxiliary connection is not available or if other students also need to hear the signal at the same time A direct connection also may be made between a device and the auxiliary input of the FM trans mitter using an appropriate input cord available from the FM manufacturer Direct connection may provide a more consistent signal to the FM user but may disable the device s external loudspeaker for other listeners The FM receiver also may be connected directly to the device This is appropriate when the student is close to the device or working alone Again connecting direct ly to the receiver may disable an external loudspeaker for other listeners Some personal FM receivers however may not have an auxiliary input In some instances spe cial cords or modifications may be required for use m a b pi i e Figure 13 Te
50. ses of extreme dis tractibility the use of a sound field FM system may be the best choice Assessment of Aided Performance After choosing an FM system the instrument will need to be set and its performance verified to assure that the system is functioning correctly and that the child will receive benefit Assessment then includes both evalua tion of FM function and verification of the child s aided performance Merely setting an FM system based on manufacturer s specification sheets is no guarantee that the child is receiving an appropriate audible signal Assessment of aided performance is essential to a suc cessful fitting Assessment Methods Before setting an FM system or verifying its perfor mance on a child it is best to make sure that the system is working properly Both a listening check and 2cc coupler evaluation of FM and environmental microphones are important Much time can be wasted setting a system that is not functioning according manufacturer s specifications in one or both channels Currently there are no ANSI specifications for FM performance but many FM manu facturers provide full on gain SSPL90 and harmonic dis tortion information for their systems with various coupling options Both behavioral and objective methods are available to the audiologist when setting and verifying FM system performance Behavioral methods include functional gain testing and sound field evaluation of word recognition perform
51. sonal receivers is an FM receiver in an audio boot figure 12 When the boot is attached to a compatible hearing instrument it allows FM reception At the time of this writ ing several companies produce wireless FM boots For some systems the antenna extends upward from the boot For at least one the antenna is internal or in a small metal ring at the bottom of the boot An additional small antenna can be added to extend the reception range The most recent advance in FM receiver boots is the open platform FM receiver This receiver snaps directly into the audio boot of many current brands and models of hearing aids with direct audio input capabilities When connected to the hearing aid the FM receiver has a switch allowing it to be set to FM only FM plus hearing aid or hearing aid only mode of operation As stated previously FM systems also may be used in conjunction with cochlear implants Special cords are available from each FM manufacturer to connect the out put of the FM receiver to the auxiliary input of the implant processor Initial attempts to connect these two systems have been hampered by interference problems but FM companies have been working to improve the compatibil ity of their systems with cochlear implant processors Recent Developments in FM Technology In addi tion to the BTE and boot receivers there have been a number of other developments in both self contained and personal FM technology over the past few
52. sroom amplification systems employ a remote microphone to improve the signal to noise ratio SNR advantage for listeners with hearing loss These include infrared systems large area induction loop systems sound field amplification systems and individual frequen cy modulated FM systems It is beyond the scope of this chapter to discuss all classroom amplification alternatives Therefore the focus will be on individual FM systems For further information on other classroom amplification choic es the reader is referred to other sources Berg 1993 Boothroyd 1981 Crandell Smaldino and Flexer 1995 DeConde Johnson et al 1997 Gilmore 1995 Lewis 1994a 1997 Figure 2 Lavalier style FM microphone transmitter photo cour tesy of AVR Sonovation Rationale and Procedures for FM System Fitting 89 Table 2 Benefits and limitations of individual FM systems Benefits e Flexible for wide range of hearing losses e May not require hearing instruments to operate e Portable e Can be used in multiple rooms in the same building Limitations e FM interference e Some systems require functioning hearing instruments with appropriate coupling e User must wear receiver e Troubleshooting maintenance may be more compli cated than with some other amplification devices Individual FM Systems In individual FM systems the talker wears a micro phone transmitter from which signals are sent via FM radio waves to a receiver worn by t
53. t s hearing instruments at school ensures that he or she will be using the same signal processing throughout the day Hearing instrument choices have increased considerably in recent years yielding a variety of style and processing schemes When used appropri ately hearing instruments allow the student to hear the teacher the voices of other students and his or her own voice although not to the same degree They also are portable and do not require the student or teacher to wear additional equipment However practical issues must be addressed when using new hearing instrument technolo gy as classroom amplification These practical issues are summarized in table 1 In addition there are many situa tions in which the ability of hearing instruments to process speech will be adversely affected by distance noise and reverberation Using Rapid Speech Transmission Index RASTI measurements Leavitt and Flexer 1991 examined the reproducibility of a soeechlike signal at various distances from a sound source in a classroom A score of 1 0 rep resents a perfect reproduction of the signal Leavitt and Flexer 1991 reported scores of 0 83 at 2 65 meters from the source front row center seat and 0 55 at 10 88 meters back row center seat In fact a score of 1 0 was obtained only at the reference location 6 inches from loudspeaker Although these results cannot be com pared directly to soeech perception scores they suggest that the s
54. tely 1 meter An input level of 65 dB SPL would represent average conversational speech originat ing at 1 meter In order to evaluate audibility of the child s own voice for self monitoring purposes input levels of 75 to 80 dB SPL are used The saturation response of the FM system in the child s ear is always of concern and should be evaluated both at use and full on volume settings using a 90 dB SPL constant level pure tone input The presence of input compression in the FM microphone needs to be consid ered once again because the maximum output in the FM mode may be less than the maximum output of the EM or the users own hearing instrument The saturation response of all microphones needs to be assessed Input Stimuli The type of signal chosen to assess an FM system may depend on the particular 2cc or probe microphone test system being used In currently available systems one or more of the following signals are available con stant level swept pure tones speech weighted swept pure tones speech weighted composite noise or a dynamic signal with pure tones of variable amplitude and duration Speech weighted inputs are more representa tive of the intended inputs to the microphones but con stant level swept pure tones can also be used Whatever type of signal is chosen consistency is the key Regardless of input level the same type of input must be used for all measurements with the exception of satura tion response Because m
55. ther using a different channel or because FM interference in a particular loca tion renders a previously used channel useless Even within a classroom different channels may be helpful if students are divided into small groups Traditionally FM transmitters were single channel and receiver channels were changed by using removable oscillator chips They required the purchase of multiple transmitters one for every transmission channel needed and many oscillator chips Several FM manufacturers now have transmitters and receivers where the channels can be changed via pushbuttons or knobs significantly increasing the ease with which different channels can be accessed Recently one FM manufacturer introduced a per sonal FM system that provides the cosmetic benefits of neckloop coupling without many of the limitations i e low frequency roll off electromagnetic interference sig nal fluctuations with head movement In this system the user wears a body style FM receiver coupled to a special neckloop A direct audio input boot is coupled to the hear ing instrument and a small pulse width modulated receiv er is attached figure 13 The signal is sent to the body style receiver via FM transmission and from the neckloop to the receiver at the ear via telemagnetic trans mission Each receiver and coupling option has its own benefits and limitations which are outlined in tables 4 and 5 Using FM Systems with Other Technology In class
56. ton Finitzo Hieber T and Tillman T 1978 Room acoustics effects on monosyllabic word discrimination ability for normal and hearing impaired children Journal of Speech and Hearing Research 21 440 458 Flexer C Millin J and Brown L 1990 Children with developmental disabilities The effect of sound field amplification on word identification Language Speech and Hearing Services in Schools 21 177 182 Frye Electronics 1993 Testing FM systems and auditory trainers with Fonix 6500 Analyzers Tigard Frye Electonics In Gegg Rosenberg G 1995 Improving classroom acoustics ICA Inservice training manual Tallahassee Fla Florida Department of State Gengel R 1971 Room acoustics effects on monosyllabic word discrimination ability for normal and hearing impaired children Journal of Auditory Research 11 219 222 Gilmore R A 1995 Orientation to the use of induction loop systems In R S Tyler and D J Schum eds Assistive devices for persons with hearing impairment pp 185 196 Needham Heights Mass Allyn and Bacon Gravel J and Wallace 1992 Listening and language at 4 years of age Effects of early otitis media Journal of Speech and Hearing Research 35 220 228 Rationale and Procedures for FM System Fitting 107 Hawkins D 1984 Comparisons of speech recognition in noise by mildly to moderately hearing impaired chil dren using hearing aids and FM systems Journal of Speech and Hearing Diso
57. uction of noise from audiovi sual equipment In the meantime Audie travels to a local health club and picks up a box of used tennis balls These are sliced on the top and placed on the feet of all of the desk chairs to reduce noise levels generated by move ment across the hard floor The three children Mary Tommy and Bobby are evaluated audiologically and a trial with FM amplification systems is initiated Bobby is fitted with a self contained FM receiver that will provide him with classroom amplifi cation even on those days when he does not wear his own hearing instruments Tommy is fitted with a personal FM receiver with nonoccluding earphones and Mary uses a personal FM receiver coupled to her hearing instruments In addition Mary is moved to a location in the classroom that will provide good visibility without as much noise interference Mr Goodteacher uses a head worn microphone to provide the best SNR advantage possible when the windows are open and fans running An in service with Mr Goodteacher and the three stu dents ensures that the systems will be used appropriate ly A classroom discussion gives the other students a better understanding of the problems associated with noise distance and reverberation as well as the effects of hearing loss As the afternoon sun filters through the windows sig naling the ending of the school day the camera pans the classroom Instead of making paper airplanes Tommy is raising his
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