Consonant Perception in Quiet : Effect of Increasing the Consonant Vowel Ratio with Compression Amplification

Transcription

1 J Am Acad Audiol 8 : (1997) Consonant Perception in Quiet : Effect of Increasing the Consonant Vowel Ratio with Compression Amplification Louise Hickson* Denis Byrner Abstract Single-channel syllabic compression amplification may increase the consonant-vowel ratio (CVR). This study was conducted to investigate the effect of CVR increases, associated with syllabic compression, on consonant perception in quiet for people with normal hearing and those with sensorineural hearing impairment. Fifteen hearing-impaired and 15 normal-hearing older people were assessed with different versions of the Nonsense Syllable Test, which had been recorded with linear and compression amplification (compression ratio = x). Overall scores did not differ significantly with type of amplification for both subject groups. Analysis of classes of sounds revealed the differential effect of type of amplification for the subject groups. This study highlights the need for audiologists to be aware that applying amplification that raises the level of consonants in relation to vowels is not always beneficial for people with hearing impairment, as the evidence indicates that CVR may be a cue to the perception of some consonants. Key Words : Amplification, compression, consonant perception Abbreviations : AMP C =compression amplification, AMP L = linear amplification, CVR = consonant-vowel ratio, MCL = most comfortable listening, NAL = National Acoustic Laboratories, NST = Nonsense Syllable Test coustic analysis of single-channel syllabic compression and linear amplification has revealed that compression A may result in changes in the intensity relationships between parts of the speech signal (Hickson and Byrne, 1995). Specifically, we found that the consonant-vowel ratio (CVR) within syllables was increased, owing to a combination of an increase in the level of the consonant and a decrease in the level of the vowel. We suggested that this increase in CVR could be expected to improve consonant perception for people with hearing impairment for two reasons. First, increasing the level of the consonant should increase the audibility of the acoustic 'Department of Speech Pathology and Audiology, The University of Queensland, Queensland, Australia, 4072 ; tnational Acoustic Laboratory, Chatswood, New South Wales, Australia, 2067 Reprint requests : Dr. Louise Hickson, Department of Speech Pathology and Audiology, The University of Queensland, Queensland, Australia, 4072 ; Tel : ; Fax : ; I.hickson@mailbox.uq.oz.au cues necessary for perception. Second, decreasing the level of the vowel should decrease the masking effects of the stronger components of the speech signal on the weaker ones, thus also increasing audibility of consonant acoustic cues. There is no clear experimental evidence to support this suggestion, however, and research in linguistics with normal-hearing subjects indicates that CVR itself may be a cue for the perception of some speech sounds (Ohde and Stevens, 1983 ; Freyman and Nerbonne, 1989 ; Balakrishnan et al, 1996). In a study by Hickson et al (1995), no significant consonant perception differences were found between linear and compression amplification in quiet for people with mild to moderate sensorineural hearing loss. Consonant perception in noise was adversely affected by compression because of the increase in the level of the noise in relation to the consonant. These results were obtained with a hearing aid that allowed for the selection of two compression conditions for comparison (compression ratios =1.3 and 1.8). Other researchers (Dreschler, 1988a ; Dreschler et al, 1984) have reported no difference in speech 322

2 Consonant Perception in Quiet/Hickson and Byrne perception in quiet between linear amplification and compression amplification with ratios up to 5. Sammeth et al (1996) reported low and nonsignificant correlations between CVRs and percent correct scores for syllables processed with linear amplification and three different nonlinear hearing aids. Increasing the CVR was not associated with significant improvements in consonant recognition in quiet or in noise for individuals with sensorineural hearing impairment. Peterson et al (1990) found that speech perception in quiet improved with compression amplification having a ratio of 10 for some subjects with sensorineural hearing loss. A possible reason for this was the much greater CVR increase that would have occurred with an aid of this kind. The present study was conducted specifically to investigate the effect of CVR changes on consonant perception in quiet. A hearing aid with a high compression ratio (-) was used in order to maximize the CVR increases for a given compression threshold and frequency response. It was hypothesized that, if CVR increases are beneficial, then speech perception in quiet should be significantly enhanced with this form of amplification. If, on the other hand, CVR is a cue to the perception of some consonant sounds, then speech perception in quiet could be expected to decrease. Individuals with normal hearing and with hearing impairment were included in order to compare the influence of CVR changes on consonant perception for these two groups. It is worthwhile to consider the performance of normal-hearing subjects since speech enhancement strategies being developed for people with hearing impairment generally start from the assumption that it is necessary to enhance aspects of the speech signal that are important for normal speech perception (Revoile and Holden-Pitt, 1993). Subjects METHOD Fifteen subjects with mild to moderate sensorineural hearing loss and 15 subjects with normal hearing participated in this study. All subjects had English as a first language and no obvious physical, psychological, or literacy problems that would affect their performance on the test procedures. The normal-hearing subject group consisted of 11 females and 4 males who were matched for age (±2 years) with the 15 hearing-impaired subjects. The mean age of the group was 67.9 years (SD = 8.2 years) with a range of 40 to 74 years. All subjects had thresholds of less than or equal to 25 db from 250 to 4000 Hz in the test ear. The air-bone gap at all frequencies from 500 to 4000 Hz was less than or equal to 10 db. Impedance audiometry showed that all subjects had normal middle ear compliance and pressure values in both ears. Stapedius reflexes were elicited with stimulation of the test ear at normal sensation levels (70-95 db) at 500, 1000, and/or 2000 Hz. Subjects had no significant history of ear disease or of noise exposure. The hearing-impaired subject group consisted of 5 females and 10 males aged between 42 and 74 years (mean = 67.9 years, SD = 8 years). The mean ages of the two subject groups were the same. As a decline in speech perception is known to occur with age independently of hearing levels (e.g., Marshall, 1981 ; Otto and McCandless, 1982 ; Dubno et al, 1984 ; Jerger et al, 1989), it was essential to have subject groups of similar ages in order to conduct a meaningful comparison of performance. All subjects in the hearing-impaired group had a bilateral sloping sensorineural hearing loss and only one ear of each subject was tested. Pure-tone air-conduction thresholds for the test ear are shown in Table 1. The air-bone gap at all frequencies from 500 to 4000 Hz in the test ear was less than or equal to 10 db, consistent with sensorineural Table 1 Air-Conduction Hearing Threshold Levels (db HTL) of Test Ears of Subjects with Hearing Impairment Subject 250 Frequency (Hz) " ' " ' ' ' ' ' SD Range "Hearing aid user.

3 Journal of the American Academy of Audiology/Volume 8, Number 5, October 1997 hearing loss. Impedance audiometry showed that subjects had normal middle ear compliance and pressure values in both ears. Stapedius reflexes were elicited with stimulation of the test ear at sensation levels of less than 60 db at 500, 1000, 2000, and/or 4000 Hz, showing evidence of recruitment consistent with cochlear hearing loss. All subjects described the onset of their hearing loss as gradual. Seven subjects had never used a hearing aid, seven wore monaural amplification, and one subject wore binaural aids. All aids were single-channel devices. Five subjects wore a hearing aid in the test ear; however, only two of these (subjects 6 and 13) used the aid consistently on a daily basis. For these two subjects, their hearing aids provided linear amplification with compression limiting for maximum level signals only. Materials and Equipment Speech The speech material consisted of seven subtests of the Nonsense Syllable Test (NST) developed by Resnick et al (1975) and first reported in detail by Levitt and Resnick (1978). The version of the test used in this study was recorded by an Australian male speaker. The NST is a closed-response speech test and each subtest contains seven to nine nonsense syllables of the consonant-vowel or vowel-consonant type. The test has a total of 62 stimulus items and this includes one repeat item in each subtest. In this study, two different versions of the test were used with randomized syllable presentation. In each version of the test, a different syllable was randomly chosen for repetition in each subtest. Each version of the NST was preceded on the tape by a 1000-Hz test tone and a passage of connected discourse. The test tone corresponded to the average peak levels of the speech signal. Hearing Aid A Bernafon/NAL SB13 programmable behind-the-ear hearing aid was used. This twomemory aid was programmed with linear amplification (AMP L) in one memory and input compression amplification (AMP C) in the other. The aid's input/output graphs obtained on the settings used in this experiment are presented in Figure 1. The characteristics of AMP C were compression threshold = 62 db SPL, compression ratio = - : 1, attack time = 6 msec, and release time = 50 msec. The aid also allows for m a e Figure 1 Input/output functions of the aid on the two different amplifier settings, measured at 1600 Hz. the selection of compression with a ratio of 2:1 ; however, this condition was not used in the present study. Test Recordings Test recordings of speech amplified by the hearing aid were made with the NST presented at a level of 75 db SPL. This level was chosen as the output of the two types of amplification was identical at this point for a mid-frequency signal (see Fig. 1) and the hearing aid was not saturated for the linear condition. The test recording and playback set-up were the same as those described in the previous study by Hickson et al (1995). Recordings were produced by placing the hearing aid, which was connected to recording equipment, 1 meter from a loudspeaker in a sound-treated room of a recording studio. The speech material was presented via the loudspeaker and this signal was picked up by the hearing aid microphone, amplified, and then recorded on to tape. The hearing aid was connected to a 2-cc coupler, which was attached to a Bruel and Kjaer sound level meter (Type 2203) with a Type 4144 condenser microphone. The output of the hearing aid passed through a Tascam M312B mixer and was recorded on a Tascam 112 cassette deck. The speech stimuli cassette recording was played on a Sony TC K370 player. Signals were mixed on the Tascam 246 mixer and amplified with a Tandberg 3002 preamplifier and a Tandberg 3003 power amplifier. The amplifier settings were set to a flat frequency response and gain was fixed throughout. The speech and noise signals were presented in the sound-treated room 324

4 Consonant Perception in Quiet/Hickson and Byrne Frequency, Hz Figure 2 Estimated insertion gain of the hearing aid on the two amplifier settings. via an Altec 9813A studio speaker. All levels were set using a Rion Sound Level Meter NA23. To estimate the insertion gain of the signal for the subjects, a pink noise signal was recorded in the same way that the test recordings were made, and the output was measured through the playback system used for the subjects. Measurements were taken in octave bands. A correction factor for ear canal resonance was then subtracted and a 6-cc coupler to eardrum factor was added at each frequency (Bentler and Pavlovic, 1989). The resulting insertion gain frequency response curves for the AMP L and AMP C are shown in Figure 2. The greatest difference between types of amplification was 4 db at 8000 Hz. Acoustic Analysis of Test Recordings Prior to presenting the test recordings to the subjects, acoustic analysis of the speech material was undertaken to determine whether or not the expected changes in level had occurred. For all recorded syllables, the root-mean-square (rms) levels of the consonant and vowel were measured in the manner described by Hickson and Byrne (1995). Recordings of each syllable were digitized using the Computerized Speech Lab (Model 4300, Kay Elemetrics Corporation). Consonant and vowel boundaries were determined initially by careful examination of the waveform and the spectrogram and by listening to the speech signal via headphones. The rms level of the consonant and vowel portions of the syllable were computed as the square root of the mean of the squared amplitudes (average power) of the sampled points within the time segment of each sound. The rms level was then converted to db SPL by taking 20 times the log (base 10). The rms levels for stops were calculated from the burst and aspiration time durations, that is, closure was not included (Freyman and Nerbonne, 1989). The CVR for each syllable was calculated by subtracting the consonant db SPL from the vowel db SPL. A summary of results for all syllables in the NST and the statistical comparisons of AMP L and AMP C values are presented in Table 2. For all measures, there were significant differences between the two types of amplification. With AMP C, the level of the signal overall was increased (i.e., for consonant and vowel), while with AMP L there was more variation in level. The greater variation in level with AMP L was evidenced by larger differences between mean values for vowels and consonants and larger standard deviations. CVR was increased with compression amplification. vowel level for all syllables was not found to decrease with AMP C for two reasons. First, the speech signal was presented to the aid at 75 db SPL only and, at this particular level, the main effect of compression amplification, combined with the low-cut frequency response, was to increase the levels of the lower-intensity components of speech rather than to decrease the high-intensity components. The data presented in Hickson and Byrne (1995) were mean results across a number of different presentation levels and it was pointed out that the effects depend on presentation level. Second, the change in vowel level was dependent on the vowel context. Increase in vowel level was least for /a/ and greatest for the lower intensity vowels /i/ and /u/, which influenced the mean vowel level result. Further acoustic analysis findings are presented later in relation to the perceptual results obtained. Procedure Pure-tone audiometry was performed on all subjects using an Interacoustics Clinical Computer Audiometer Model AC5 with Telephonics Table 2 Summary of Acoustic Analysis Data for Nonsense Syllables with Linear and Compression Amplification AMP C (SD) AMP L (SD) t(54) P Consonant level, <.001 db SPL (3.04) (7.47) Vowel level, <.001 db SPL (1.36) (3.83) CVR, db <.05 (3.18) (9.00) 325

5 Journal of the American Academy of Audiology/Volume 8, Number 5, October 1997 TDH39 headphones. Impedance audiometry was conducted with an Amplaid 770 Impedance Meter. The NST recordings were presented via the audiometer headphones to the test ear at each subject's most comfortable listening (MCL) level, determined separately for the linear and compression conditions. A sample of connected discourse, which had been recorded through the hearing aid in the same way as the following NST recording, was used for the selection of MCL. Recordings were played on a Yamaha Natural Sound Stereo Cassette Deck K1020. Presentation levels of the NST recordings for the hearing-impaired subject group ranged from 85 to 100 db SPL, measured in a 6-cc coupler (mean = 94.3 db, SD = 4.2 db). For the normal-hearing group, presentation levels ranged from 65 to 75 db SPL (mean = 69 db, SD = 4.3 db). Testing was done in a counterbalanced order, with subjects 1 to 7 tested with AMP L first and subjects 8 to 15 tested with AMP C first. RESULTS Consonant Perception Subjects with Normal Hearing A summary of the statistical analysis of the results for the group of subjects with normal hearing is contained in Table 3. NST scores and scores for classes of sounds in the NST with AMP L and AMP C were compared using paired t-tests. Where the difference between means was less than 2 percent, the mean differences were not statistically compared. As multiple comparisons were made on the same data set, the Bonferroni procedure was used to maintain the overall comparison error rate at a 0.05 significance level (SYSTAT, 1985). Scores for fricatives and voiceless consonants were higher with AMP C than AMP L. Differences for overall scores and for other classes of sounds were not significant. The overall NST difference score between amplifiers for each subject is shown in Figure 3 : all except two subjects showed an increase in consonant perception with compression amplification. Subjects with Hearing Impairment Subjects were initially divided into two subgroups on the basis of their suitability for the frequency response provided by the amplification. This was done as it was recognized that frequency response suitability may influence results Table 3 Summary of Bonferroni Protected Paired T-tests of NST Scores for Subjects with Normal Hearing NST Score AMP C AMP L (SD) (SD) t(14) P Overall NS (5.83) (9.56) Fricatives <.05 (6.85) (9.92) Stops NS (7.98) (9.01) Affricates Not (0.00) (0.00) tested Nasals NS (10.78) (21.32) Liquids/glides Not (10.35) (16.33) tested Voiceless <.05 consonants (8.74) (11.54) Voiced NS consonants (4.04) (9.18) Initial consonants NS (3.69) (8.39) Final consonants NS (8.56) (11.54) NS = not significant. obtained with linear and compression amplification. One group consisted of subjects 1, 3, 4, 7, 10, 14, and 15 (see Table 1), whose hearing losses were best suited to the frequency response of the system according to the NAL prescription (Byrne and Dillon, 1986). The remaining eight subjects were assigned to the other group. The same pattern of results was obtained for the two subgroups as was found for the group as a C Subjects with normal hearing Figure 3 NST difference scores (AMP L minus AMP C) for each subject with normal hearing. The bar falling above the line indicates that performance was better with AMP L. The bar falling below the line indicates that performance was better with AMP C. 326

6 Consonant Perception in Quiet/Hickson and Byrne whole. No significant differences were found between the mean overall NST scores for the two groups with the two types of amplification (for AMP C, t[131 = 1.90, p =.08 ; for AMP L, t[131 = 0.76, p =.458). Therefore, the results from all 15 subjects were combined for further analysis. Results for the group of subjects with hearing impairment are summarized in Table 4. The only significant difference between scores with the two types of amplification occurred for stop consonants where scores were higher with AMP L than with AMP C. The difference between AMP L and AMP C was not significant for the overall NST scores, although it is important to point out that there was considerable individual variation in scores with compression amplification. This is evidenced by the larger standard deviation for the AMP C results. Figure 4 shows the difference scores for the 15 subjects and the direction of the difference. Five of the subjects had more than a 9 percent improvement in performance with linear amplification. Rationalized arcsine units (raus : Studebaker, 1985) were calculated for both subject groups in order to check that the greater variability in the group with hearing impairment was not a function of the fact that the scores for the normal hearing group were generally higher. Minimal differences were found between percentage scores Table 4 Summary of Bonferroni Protected Paired T-tests of NST Results for Subjects with Hearing Impairment NST Score AMP C AMP L (SD) (SD) t(14) p Overall NS (11.45) (5.63) Fricatives NS (13.85) (6.37) Stops <.05 (16.06) (8.17) Affricates NS (20.70) (0.00) Nasals NS (17.84) (25.71) Liquids/glides NS (0) (12.6) Voiceless NS consonants (15.29) (5.75) Voiced NS consonants (8.93) (10.32) Initial consonants NS (7.41) (9.62) Final consonants NS (15.14) (8.30) NS = not significant Subjects with hearing impairment Figure 4 NST difference scores (AMP L minus AMP C) for each subject with hearing impairment. The bar falling above the line indicates that performance was better with AMP L. The bar falling below the line indicates that performance was better with AMP C. and raus. For example, for AMP C, the mean percentage score for the hearing-impaired group was (SD = 11.45) and the mean rau was (SD = 11.21) ; the mean percentage score for the normal-hearing group was (SD = 5.83) and the mean rau was (SD = 6.73). Summary of Consonant Perception Findings For both normal-hearing and hearingimpaired subject groups, overall NST scores did not differ significantly with type of amplification. Results were more variable in the hearingimpaired subject group. Analysis of classes of sounds revealed the differential effect of type of amplification for the subject groups. For the subjects with normal hearing, scores for fricatives and voiceless consonants were higher with compression amplification. For the subjects with hearing impairment, scores for stop consonants were higher with linear amplification. Confusion Matrices Error patterns were further investigated using confusion matrices. Since the effects of type of amplification were most evident for voiceless consonants, compared to voiced consonants, matrices for voiceless sounds only are presented. In addition to the confusion matrix data, scores for particular sounds were analyzed on the basis of the results presented above. For example, for the hearing-impaired subject group, scores for stop consonants were analyzed as this class of

7 Journal of the American Academy of Audiology/Volume 8, Number 5, October 1997 sound was the only one found to be significantly affected by type of amplification. Res p onse /Y /W n=15 /s/ Stimulus /tj/ n=15 /0/ /p/ /W /t/ Subjects with Normal Hearing The voiceless consonant confusion matrices of subjects with normal hearing with the two different forms of amplification are presented in Figures 5 and 6. Statistical comparisons were then made between results obtained with AMP L and AMP C for each fricative sound using paired t-tests. scores for all fricatives with AMP C were either higher than or the same as scores with AMP L; however, the difference between scores was not significant for any particular sound (p >.05). The type of errors made by subjects was also examined from the confusion matrix data. Overall, the majority of errors involved confusions of place of articulation (72% for AMP L, 85%a for AMP C). More place/manner errors were made with AMP L (25%) than with AMP C (15%), and a proportions test indicated that this difference was significant (z = 2.11, p <.05). Subjects with Hearing Impairment The voiceless consonant confusion matrices for subjects with hearing impairment with compression and linear amplification are shown in Figures 7 and 8, respectively. Paired t-test results comparing stop consonant scores with AMP L and AMP C are presented in Table 5. Scores for all stop consonants with AMP L were equal to or higher than scores with AMP C, but the difference between mean scores was significant for /t/ only. The mean scores for consonants /p/ and /g/ were not compared since the difference between means was less than 2 percent for Response /Y /W n=15 /s/ /tf/ n=15 /f/ /6/ /h/ 100 /s/ /tj/ 100 /f/ /0/ Stimulus /p/ 79 3 /k/ /t/ I 9 I 8 I 95 Figure 5 Confusion matrix, in percentages, for the perception of NST voiceless consonants by the normalhearing subject group with AMP C. /J/ 89 5 /W 93 /S/ 5 85 /tf/ /f/ /B/ /p/ /k/ /t/ Figure 6 Confusion matrix, in percentages, for the perception of NST voiceless consonants by the normal hearing subject group with AMP L. these consonants. Examination of the confusion matrices shows that the consonant /t/ was most commonly confused with /0/ for both types of amplification ; however, more diverse errors were made with AMP C. Examination of the type of error made by subjects with hearing impairment for all NST consonant sounds showed that the majority of errors for both AMP L (74%) and AMP C (80%) were errors of place of articulation. Slightly more place/manner errors occurred with AMP L (23%) than with AMP C (17%), but a proportions test indicated that the difference between these values was not significant (z = 1.60, p =.109). Comparison of Perceptual Results and Acoustic Analysis The results were further analyzed in relation Res onse to the acoustic analysis data on the test material. Stimulus /J/ /W /s/ /d/ /0 /0/ /p/ /k/ /t/ n=45 n=15 n=15 n=50 /J/ 89 1 /h/ 93 /s/ /tj/ /fl /0/ /p/ 4 55 /k/ /t/ Figure 7 Confusion matrix, in percentages, for the perception of NST voiceless consonants by the hearingimpaired subject group with AMP C. 328

8 Consonant Perception in Quiet/Hickson and Byrne Response di n=45 ml n=15 5 5_11, Stimulus /f/ n=15 /f/ /0/ /p/ /k/ /f/ /h/ 93 /s/ /tf/ 2 80 /f/ /u n--75 /0/ /p/ /W /t/ Figure 8 Confusion matrix, in percentages,, for the perception of NST voiceless consonants by the hearingimpaired subject group with AMP L. Subjects with Normal Hearing For the normal-hearing subject group, the perception of fricative consonants was improved with compression amplification. differences for individual sounds with the different types of amplification were not significant. The syllables containing the consonants /f/ and /0/ were selected for acoustic analysis because they were associated with the largest mean increases in perception with AMP C (11.7% for /f/ and 11.6% for /0/). increases for the remaining fricative consonants ranged from 0 to 7 percent. Acoustic analysis data for syllables containing /f/ and /0/ are presented in Table 6. The mean CVR for syllables containing /f/ and /0/ was db (SD = 2.13 db) with AMP C and db (SD = 6.23 db) with AMP L; the difference between these mean values was significant (t[71= 54.13, p <.01). The difference in CVR with type of amplification depended on the context of the consonant with the largest CVR increases with compression evident for syllables containing the vowel /a/. For example, in the Table 5 Bonferroni Protected Paired T-test Results for Scores Obtained by Hearing-impaired Subjects for Each of the NST Stop Consonants AMP C (SD) AMP L (SD) t(14) P /p/ (20.70) (31.60) Not tested /k/ (35.50) (11.90) 2.22 NS /t/ (24.90) (11.20) 3.20 <.05 /b/ (12.00) (15.40) 0.79 NS /g/ (16.00) (16.10) Not tested /d/ (34.30) (24.60) 0.80 NS NS = not significant. case of syllables containing /f/, the largest increases in CVR with compression were found for the syllables /fa/ and /af/ (10.92 db and 8.2 db respectively). In contrast, CVR improvement for /if/ was 3.24 db and for /uf/ was only 1.03 db. Examination of perceptual results for each syllable showed consistent improvement with compression for the syllables /fa/, /af, and /if/. Scores for the syllable /uf/ were identical with both types of amplification. These results suggest that CVR increases may account for improvements in the perception of the consonant /f/ by normal-hearing subjects. Increases in consonant level alone did not account for the perceptual results, as it can be seen from Table 6 that the greatest increases occurred for the syllables /fa/ and /uf/. Despite similar acoustic results for syllables containing /f/ and /0/, a different perceptual pattern emerged. Large increases in CVR with AMP C for the syllables /0a/, /a0/, and /i0/ (range = db) had little effect on perceptual results. scores for /0a/ were identical with both types of amplification ; scores for /a0/ were 20 percent with AMP L and 33 percent with AMP C; and scores for /i0/ were 43 percent with AMP L and 33 percent with AMP C. The greatest perceptual difference between types of amplification occurred for the syllable /u0/ that was associated with the smallest change in CVR Table 6 Consonant and Vowel Levels and CVRs for Syllables Containing /f/ and /0/ AMP C AMP L /fa/ Consonant level, db SPL Vowel level, db SPL CVR, db /af/ Consonant level, db SPL Vowel level, db SPL CVR, db /uf/ Consonant level, db SPL Vowel level, db SPL CVR, db /if/ Consonant level, db SPL Vowel level, db SPL CVR, db /9a/ Consonant level, db SPL Vowel level, db SPL CVR, db /a0/ Consonant level, db SPL Vowel level, db SPL CVR, db /uo/ Consonant level, db SPL Vowel level, db SPL CVR, db JO/ Consonant level, db SPL Vowel level, db SPL CVR, db

9 Journal of the American Academy of Audiology/Volume 8, Number 5, October 1997 (1.6 db). The mean score with AMP L for this syllable was 15 percent, compared to 73 percent with AMP C. Possible reasons for this perceptual improvement with compression were the increase in consonant level and the minimal change in CVR. Therefore, in contrast to the findings for /f/, the perception of /0/ was not enhanced for normal-hearing subjects by increasing CVR. CVR may serve as a cue to the perception of /0/ for normal-hearing subjects. Subjects with Hearing Impairment The perception of the stop consonant /t/ was affected by type of amplification for the hearingimpaired subject group. Scores were significantly higher with AMP L than with AMP C. Acoustic analysis data for syllables containing /t/ and /p/ are presented in Table 7. The results obtained for /p/ are included for comparison. scores for /p/ with AMPs L and C differed by only 0.3 percent. For the four syllables containing /t/, consonant level was significantly higher with AMP C (t[3] = 4.25, p <.05) and CVR was not significantly affected by amplification type (t[31= 0.77, p =.496). CVR was higher with AMP C for /at/ and /it/ and lower for /ta/ and /ut/. Examination of the errors for each syllable revealed little dif- Table 7 Consonant and Vowel Levels and CVRs for Syllables Containing /t/ and /p/ AMP C AMP L /ta/ Consonant level, db SPL Vowel level, db SPL CVR, db /at/ Consonant level, db SPL Vowel level, db SPL CVR, db /ut/ Consonant level, db SPL Vowel level, db SPL CVR, db /it/ Consonant level, db SPL Vowel level, db SPL CVR, db /pa/ Consonant level, db SPL Vowel level, db SPL CVR, db /ap/ Consonant level, db SPL Vowel level, db SPL CVR, db /up/ Consonant level, db SPL Vowel level, db SPL CVR, db /ip/ Consonant level, db SPL Vowel level, db SPL CVR, db ference between types of amplification for /ta/ and /ut/. The syllable /ta/ was perceived correctly by all subjects with both types of amplification and /ut/ was 93 percent correct with AMP L and 80 percent correct with AMP C. For the syllable /at/, subjects scored 93 percent correct with AMP L and only 50 percent correct with AMP C; for the syllable /it/, subjects scored 93 percent correct with AMP L and 73 percent correct with AMP C. This suggests that it was the CVR increase associated with compression that led to a decrease in perception of the consonant /t/. The perception of the voiceless stop /p/ was the same with both types of amplification. It is interesting to note that syllables containing /p/ showed a more consistent change in CVR than those containing /t/ (see Table 7). CVR was increased with compression for all /p/ syllables (range = db). Despite the large increase in CVR for the syllable /ap/, percentage correct scores were exactly the same with both types of amplification. DISCUSSION he aim of this study was to examine the T effect of CVR changes on consonant perception. The experimental aid had a compression threshold of 60 db and a low-cut frequency response, as have many hearing aids currently being fitted. However, a very high compression ratio (rarely used in clinical practice) was chosen to ensure substantial CVR changes. The results indicate that the effect of CVR change depends on the consonant sound and on the characteristics of the listener. No significant difference in overall NST scores was found for either subject group. However, there were differences in performance for different classes of sounds (and for sounds within those classes). This result for overall scores is the same as found by Hickson et al (1995) for subjects with hearing impairment using an aid with lower compression ratios (1.3 and 1.8). However, unlike the results of the previous experiment, differences were found for particular classes of sounds. Acoustic analysis indicated that consonant level was increased consistently with compression amplification. CVR also increased ; however, the extent of the increase varied with context. The normal-hearing subjects had significantly higher scores for fricative consonants with compression amplification, and the largest effects were seen for the consonants /f/ and /0/. Close examination of the acoustic analysis data indicated a different relationship between 330

10 Consonant Perception in Quiet/Hickson and Byrne acoustic changes and consonant perception for the two sounds. For /f/, the greatest improvement in perception occurred for the syllables showing the largest CVR increases. Presumably, this was due to the increased audibility of the spectral cues of the fricatives, which is consistent with the assertion of Behrens and Blumstein (1988) that spectral properties are the primary acoustic cues for fricative perception in people with normal hearing. Spectral properties were further enhanced in the present study by the high-frequency emphasis of the amplification used. The result for /f/ conflicts with the finding of Freyman and Nerbonne (1989) and Balakrishnan et al (1996) that the intelligibility of /f/ decreased with increasing CVR for normal-hearing subjects. There is no obvious explanation for the disagreement between those studies and the present investigation, but it may be noted that the other researchers used much younger adult subjects, speech was presented with a background noise, and, in the case of Balakrishnan et al (1996), the speech signal was spectrally degraded. These more difficult listening conditions may have meant that subjects used the secondary amplitude cue as the spectral properties were not clear. In contrast to the findings for /f/, the perception of /0/ did not improve significantly with increasing CVR in the normal-hearing subjects. The largest improvement in scores with compression amplification occurred for a syllable (/u0/) in which the consonant level was increased but the CVR change was minimal. Little change was observed for the perception of /0/ in syllables where compression amplification resulted in marked increases in both consonant level and CVR. Hence, increasing the level of the consonant was beneficial provided that the "normal" CVR was retained. It seems that, for normalhearing subjects, amplitude information may serve as a distinctive cue for the perception of this particular fricative. It is interesting to note that the significant increases in CVR that occurred for low-intensity fricative sounds (e.g., /f/ and /0/) did not result in improved perception of these sounds by subjects with hearing impairment. Spectral cues should have become clearer as the level of the consonant was increased, and this suggests that these cues are not the only ones necessary for identification of fricatives. It is possible that the increases were not sufficient to make the consonants audible ; however, this seems unlikely as the level increases were more than 10 db in many instances. For the hearing-impaired subject group, it was voiceless stop consonant perception that was affected by type of amplification : scores were higher with linear amplification than with compression. The same pattern of performance was found in the normal-hearing subject group ; however, the differences between types of amplification were not significant. The result for the stop consonants is in line with that of Dreschler (1988b), who reported that the perception of "plosiveness" in a quiet listening condition was worse with syllabic compression than with linear amplification for subjects with hearing impairment. He hypothesized that this occurred because of temporal distortions associated with compression. Results of other research studies on the effect of CVR changes on the perception of stop consonants have been variable. Turner and Robb (1987) reported an improvement in perception with increased CVR in normal-hearing subjects, whereas Freyman and Nerbonne (1989) found no change. Gordon-Salant (1986, 1987) investigated the influence of increasing CVR by 10 db in normal-hearing young and elderly subjects and in elderly subjects with mild to moderate sensorineural hearing loss. They found that the performance of all subject groups was significantly improved for stop consonants, a result that conflicts with that of the study described here. A possible reason for this is the different nature of the speech material used. Gordon-Salant used consonants in the initial position of syllables only. In the present study, CVR was not enhanced with compression amplification for initial stop consonants and perception was not affected. The difference between types of amplification was most apparent for stops in the final position of a syllable. Although there was not a significant difference in overall NST scores for either group, it is interesting to note that there was greater variability in subject performance in the group with hearing impairment. In the normal-hearing group, scores were generally higher with compression amplification, with only one subject showing a small improvement with linear amplification (see Fig. 3). The performance of the subjects with hearing impairment was variable (see Fig. 4), a finding consistent with that of Sammeth et al (1996), who reported marked variability in consonant perception across subjects with linear and three different types of nonlinear amplification. This was despite the subjects having essentially the same audiograms. Thus, the need to determine the relationship between subject characteristics and perceptual results is evident. Such analysis was beyond the scope ofthe present study. 331

11 Journal of the American Academy of Audiology/Volume 8, Number 5, October 1997 Further research is necessary in this area to determine if improved performance with compression can be predicted on the basis of subject characteristics such as age, degree of loss, dynamic range, and frequency resolution. This may be complicated by the specific characteristics of the compression system and their interaction with frequency response. In summary, the results obtained suggest a complex relationship between CVR changes and consonant perception, which depends on the consonant sound and the characteristics of the listener. The subjects with normal hearing showed improved perception of voiceless fricatives with compression. This appeared to be due to increases in consonant level and hence the audibility of these low-intensity fricatives. For subjects with hearing impairment, the perception of voiceless stop consonants was adversely affected by increasing the CVR with compression amplification. These findings indicate that CVR is a cue to the perception of some consonant sounds for listeners with hearing impairment. Therefore, in the clinical situation, audiologists need to be aware that single-channel syllabic compression amplification may have an adverse effect on consonant perception in quiet in that it is changing important acoustic cues of the speech signal. The pattern of results varied for individual hearing-impaired subjects, and further research is necessary to determine reasons for this variability. Overall, the results of this study challenge the appropriateness of using information about the perception of speech by normal-hearing listeners when developing signal processing strategies for people with hearing impairment. Acoustic cues, little used by people with normal hearing, may be important for those with hearing impairment. REFERENCES Balakrishnan U, Freyman RL, Chiang Y-C, Nerbonne GP, Shea KJ. (1996). 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