Effect of Preferred Volume Setting on Speech Audibility in Different Hearing Aid Circuits

Transcription

1 J Am Acd Audiol 12 : (2001) Effect of Preferred Volume Setting on Speech Audibility in Different Hering Aid Circuits Pmel E. Souz* Virgini J. Kitch* Abstrct This study compred preferred volume setting for liner pek clipping, compression limiting, nd wide dynmic rnge compression (WDRC) mplifiction nd quntified speech udibility t the preferred volume setting for ech mplifiction type. Ten listeners with mild-to-moderte hering loss were fitted monurlly with behind-the-er hering id progrmmed sequentilly with WDRC, compression limiting, nd liner pek clipping mplifiction. Speech ws presented in quiet nd in noise t rnge of input levels. In ech condition, the listener djusted the volume for mximum clrity. Signl levels were mesured using probe microphone system. There ws no significnt difference in speech udibility between mplifiction strtegies for ny speech level regrdless of the presence or bsence of bckground noise. These results imply tht the improved udibility dvntge of WDRC over liner mplifiction demonstrted in controlled environments my not be mintined in situtions where the hering id werer controls the volume. Key Words: Amplifiction, udibility, clrity, preferred volume Abbrevitions : AAI = Aided Audibility Index, AI = Articultion Index, ANOVA = nlysis of vrince, NAL-R = Ntionl Acoustic Lbortories-Revised, SIR = Speech Intelligibility Rting, WDRC = wide dynmic rnge compression he incresed use of wide dynmic rnge compression (WDRC) mplifiction hs T generted considerble discussion of its benefits reltive to liner mplifiction. The theoreticl dvntge of WDRC is improved udibility of low-level speech cues without discomfort from high-level sounds. However, the results of studies with werble ids hve been mixed : some studies demonstrte superior speech recognition performnce with WDRC ids, wheres others show limited or no benefit from this strtegy (see Hickson, 1994, nd Dillon, 1996, for reviews). One possible influence on these results is tht the theoreticl benefits of improved udibility my not be mintined in situtions in which the listener controls the hering id volume setting. In prt, this rgument is the bsis for the exclusion of volume controls in mny *Deprtment of Speech nd Hering Sciences, University of Wshington, Settle, Wshington Reprint requests Pmel E. Souz, Deprtment of Speech nd Hering Sciences, University of Wshington, th Avenue NE, Settle, WA WDRC hering ids. However, bout hlf of hering id werers fit with WDRC ids tht do not include mnul volume djustment would prefer to hve volume control (Knebel nd Bentler, 1998 ; Vlente et l, 1998 ; Kochkin, 2000). It is importnt to understnd the effects of such volume djustments, which cn lter the reltive output levels of speech mplified by different hering id circuits. Severl recent studies support the ide tht vritions in volume setting cn significntly ffect the dvntge of improved udibility offered by WDRC mplifiction. In fct, the benefits of WDRC hve been demonstrted most clerly in controlled studies in which the experimenter sets the mount of gin nd thus the udibility of the speech signl. In this sitution, WDRC cn improve speech udibility, with corresponding improvements in speech recognition, over liner mplifiction. For exmple, when test conditions re designed to provide greter udibility for WDRC-mplified thn for linerly mplified speech, speech recognition is higher with the WDRC hering id (Moore nd Glsberg, 1986 ; Souz nd Turner, 1998). However, when speech udibility is equted between 415

2 Journl of the Americn Acdemy of Audiology/Volume 12, Number 8, September 2001 hering ids, listeners show similr performnce for liner versus WDRC mplifiction (Souz nd Turner, 1996, 1998). The recent Ntionl Institute on Defness nd Other Communiction Disorders/Veterns Affirs clinicl tril found miniml differences in speech recognition between WDRC nd liner (compression limiting or pek clipping) hering ids when hering id volume ws djusted to Ntionl Acoustic Lbortories-Revised (NAL-R) trgets (Lrson et l, 2000). Presumbly, NAL-R trgets were similr cross mplifiction types for moderte input levels, resulting in similr speech udibility nd no net dvntge for the WDRC hering id in tht sitution. In comprisons of compression nd liner mplifiction where the listener djusted the volume, the results were mixed : some investigtors found superior performnce with the WDRC id (e.g., Lurence et l, 1983), wheres others found no difference between circuits (Dreschler et l, 1984 ; Tyler nd Kuk, 1989). Becuse speech output level t the preferred volume ws not specificlly mesured in these studies, it is difficult to distinguish between the effects of volume djustment on speech udibility nd other procedurl fctors (e.g., frequency response, compression prmeters). However, the studies described bove do suggest tht when the listener hs control over the volume setting, listener who selects higher volume setting for WDRC processing thn for liner processing my show better performnce with the WDRC id, wheres listener who selects similr output levels for both ids my show no difference in recognition. There is considerble informtion vilble bout how listeners select gin for liner hering ids (Cox, 1984 ; Byrne nd Cotton, 1987 ; Kuk, 1990 ; Leijon et l, 1990 ; Cox nd Alexnder, 1991 ; Kuk nd Lu, 1996 ; Humes et l, 2000 ; Scollie et l, 2000). Preferred volume setting depends on the listening environment, including tlker level, signl-to-noise rtio, mount of reverbertion, nd degree of distortion (Wlden et l, 1977 ; Cox nd Alexnder, 1994). For exmple, liner hering id werers select higher gin in quiet thn in noisy or reverbernt environments (Cox nd Alexnder, 1991). Overll, ptients select lower volume levels for everydy listening thn those prescribed by clinicins (Leijon et l, 1984, 1990 ; Cox nd Alexnder, 1991 ; Humes et l, 2000). We know less bout how listener djusts gin for WDRC hering ids. In recent study, Neumn et l (1995) compred preferred lis- tening levels for slow-cting compression versus liner lbortory-bsed mplifiction systems. Input-output functions were equted reltive to 70-dB input signl presented t the most comfortble level, providing successively higher gin for low-intensity speech s compression rtio incresed. At low compression rtios, preferred listening levels were similr for liner nd slow-cting compression mplifiction. At higher compression rtios, the listeners selected higher listening levels for the liner mplifiction. Becuse this study used long time constnts nd higher kneepoint thn typicl for WDRC systems, it is not possible to pply these results directly to WDRC hering ids. Also, becuse lbortory-bsed system ws used, mny of the effects present in werble ids (e.g., ermold nd venting effects, distortion owing to output limiting) were not incorported. Such effects re importnt to consider becuse they cn influence volume setting (Kuk, 1990). To explore these issues, the gols of this study were to (1) compre the subject's preferred volume setting for liner pek clipping, compression limiting, nd WDRC mplifiction nd (2) to quntify speech udibility t the ptient's preferred volume setting for ech type of mplifiction. Prticipnts METHOD Prticipnts were 10 listeners (7 femle, 3 mle), ged 23 to 70 yers (men = 52 yers), with bilterl sensorineurl hering loss. All were ntive spekers of English. Six of the listeners were current binurl hering id werers ; of these, 3 used compression-limiting hering ids, 2 WDRC hering ids, nd 1 liner pek clipping id. Three hd never worn mplifiction nd 1 ws prior hering id user, but the id ws not vilble for testing. Becuse listenerselected gin ppered to be stble over time for both the new nd experienced hering id users (Leijon et l, 1990 ; Cox nd Alexnder, 1992 ; Bentler et l, 1993 ; Humes et l, 2000), the prticipnts were considered single test group without regrd to prior mplifiction use. Frequency-specific loudness discomfort levels were mesured for wrble tones presented t 0.5, 1, 2, nd 4 khz vi n ER-3 insert erphone. As recommended by Hwkins et l (1987), loudness discomfort level ws defined s the lowest intensity tht evoked response of "uncomfortbly loud" in two of three scending trils, minus 416

3 Preferred Volume Setting nd Speech Audibility/Souz nd Kitch 110 J 2 m J U) m r O ~ i -, r.+...~ ,000 Frequency (Hz) Figure 1 Men hering thresholds nd loudness discomfort levels (LDL). Error brs represent ± 1 SD bout the men. 1 db. The men pure-tone thresholds nd loudness discomfort levels re shown in Figure l. Hering Aid During the test sessions, ech listener ws fitted monurlly with progrmmble, multimemory, behind-the-er hering id with omnidirectionl microphone nd remote control. In cses of symmetric loss, the better er ws selected. In cses of symmetric loss, one er ws selected t rndom. The nontest er ws occluded with fom erplug. This method ws purposely used rther thn introduction of msking noise to the nontest er becuse the presence of noise cn influence the selection of the preferred volume setting (Cox nd Alexnder, 1991). The hering id ws progrmmed sequentilly (in rndom order) with WDRC, compression limiting, nd liner pek clipping mplifiction. The hering id frequency response, output sound pressure level with 90 db SPL input, nd WDRC rtio were individully set for ech listener ccording to the Desired Senstion Level (Cornelisse et l, 1995) prescriptive procedure nd progrmmed using the mnufcturer's fitting softwre. For WDRC mplifiction, the hering id operted s n input compressor with compression threshold fixed t 50 db SPL nd compression rtios rnging from 1 :1 to 2.7 :1. For compression-limiting mplifiction, the hering id operted s n output compressor with compression thresholds between 65 nd 85 db SPL nd compression rtios between 8 :1 nd 20 :1, depending on the volume setting. Representtive output nd gin functions for the three hering id circuits re shown in Figure 2. c (V 0 -PC --CL WDRC Input (db SPL) Figure 2 Comprison of representtive output (top pnel) nd gin (lower pnel) functions for the three hering id circuits. All vlues re 2-cc coupler levels for 2-kHz input signl with the hering id progrmmed for the men prticipnt udiogrm. PC = pek clipping; CL = compression limiting; WDRC = wide dynmic rnge compression. The hering id ws coupled to lucite skeleton ermold with select--vent. Trget mtch ws verified using probe microphone mesurements, nd the frequency-gin response nd/or vent size ws djusted if necessry to obtin the closest possible mtch to rel-er insertion gin trget (Fig. 3). Mterils The test mteril ws compct disc recording of the Speech Intelligibility Rting (SIR) test, developed by Cox nd McDniel (1989). The SIR test consists of 20 pssges on everydy topics, djusted for equl intelligibility. The SIR test lso contins multitlker bbble with spectrum mtched to the speech spectrum. The speech nd noise outputs from the compct disc plyer (Tscm CD301) were routed to seprte ttenutors (TDT PA4), mixed (TDT SM3), mplified (Crown D75), nd plyed through speker (Optimus XTS36) for presenttion to the listeners. The speker ws plced t hed level, 1 meter in front of the listener t 0 degrees zimuth in double-wlled sound booth. 417

4 Journl of the Americn Acdemy of Audiology/Volume 12, Number 8, September 2001 Speech ws presented t three input levels : 50, 65, nd 80 db SPL, representing low, moderte, nd high-level speech, respectively. Sound levels were root men squre levels specified t the position corresponding to the center of the listener's hed with the subject removed from the sound field nd were mintined through dily clibrtions. Ech speech level ws presented in quiet nd in noise t +7 signl-to-noise rtio for totl of six conditions. Procedure For ech subject, the order of the three mplifiction strtegies ws counterblnced. The order of the six test conditions ws rndomized within ech type of mplifiction. In ech condition, the listener ws instructed to djust hering id volume, using the remote control, for mximum speech clrity. Specificlly, the listener ws instructed to set the hering id to level tht would llow them to follow the converstion without strin. Clrity ws chosen s the criterion rther thn overll preference becuse hering id werers describe this to be the most importnt property of mplified speech (Hgermn nd Gbrielsson, 1985). The listener ws llowed to djust the volume freely until the criterion ws reched. Typiclly, the desired volume setting ws reched within one pssge; however, the listener ws llowed to listen to s mny pssges s desired. The remote control volume djustment offered rnge of 26 db with 1.3-dB step size. If the listener reched the top or bottom limit of the rnge before chieving the desired volume setting in ny condition, the overll gin ws reprogrmmed to llow the listener to rech the desired level. When the listener indicted tht the preferred volume setting hd been reched, output of the hering id t the tympnic membrne ws mesured using probe microphone system (Etymotic Reserch 7C) routed to spectrum nlyzer (HP35665A). Probe microphone rther thn coupler mesurements were used becuse they provide more ccurte mesure of sound pressure level (Brlow et l, 1988). The test signl for probe microphone mesurements ws the speech-shped clibrtion noise provided on the compct disc recording of the SIR test. This noise is shped to hve the sme long-term verge spectrum s the test stimulus. Without ltering the test environment (i.e., speker position nd zimuth or hering id volume setting), overll root men squre levels of the Figure 3 Men trget (shded brs) nd mesured (open brs) rel-er insertion gin (REIG). Drk solid brs show the men devition from trget t ech frequency. Error brs represent one stndrd error of the men. signl were mesured for ech volume tril. One-third octve bnd levels were lso obtined for use in the speech udibility clcultions described below. Two trils were conducted in ech test condition nd verged for nlysis. Test-retest vrition ws pproximtely 1 db cross trils. The hering id volume ws reset to minimum fter ech tril. RESULTS Preferred Volume Setting The mesured output t the listeners' preferred volume setting is shown in Figure 4. The top pnel shows the results in quiet nd the lower pnel shows the results in noise. The results were nlyzed using three-wy, repeted-mesures nlysis of vrince (ANOVA). Within-subject fctors were mplifiction type (liner pek clipping, compression limiting, WDRC), speech level (50, 65, 80 db SPL), nd signl environment (presence or bsence of bckground noise). The three-wy interction ws nonsignificnt (F = 1.65, df = 4, 36, p =.183). There ws no interction between the speech input level nd the signl environment (quiet or noise) (F = 1.54, df = 2, 18, p =.241) or between input level nd mplifiction type (F = 1.23, df = 4, 36, p =.314). The min effect of input level ws nonsignificnt (F = 1.05, df = 2, 18, p =.370). In other words, ptients djusted gin from pproximtely 4 db for loud speech to 35 db for soft speech to provide constnt output cross 418

5 Preferred Volume Setting nd Speech Audibility/Souz nd Kitch J 90 N m S 85 Quiet Q 0.95 Liner pek clipping 11 Compression limiting " WDRC 7 O N E 80 C I ~ N I- 90 J m S85 S O E 80 -W- Liner pek clipping m 2 -F]-- Compression limiting -A-WDRC 75 C--_-_ Input Level (db SPL) Noise i Figure 4 Men root men squre (rms) levels mesured t the tympnic membrne with the id set to the preferred volume for ech mplifiction type. The top pnel shows the results in quiet nd the lower pnel shows the results in noise. Error brs represent - one stndrd error bove the men. WDRC = wide dynmic rnge compression. rnge of input levels, regrdless of the type of mplifiction or the presence of bckground noise. There ws significnt interction between the type of mplifiction nd the signl environment (F = 8.51, df = 2, 18, p =.003). Simple min effects were nlyzed further by completing seprte one-wy, repeted-mesures ANOVAs compring the type of mplifiction within ech signl environment (quiet or noise). The results indicted tht listeners chose similr output levels for ech mplifiction type in quiet (F = 3.08, df = 2, 18, p =.071) nd in noise (F =.54, df = 2, 18, p =.594). For ech type of mplifiction, listeners selected greter gin (pproximtely 3 db more) in noise thn in quiet (p <.005). Speech Audibility A second gol in this study ws to exmine speech udibility t the listener's preferred volume setting. Becuse differences in mplifiction processing cn ffect the extent to which low-intensity cues re mplified reltive to Input Level (db SPL) Figure 5 Comprison of speech udibility, clculted t the preferred volume setting, for ech type of mplifiction nd speech input level. Error brs represent one stndrd error bove the men. AAI = Aided Audibility Index. high-intensity cues, the selection of similr long-term speech output levels cross mplifiction types does not gurntee equivlent udibility of the short-term intensity rnge. Put nother wy, the preferentil mplifiction of lower-intensity signls by WDRC circuit likely increses the overll udibility of the signl over tht of liner circuit. The 1/3-octve bnd levels mesured t the tympnic membrne in the quiet condition were used to clculte Aided Audibility Index (AAI) vlues (Stelmchowicz et l, 1994). The AAI is n importnce-weighted mesure of udibility tht indictes the proportion of the speech mplitude rnge vilble to the listener. Clcultion of the AAI is similr to the Articultion Index. (AI ; French nd Steinberg, 1947 ; Fletcher nd Glt, 1950) nd its counterprt, the Speech Intelligibility Index (ANSI, 1995). In contrst to those mesures, which ssume 30-dB rnge for linerly mplified speech without distortion (i.e., pek clipping), the AAI clcultion lso ccounts for the effects of output limiting nd for the reduction of the speech mplitude rnge owing to compression. As for the AI, n AAI of 0.0 indictes tht the entire rnge of speech levels is below the listener's threshold. An AAI of 1.0 indictes tht the entire rnge of speech levels is udible to the listener. Severl recent rticles provide detiled description of the AAI clcultion (Stelmchowicz et l, 1998 ; Souz nd Turner, 1999 ; Souz nd Bishop, 2000). Men AAI vlues t the listener's preferred volume settings in quiet re shown in Figure 5. An ANOVA ws used to compre speech udi- 419

6 Journl of the Americn Acdemy of Audiology/Volume 12, Number 8, September 2001 bility s function of mplifiction type nd speech input level. There ws no interction between mplifiction type nd input level (F =.44, df = 4, 36, p =.782). Speech udibility did not vry s function of speech input level (F =.34, df = 2, 18, p =.718) or type of mplifiction (F = 2.81, df = 2, 18, p =.087). DISCUSSION hen sked to set their hering id volume W to mximize speech clrity, listeners with mild-to-moderte loss set the hering id volume to provide listening level (mesured t the tympnic membrne) of pproximtely 83 db SPL, regrdless of input level. This pttern of incresing gin with decresing input level grees with previous reserch by Cox nd Alexnder (1994) for liner mplifiction nd Neumn et l (1995) for slow-cting compression hering ids. In the present study, this produced smll, sttisticlly equivlent rnge of output levels for low, moderte, nd high input levels, wheres previous reserch indicted slightly greter output for higher input levels. For speech in noise, listeners incresed gin by pproximtely 3 db over speech in quiet to mintin clrity. This finding conflicts with some previous work tht suggested lower volume setting for speech in noise versus speech in quiet (Cox nd Alexnder, 1991). Although Neumn et l (1995) lso demonstrted generl trend towrd decresing gin s the noise level increses, the results showed considerble vribility. For exmple, mesures of speech presented in bckground of cfeteri noise, the bckground noise most similr to our study, showed U-shped pttern, with the lowest gin for intermedite signl-to-noise rtios nd the higher gin t higher nd lower signl-tonoise rtios. In the present study, criterion of mximum clrity rther thn comfort likely cused the subjects to increse the volume in the presence of bckground noise. Anecdotlly, some listeners noted tht they would hve djusted the volume differently given different instructions. Clrity ws specificlly chosen s the criterion becuse hering id listeners rte this s the most importnt chrcteristic of their fittings. In everydy listening situtions, the criterion for djusting volume my vry with the level of bckground noise. For exmple, Brker nd Dillon (1999) suggested tht comfort my be reltively more importnt thn speech intelligibility in high levels of noise. Kuk et l (1994) lso emphsized clrity s criterion for volume djustment but found tht subjects selected less gin s bckground noise levels worsened pst signl-to-noise rtio of +5 db. However, in tht study, listeners djusted low-frequency gin independently. Mnipultion of the lower frequencies my hve effectively reduced the influence of bckground noise, resulting in n outcome similr to incresing the overll level of the signl. There ws no significnt difference in speech udibility (i.e., AAI) between hering id processing strtegies for ny speech level regrdless of the presence or bsence of bckground noise. Although speech recognition ws not directly mesured in this study, equivlent AAIs cross circuit types predict equivlent recognition scores cross circuit types (Souz nd Turner, 1999). Becuse lower compression thresholds re generlly ssocited with higher gin for low-intensity sounds, they re presumed to improve recognition of low-intensity consonnts (Dreschler, 1988 ; Kuk nd Ludvigsen, 1999) nd thus increse recognition scores over liner mplifiction. Insted, our results imply tht the dvntge of improved udibility over liner mplifiction demonstrted with WDRC ids in restrictive lbortory environment my not be mintined in more relistic situtions, when the hering id user hs control over the precise gin. Perhps more importntly, even for WDRC mplifiction tht theoreticlly reduces the need for volume control, listeners required lrge djustment in volume setting s the presenttion level chnged to mintin mximum clrity. These results support those noted by Brker nd Dillon (1999), for whom the mjority of subjects wering low compression threshold ids reported using their volume controls. This is n importnt point, prticulrly given the lck of mnul volume controls in mny WDRC hering ids nd the stted preference for mnul volume controls in previous work (Knebel nd Bentler, 1998 ; Vlente et l, 1998 ; Kochkin, 2000). It is uncler to wht extent this preference is influenced by previous experience with liner versus WDRC mplifiction s there re few dt vilble on cclimtiztion to circuit type. Future work should be expnded to consider these issues. Acknowledgment. This reserch ws supported by the Defness Reserch Foundtion. The uthors would lso like to thnk Rich Folsom nd Tom Rees for their ssistnce with study design nd Kte Drummond for her work on dt nlysis. 420

7 Preferred Volume Setting nd Speech Audibility/Souz nd Kitch REFERENCES Americn Ntionl Stndrds Institute. (1995). Methods for the clcultion of the speech intelligibility index. (ANSI S ). New York : ANSI. Brker C Dillon H. (1999). Client preferences for compression threshold in single-chnnel wide dynmic rnge compression hering ids. Er Her 20 : Brlow N, Auslnder M, Rines D, Stelmchowicz P (1988). Probe-tube microphone mesurements in heringimpired children nd dults. Er Her 9: Bentler R, Niebuhr D, Gett J, Anderson C. (1993). Longitudinl study of hering id effectiveness. I : objective mesures. J Speech Her Res 36 : Byrne D, Cotton S. (1987). Preferred listening levels of sensorineurlly hering-impired listeners. Aust JAudiol 9:7-14. Cornelisse LE, Seewld RC, Jmieson DG. (1995). The input/output formul : theoreticl pproch to the fitting of personl mplifiction devices. J Acoust Soc Am 97 : Cox RM. (1984). Reltionship between ided preferred listening level nd long-term listening rnge. Er Her 5: Cox RM, Alexnder GC. (1991). Preferred hering id gin in everydy envinroments. Er Her 12 : Cox RM, Alexnder GC. (1992). Mturtion of hering id benefit : objective nd subjective mesurements. Er Her 13 : Cox RM, Alexnder GC. (1994). Prediction of hering id benefit : the role of preferred listening levels. Er Her 15 : Cox RM, McDniel R. (1989). Development of the Speech Intelligibility Rting (SIR) test for hering id comprisons. J Speech Her Res 32 : Dillon H. (1996). Compression? Yes, but for low or high frequencies, for low or high intensities, nd with wht response times? Er Her 17 : Dreschler WA. (1988). The effect of specific compression settings on phoneme identifiction in hering-impired subjects. Scnd Audio( 17 : Dreschler WA, Eberhrdt D, Melk PW. (1984). The use of single-chnnel compression for improvement of speech intelligibility. Scnd Audiol 13 : Fletcher H, Glt RH. (1950). The perception of speech nd its reltion to telephony. JAcoust Soc Am 22 : French NR, Steinberg JC. (1947). Fctors governing the intelligibility of speech sounds. JAcoust Soc Am 19 : Hgermn B, Gbrielsson A. (1985). Questionnires on desirble properties of hering ids. Scnd Audiol 14 : Hwkins DB, Wlden BE, Montgomery A, Prosek RA. (1987). Description nd vlidtion of n LDL procedure designed to select SSPL90. Er Her 8: Hickson LMH. (1994). Compression mplifiction in herine ids. Am J Audiol 3 : Humes LE, Brlow NN, Grner CB, Wilson DL. (2000). Prescribed clinicin-fit versus s-worn coupler gin in group of elderly hering-id werers. J Speech Lng Her Res 43 : Knebel SB, Bentler RA. (1998). Comprison of two digitl hering ids. Er Her 19: Kochkin S. (2000). MrkeTrk V: consumer stisfction revisited. Her J 53 : Kuk FK. (1990). Preferred insertion gin of hering ids in listening nd reding-loud situtions. J Speech Her Res 33 : Kuk FK, Hrper T, Doubek K. (1994). Preferred rel-er insertion gin on commercil hering id t different speech nd noise levels. J Am Acd Audiol 5: Kuk FK, Lu C. (1996). Effect of hering id experience on preferred insertion gin selection. J Am Acd Audiol 7: Kuk FK, Ludvigsen C. (1999). Vribles ffecting the use of prescriptive formule to fit modern nonliner hering ids. J Am Acd Audiol 10 : Lrson VD, Willims DW, Henderson WG, Luethke LE, Beck LB, Noffsinger D, Wilson RH, Dobie RA, Hskell GB, Brtt GW Shnks JE, Stelmchowicz P, Studebker GA, Boysen AE, Donhue A, Cnlis R, Fusti SA, Rppport BZ. (2000). Efficcy of 3 commonly used hering id circuits : crossover tril. JAMA 284: Lurence RF, Moore BCJ, Glsberg BR. (1983). A comprison of behind-the-er high-fidelity liner hering ids nd two-chnnel compression ids, in the lbortory nd in everydy life. Br JAudiol 17 : Leijon A, Eriksson-Mngold M, Bech-Krlsen A. (1984). Preferred hering id gin nd bss-cut in reltion to prescriptive fitting. Scnd Audio( 13 : Leijon A, Lindkvist A, Ringdhl A, Isrelsson B. (1990). Preferred hering id gin in everydy use fter prescriptive fitting. Er Her 11 : Moore BCJ, Glsberg BR. (1986). A comprison of twochnnel nd single-chnnel compression hering ids. Audiology 25 : Neumn AC, Bkke MH, Hellmn S, Levitt H. (1995). Preferred listening levels for liner nd slow-cting compression hering ids. Er Her 16: Scollie SD, Seewld RC, Moodie KS, Dekok K. (2000). Preferred listening levels of children who use hering ids : comprison to prescriptive trgets. JAm Acd Audiol 11 : Souz PE, Bishop RD. (2000). Improving udibility with nonliner mplifiction for listeners with high-frequency loss. J Am Acd Audiol 11 : Souz PE, Turner CW (1996). Effect of single-chnnel compression on temporl speech informtion. J Speech Her Res 39 : Souz PE, Turner CW. (1998). Multichnnel compression, temporl cues nd udibility. J Speech Lng Her Res 41: Souz PE, Turner CW. (1999). Quntifying the contribution of udibility to recognition of compression-mplified speech. Er Her 20 :

8 Journl of the Americn Acdemy of Audiology/Volume 12, Number 8, September 2001 Stelmchowicz P, Lewis D, KlbererA, Creutz T. (1994). Situtionl Hering Aid Response Profile Users Mnul (SHARP, v. 2.0). Omh, NE : Boys Town Ntionl Reserch Hospitl. Stelmchowicz PG, Dlzell S, Peterson D, Kopun J, Lewis DL, Hoover BE. (1998). A comprison of threshold-bsed fitting strtegies for nonliner hering ids. Er Her 19 : Tyler RS, Kuk FK. (1989). The effects of "noise suppression" hering ids on consonnt recognition in speech-bbble nd low-frequency noise. Er Her 10 : Vlente M, Fbry DA, Potts LG, Sndlin RE. (1998). Compring the performnce of the Widex SENSO digitl hering id with nlog hering ids. J Am Acd Audiol 9: Wlden BE, Schuchmn GI, Sedge RK. (1977). The relibility nd vlidity of the comfort level method of setting hering id gin. J Speech Her Disord 44 :