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J Am Acad Audiol 10 : 445-457 (1999)

Comparison of Performance with Wide Dynamic Range Compression and Linear Amplification Anna C. S . Kam*' Lena L . N . Wong*

Abstract

This study compared subject performance and preference using a compression-limiting hear-ing aid set to linear amplification (program 1) and wide dynamic range compression (WDRC, program 2) . The frequency responses of the hearing aid were matched to a 65 dB SPL signal

and maximum output to a 90 dB SPL signal . Twenty subjects with moderate to moderately severe sensorineural hearing loss were tested . Speech recognition scores and speech recep-tion thresholds were obtained both in quiet and in noise . Subjective preference for WDRC

or linear amplification was measured via a paired-comparison procedure on "loudness appro-priateness," "clarity," and "pleasantness" to continuous discourse presented in quiet and in noise . Results suggested that WDRC yielded better speech intelligibility in quiet for low-level signals and no difference in speech intelligibility in noise compared to linear amplification . Subjects preferred WDRC for loudness to both high- and low-level signals and for pleas-antness to high-level signals .

Key Words: Linear amplification, speech intelligibility, subjective preference, wide dynamic range compression

Abbreviations : ANOVA= analysis of variance, AVC =automatic volume control, CL= com-pression limiting, CVR = consontant-vowel-ratio, HINT = Hearing In Noise Test, P = program, REIG = real-ear insertion gain, REIR = real-ear insertion response, SNR = signal-to-noise ratio, SRS = speech recognition score, SRT = speech reception threshold, WDRC = wide dynamic range compression

N

onlinear amplification or compression systems were originally incorporated in hearing aid design to limit the max-

imum output of a hearing instrument to below the wearer's loudness discomfort level. Before the introduction of compression, peak clipping was the most frequently used method of output lim-iting (Hawkins and Naidoo, 1993). However, this method causes a considerable amount of dis-tortion that is unpleasant to the listener and may even reduce speech intelligibility (Dreschler, 1988a). To eliminate or minimize these unde-sirable effects, the use of compression limiting was introduced as an alternative to peak clip-

*Department of Speech and Hearing Sciences, The University of Hong Kong, Hong Kong ; tcurrently affiliated with Phonak Hearing Centre, Hong Kong Limited

Reprint requests : Lena L. N. Wong, Department of Speech and Hearing Sciences, Prince Philip Dental Hospital, 5/F, 34 Hospital Road, Hong Kong

ping. Except for people with very profound hear-ing loss, compression limiting is superior to peak clipping both subjectively and objectively (Gioannini and Franzen, 1978 ; Dreschler, 1988a ; Hawkins and Naidoo, 1993; Dillon, 1996) .

The use of compression to limit hearing aid output is just one of its applications in hearing aid design . Based upon its function and characteris-tics, compression used in modern hearing aids can be categorized into three main types: compression limiting (CL), slow-acting automatic volume con-trol (AVC), and wide dynamic range compres-sion (WDRC) (Dillon, 1988 ; Hickson, 1994). The characteristics of the three types of compression systems are summarized in Table 1 .

CL is characterized by high compression ratios, high compression thresholds, and short time constants (attack time and release time). This type of compression is designed to limit the output below the listeners' tolerance level, minimize temporal and spectral distortion of

445

Journal of the American Academy of Audiology/Volume 10, Number 8, September 1999

Table 1 Characteristics of Three Common Types of Compression System (after Fortune, 1996)

Compression Compression Compression Attack Release System Threshold Ratio Time Time

(dB SPL) (msec) (msec)

CL >80 >5 <5 50-100 AVC <65 >5 10-50 150-2000 WDRC <60 <5 <5 10-100

CL = compression limiting, AVC = automatic volume control, WDRC = wide dynamic range compression .

the acoustic input at low and intermediate lev-els by providing linear amplification to sounds below the compression threshold, and minimize distortion that may be generated by peak clip-ping (Dreschler et al, 1984 ; Boothroyd et al, 1988 ; Dillon, 1988; Preves, 1991).

Hearing aids with AVC typically have inter-mediate to high compression ratios, low com-pression thresholds, short attack time, and a very long release time . Because of the long release time, the output of the signal remains relatively constant in the presence of input fluctuations. As a consequence, the need to adjust the volume control of the aid is reduced (Dillon, 1988 ; Hick-son, 1994 ; Kuk, 1996).

WDRC is characterized by low compression ratios, low compression thresholds, and short time constants. Kuk (1996) and Dillon (1988) lim-ited the definition of WDRC to a system with short release time . This syllabic compression system allows the compression aid to "follow" the envelope fluctuation among syllables seen in speech (Kuk, 1996). The rationale behind WDRC is to match or "squeeze" the normal speech range (from soft to loud speech) to the reduced dynamic range of hearing-impaired people (Steinberg and Gardner, 1937 ; Dillon, 1988). People with sensorineural hearing loss typically have reduced auditory dynamic range (the difference in dB between detection threshold to threshold of dis-comfort) . In the unaided condition, low-intensity sounds will be inaudible while high-intensity sounds remain loud . Unlike linear amplifica-tion, which provides constant gains regardless of the input sound levels, WDRC gives more gain to low-intensity sounds and less gain (gain reduction) to high-intensity sounds . As a result, soft sounds should become audible while lis-tening comfort is ensured even with loud sounds .

During the past 2 decades, numerous stud-ies have been carried out to investigate the per-formance of different amplification systems . Lines of research include comparison of perfor-mance between CL and peak clipping (e.g ., Gioannini and Franzen, 1978 ; Dreschler, 1988a;

Hawkins and Naidoo, 1993), linear amplification and AVC (e.g., Moore et al, 1991; Neuman et al, 1994), linear amplification and WDRC (e.g ., Nabelek, 1983 ; Dreschler, 1988b ; Peterson et al, 1990), and evaluation of multiband com-pression (e.g ., Plomp, 1988 ; Moore et al, 1992 ; Hohmann and Kollmeier, 1995 ; Yund and Buck-les, 1995). However, the findings in most areas are inconclusive .

Theoretical advantages of WDRC over lin-ear amplification are well documented (e.g ., Hickson, 1994; Dillon, 1996; Kuk, 1996). How-ever, due to problems and differences in the design of various empirical studies, results doc-umenting the advantages are conflicting or inconclusive . Verschuure et al (1993) investi-gated the effect of syllabic compression (i .e ., WDRC) on speech intelligibility in 19 hearing-impaired listeners. Results obtained via a non-sense consonant-vowel-consonant syllable test in quiet revealed better speech intelligibility with WDRC. The study used experimental hear-ing aids that were specifically designed so that many parameters of the instrument could be manipulated. Most of the parameters studied could not be altered in commercial hearing aids, making comparisons to other studies very diffi-cult, if not impossible . Accordingly, generaliza-tion of the benefits of compression from experimental hearing aids is not readily applic-able to commercially available instruments.

There are some tentative experimental find-ings for speech intelligibility improvement in quiet with the use of WDRC instead of linear amplification. For example, Dreschler (1988a) compared the performance using a syllabic com-pressor with compression limiting and a linear amplifier with peak clipping. Sixteen hearing-impaired subjects with hearing loss ranging from mild to moderately severe participated in a phoneme perception task carried out in quiet. It was found that compression yielded signifi-cantly better phoneme identification scores than linear amplification . Nabelek (1983) showed that WDRC was superior to linear amplification

446

Comparison of WDRC and Linear Amplification/Kam and Wong

in quiet for 13 subjects with mild-to-severe sen-sorineural hearing loss . When speech-spectrum-

shaped noise was introduced, performance with

linear amplification was better than with WDRC.

An insignificant or negative effect of com-

pression was reported in some studies . Dreschler

et al (1984) compared the performance among

five hearing aids (a linear aid, two input com-

pressors, two output compressors) . Twelve hear-ing-impaired subjects were recruited . The speech

reception thresholds (SRTs) for sentence mate-

rial were obtained in quiet and in noise . No sig-nificant difference in performance was observed

among these hearing aids . Using 16 subjects

with mild-to-severe sensorineural hearing loss,

Tyler and Kuk (1989) failed to find any signifi-

cant improvement in consonant identification in

babble noise using a single-channel syllabic

compressor and its linear version . Hickson et al (1995) evaluated the consonant

perception of 15 subjects with mild-to-moderate sensorineural hearing loss using linear ampli-fication and compression amplification with two different compression ratios (1 .3 and 1 .8) . No sig-

nificant difference was found in the scores obtained in a nonsense syllable test in quiet. In the background of babble noise, consonant per-

ception was significantly better with linear amplification than with either form of com-

pression . In this study, speech material was

first processed by the hearing instruments and then recorded and played back via headphones during testing. As the hearing aid was not indi-vidually fitted, the hearing-impaired subjects' aided hearing ability was not optimized. This also hindered the generalization of the research find-

ings to real-life situations . Different experimental tasks and conditions

were employed in the studies, making it difficult

to compare findings . For example, some studies

have used phonemes (e.g ., Dreschler, 1988b),

nonsense syllables (e .g ., Vershuure et al, 1993), and sentences (e.g., Dreschler et al, 1984) as

the experimental speech material . When speech intelligibility was measured in noise, some experiments employed multitalker babble (e.g., Hickson et al, 1995), and some used speech-spectrum-shaped noise (e.g ., Nabelek, 1983). These differences in experimental tasks and

conditions also contributed to the inconsistency of the experimental findings .

has already been used in many commercially available hearing aids for some time . Without

any well-validated evidence or rationale, what

can hearing aid dispensers do when they must

select the most appropriate hearing aid or cir-

cuit for their clients? Byrne (1996) suggested that

the hearing aid dispenser is losing control of

the fitting process . The most basic question, "Is

compression beneficial to hearing-impaired peo-

ple?" needs to be answered . An interesting follow-up question to the

above is "Is compression beneficial to Cantonese-speaking hearing-impaired people?" In com-parison with English, which is an intonation language (i .e ., language uses pitch variations over phrases and sentences to distinguish mean-ing differences), Cantonese is a tone language, which uses the pitch of individual syllables to contrast meanings (Fromkin and Rodman,1988) . According to Ladefoged (1993), English is a stress-timed language, whereas Cantonese is a syllable-timed language in that syllables tend to recur at regular intervals of time and stress is less important than other prosodic features such

as tone. These differences in suprasegmental feature contribute to the difference in spectral and temporal cues of the two languages. Accord-

ingly, it is worthwhile to investigate the effect of WDRC (i .e ., syllabic compression) on Can-tonese perception. Previously, performance of

two-channel WDRC and one-channel linear amplification on Cantonese perception had been compared in a study done by Wong et al (1996) . In the study, better speech intelligibility using the two-channel aid was found in noise and in

quiet. To date, no information is available about the performance of single-band WDRC in Can-

tonese perception . The main aim of the present study was to

investigate the difference in performance with

single-channel WDRC and linear amplification in (a) Cantonese speech intelligibility in quiet, (b) Cantonese speech intelligibility in noise, and

(c) subjective preference . It was hoped that, with

the objective and subjective data, a better com-parison of WDRC and linear amplification per-formance can be obtained .

METHOD

Subjects

Experimental Rationale

Although the efficacy of WDRC has not been fully validated, such a signal-processing method

Twenty subjects (9 male and 11 female) ranging in age from 16 to 70 (mean = 44.6, SD = 17.4) were selected from among experienced hearing aid users who were visiting the

447


Table 2 Pure-Tone Hearing Thresholds of the Subject's Better Ears (Test Ears)

Subject Number

Hearing Level (dB HL) (re : ANSI S3.6-1989)

250 Hz

500 Hz

1000 Hz

2000 4000-8000 Hz Hz Hz

1 65.00 55.00 65.00 65.00 70.00 80.00 2 65.00 55.00 60.00 70.00 70.00 80.00 3 60.00 70.00 60.00 65.00 60.00 75.00 4 65 .00 65.00 55.00 55.00 60.00 70.00 5 45 .00 45.00 55.00 60.00 60.00 70.00 6 55.00 50.00 55.00 50.00 55 .00 70.00 7 55.00 55.00 60.00 50.00 50.00 65.00 8 60.00 60.00 65.00 65.00 65.00 75 .00 9 55.00 55.00 55.00 60.00 60.00 65 .00 10 45.00 50.00 50.00 60.00 65.00 70.00 11 40.00 45.00 50.00 55.00 55.00 65.00 12 65.00 60.00 65.00 60.00 50.00 65.00 13 45 .00 45.00 45.00 45.00 50.00 55.00 14 55.00 60.00 65.00 60.00 60.00 70.00 15 65.00 65.00 65.00 70.00 70 .00 75.00 16 60.00 55.00 55 .00 55 .00 60.00 70.00 17 50.00 50.00 60 .00 60.00 65.00 70.00 18 45.00 40.00 40.00 45.00 50.00 65.00 19 40.00 45.00 45.00 50.00 50.00 65.00 20 65.00 65.00 70.00 70.00 65.00 75.00 Mean 55.00 54.50 57.00 58.50 59.50 69.75 SD 9.03 8.26 8.01 7.80 7 .05 5.96

Audiology Clinic of the Department of Speech and Hearing Sciences at the University of Hong Kong or the Phonak Hearing Centre Hong Kong Limited. All had bilateral sensorineural hearing loss . To be included in the study, the degree of loss in the better ear (i .e ., the test ear) must be in the moderate to moderately severe range (i .e .' the pure-tone average for 0.5, 1, and 2 kHz lies between 42 to 68 dB HL) and the configuration of the loss must be flat (i .e ., the difference in hearing level between 500 and 4 kHz was 15 dB or less). Table 2 shows the pure-tone hearing thresholds of the subjects' better ears (test ears). Average pure-tone thresholds ranged from 40 to 67 dB HL (mean = 56.7, SD = 7.3). All subjects were experienced hearing aid users with hear-ing aid experience of 1 to 23 years (mean = 7.9, SD = 6.6) . Only Cantonese speakers with good oral communication abilities were recruited. The profile of the subjects is shown in Table 3.

was programmed for the test ear (better ear) before the subjects arrived. Hearing re-evalua-tion, hearing aid verification, speech intelligi-bility measurement both in quiet and in noise, and subjective rating were done within the same session.

Hearing Instrument

A Phonak behind-the-ear instrument, model Piconet2 P2 AZ, was used as the experimental hearing aid. Maximally, three hearing programs could be set and the circuit type could be selected independently in each program. The programs set could be switched via a digital remote con-trol . It had a Multi Dynamic Compression Con-trol, which could be set to yield linear amplification or WDRC. In this study, Program 1 (Pl) was set to linear amplification and Pro-gram 2 (P2) was set to WDRC. Both programs exhibited compression limiting with adaptive release time . The static and dynamic charac-teristics of the circuits are shown in Table 4.

Hearing Aid Programming

Before the subjects arrived, the target 2-cc coupler gain based on the subjects' most recently obtained audiogram was calculated using the

Table 3 Profile of Subjects (N = 20 )

Factor

Gender Age Type of hearing loss Degree of hearing loss 6 moderate (pure-tone average

Audiogram configuration Cause of hearing loss

Equipment and Procedure

All testing was carried out in a sound-treated room at the Audiology Clinic of the Department of Speech and Hearing Sciences at the Univer-sity of Hong Kong . Each subject was tested for about 2 hours in each session. The hearing aid

Hearing aid experience Years worn hearing aids Previous hearing aid circuit type

Characteristics Data

9 male, 11 female 16-70 years old (mean = 44.6) 20 sensorineural hearing loss

of 0 .5, 1 kHz and 2 kHz = 41-55 dB HL) 14 moderately severe (pure-tone average of 0 .5, 1 kHz, and 2 kHz = 56-70 dB HL)

20 flat (<15 dB difference between 1 and 2 kHz)

3 congenital 1 head trauma 6 presbycusis 7 viral infection 3 unknown 10 monaural, 10 binaural

behind-the-ear users 1-23 (mean = 7.9)

2 WDRC with CL, 18 linear with CL

WDRC = wide dynamic range compression, CL = compression limiting .

448


Table 4

Circuit

Amplification Char

Compression Charact

acteristics

eristics

of Diffe rent Circuits in the Exp

Compression

erimental Hearing Aid

Limiting

CT CR AT RT CT CR AT RT

Linear + CL N/A N/A N/A N/A

1 80 8 :1 5 Adaptiv

WDRC + CL 45 1 .1 :1 up 5 30 to 2.7 :1

CT = compression threshold, CR = compression ratio, AT = attack time (msec), RT = release time (msec), CL = compression

limiting, WDRC = wide dynamic range compression, N/A = not applicable .

FIG6 (3.0, Rev L) program. P1 of the instru-ment was set to linear amplification and P2 was set to WDRC. The 2-cc coupler gain of P1 and P2 was matched to the prescribed gain for 65 dB SPL input. Because the Phonak aid is a single-channel device, the compression ratio for WDRC setting was set to the average value of the pre-scribed ratios for both the high- and low-fre-quency bands. The mean compression ratio used for the group is 2.35 (SD = 0 .30) . FIG6 was selected since it is designed for prescribing gains and compression ratios for compression hearing aids . The prescribed gain for 65 dB SPL input was also used to prescribe gain for the linear amplification to ensure similar response between the two programs . As the OSPL 90 of P2 would be set automatically according to the selected compression ratio, no adjustment was made . OSPL 90 of P1 was set to match that of P2. The volume control wheel and the on-off switch on the instrument was deactivated.

Frequency Response Fine Adjustment

With the loudspeaker positioned 1 meter from the subject at 0" azimuth, subjective feed-back to 65 and 80 dB SPL (root mean square) continuous discourse was collected. Subjects were asked to rate the signals using a 7-point scale: "cannot hear," "very soft," "soft," "com-fortably loud," "loud," "very loud," and "intoler-ably loud ." The objective was to ensure that conversational level (65 dB SPL) speech signals were perceived as "comfortably loud" and high input level (80 dB SPL) signals were not "intol-

erably loud." It was found that no adjustment was necessary for these subjects .

Hearing Aid Veriffcation

At the beginning of the session, a routine re-evaluation of the subject's hearing was done .

The hearing thresholds of all subjects across all of the tested octave frequencies were within ±5 dB of the previously obtained value. After con-firmation of the subject's hearing status, hear-ing aid verification was done . Real-ear insertion response (REIR) was measured using the com-posite noise signal from the Fonix 6500 real-ear analyzer at three input levels : 50, 65, and 80 dB SPL. These steps were performed to verify the electroacoustic performance of the hearing aid and to verify that the frequency response actu-ally matched the target REIR . These input lev-els were also used to assess speech intelligibility.

Speech Material for Speech Intelligibility Measure

The Monosyllabic Cantonese Word Lists (Lau and So, 1988) and the Hearing In Noise Test (HINT, Chinese version) (Nilsson et al, 1994 ; Wong et al, in preparation) were used . The test stimuli of both tests were recorded by a male native Cantonese speaker. The speech stimuli were recorded on a CD-ROM. The recordings were played via a personal computer through a GSI-16 audiometer to a loudspeaker located 1 meter from the subject at 0° azimuth.

The monosyllabic word lists were used to measure word recognition ability. There were 10 lists of monosyllabic words and 1 list was pre-sented in each testing condition to determine the speech recognition score (SRS), which was defined as the percent of syllables correctly repeated .

The HINT (Chinese version) was used to determine SRT, which was defined as the pre-sentation level necessary for a listener to rec-ognize the speech materials correctly 50 percent of the time . Four 10-sentence lists, each with sen-tences of equal level of difficulty and phonemic content, were used . One list was presented in each testing condition.

e

449


Speech Intelligibility in Quiet

One monosyllabic word list was presented at each of three sound levels : 50, 65, and 80 dB SPL. The presentation levels were selected to evaluate the performance with sounds of low input level just above the compression thresh-old of WDRC (50 dB SPL), high input level above the compression threshold of compression lim-iting (80 dB SPL), and at everyday conversa-tional speech level (65 dB SPL) . The subjects were required to repeat aloud what they have heard. SRSs were obtained for both hearing aid programs .

For the HINT, one sentence list was pre-sented with each hearing aid program. The sub-jects were instructed to listen and repeat aloud whatever was heard or understood. An adaptive up-down strategy described by Nilsson et al (1994) was used to adjust the sentence presen-tation levels . The first sentence of a list was presented below threshold and the level was increased in 2-dB steps until the sentence was repeated correctly. The subsequent sentences were presented once each, with the presentation level dependent upon the accuracy of the pre-ceding response . Presentation levels were decreased by 2 dB after a correct response and raised by 2 dB after an incorrect response . SRT using each program was estimated as the mean presentation level calculated from the fifth to tenth sentences in the list .

Speech Intelligibility in Noise

One monosyllabic word list was presented in each testing condition. SRSs in noise with noise fixed at 65 dB SPL and at -9, -6, -3, 0, +3, +6, +9 dB SNRs were obtained. The levels were chosen to survey a range of performance across SNR. A four-talker babble (two female, two male) was used as competition signal during tests for speech recognition in noise.

The sentence lists of HINT were presented in a background of spectrally matched noise fixed at 65 dB SPL. SRTs (in terms of SNRs) were obtained by a similar procedure as that was used in quiet test . For tests in noise, both sig-nals and noise were presented from the same loudspeaker.

For both tests in quiet and in noise, the sequence of presentation of word lists and sen-tence lists and the order of the program being evaluated were randomized . The sequence of testing conditions or presentation levels was also randomized to counterbalance any prac-

tice and fatigue effect . The subject was blinded to the program in use and was not informed of the difference in the programs before finishing the experiment .

Subjective Preference Measure

Another set of 12 HINT sentences was used for sound quality rating. One sentence was pre-sented in each testing condition and 12 listen-ing conditions were evaluated. Preference of "loudness appropriateness," "sound clarity," and "sound pleasantness" was rated for the pro-grams in quiet at 50, 65, and 80 dB SPL and at +6 dB SNR with the spectrally matched noise fixed at 65 dB SPL. The three rating categories were chosen as they were relatively more con-crete and easier to define compared to other dimensions, such as brightness, sharpness, spa-ciousness, or fullness, which had been commonly measured in other studies (e .g., Balfour and Hawkins, 1992; Lundberg et al, 1992). This made the task simpler, especially for the older subjects . The SNR of +6 dB was selected as the test condition in noise because the speech intel-ligibility at this SNR had been rated as satis-factory to good by normal-hearing and hearing-impaired subjects in Lazarus's (1985) study (cited in Bachler and Vonlanthen, 1994). Sound quality judgment on unintelligible speech might be very difficult and unreliable .

A paired-comparison procedure was used to obtain subject preferences for linear amplifi-cation (Pl) and WDRC (P2) . The same sentence was presented in a listening condition while the hearing aid was switched to either programs via the remote control held by the experimenter. In evaluating the preference for loudness appro-priateness, the subject had to indicate which presentation of the sentence sounded more suit-ably loud . For clarity, the subject had to indicate which presentation of the sentence sounded clearer, that is, from which of the two he/she could extract the text more easily. Pleasantness is independent of intelligibility. Hence, the sub-ject had to indicate which presentation sounded pleasanter regardless of the ease of under-standing.

In each listening condition, the subject was allowed to switch back and forth between pre-sentations with P1 and P2 as many times as was necessary before a decision was made. After the subject had decided a preference for either pro-gram, he/she was asked to assign a strength to that preference . That is, the subject had to indi-cate whether the preferred program was (a)

450


t 50 dB SPL

- 65 dB SPL

-+-80 dB SPL

t 50 dB WDRC

- 65 dB WDRC

t 80 dB WDRC

.n-- 50 dB Lmea

o - 65 dB r Linear 80 &B L-

0

750 500 1000 1500 2000 3000 4000 6000

FrequencYLHz;

Figure l Mean REIGs for the WDRC program at three

input levels (N = 20).

much better, (b) moderately better, or (c) just

slightly better than the other program . The sub-

ject was also encouraged to explain the prefer-

ences . The subjects were blinded to the program

in use to minimize any subject bias . The sequence

of listening conditions and the order of pro-

grams being evaluated were randomized to

counter any task order effect .

RESULTS

Real-Ear Measurement

Mean real-ear insertion gains (REIGs) at

three tested input levels are shown in Figures

1 and 2 . REIGs using the two amplification

schemes are overlaid in Figure 3 . The gains

across all tested octave frequencies at 50 dB

SPL were significantly higher than that at 65 dB

SPL (p < .01) with the WDRC program, whereas

the gains at these two levels did not differ sig-

nificantly with the linear program . Both pro-

grams provided significantly less gain at 80 dB

SPL than that at 65 dB SPL (p < .01) across all

tested frequencies . When REIGs between the programs were

compared (see Fig . 3), significantly more gain

250 500 1000 1500 2000 3000 4000 6000 Frequency (Hz',

t 50 dB SPL

--~ -- 65 dB SPL

--r--80 dB SPL

Figure 2 Mean REIGs for the linear program at three

input levels (N = 20).

5O 500 1000 1500 2000 3000 4000 6000

Frequency (FLz ;

Figure 3 Mean REIGs for the linear and WDRC pro-

grams at various input levels .

was provided by the WDRC program to 50 dB

SPL input at 250, 500, 1000, 1500, and 2000 Hz

(F = 7.94, p < .05) . There was no significant dif-

ference in REIG at the input level of 65 dB SPL.

At 80 dB SPL, the WDRC program provided significantly less gain at 250 and 500 Hz (F =

7 .94, p < .05) than the linear program.

Speech Intelligibility In Quiet

As shown in Figure 4, the mean SRSs obtained using the monosyllabic word test with

WDRC were better than those obtained with

linear amplification at all presentation levels in

quiet. A two-way analysis of variance (ANOVA)

with repeated measures on both factors (program

and presentation level) was performed to inves-

tigate the effects of type of amplification and test

condition. Results are presented in Table 5. Both

factors and their interaction were significant,

indicating that the effect of program is signifi-

cantly different for distinct presentation levels .

As shown in Table 6, a matched-pair t-test

revealed a significant difference between pro-

grams only at the presentation level of 50 dB SPL

(t = 4.54, p < .01) . Mean SRTs obtained using the HINT in

quiet with the two programs are shown in Fig-

ure 5 . Results of a matched-pair t-test revealed

a significantly better SRT with WDRC (t = 2.36,

p < .05) .

Speech Intelligibility In Noise

Mean SRSs obtained using the monosyl-

labic word test in noise are shown in Figure 6.

Results of ANOVA of the data, as presented in

Table 7, revealed a significant effect of test con-

dition (SNR) and a significant interaction of

program and test condition. This suggests that

the effect of program is significant in certain test

451


Table 5 Analysis of Variance for Speech Recognition Scores Obtained in Quiet with Both Programs

Source df Effec t MS Effect df Error MS Error F

Program 1 1880.21 19 136.35 13 79" Presentation level 2 9799.38 38 259.02 .

37 83" Program x presentation level 2 473.96 38 51 .15 .

9 .27" MS = mean square .p < .01 .

conditions only. A matched-pair t-test was per-formed to investigate any significant difference in performance in each test condition. Results are summarized in Table 8. Better SRSs were obtained with "RC at a SNR of +6 dB (t = 2.49, p < .05) and with linear amplification at SNRs of 0and-6 dB (t=2 .87,p< .01;t=3.24,p< .01) .

Mean SRTs obtained using the HINT in noise are also shown in Figure 5. No significant difference was found with matched-pair t-tests.

Subjective Preference

Subjective preference for the two different hearing aid programs in the different test con-ditions is shown in Table 9. The strength of preference for loudness, clarity, and pleasantness under various test conditions is shown in Fig-ures 7, 8, and 9, respectively. A sign test was car-ried out to investigate any significant difference in preference. Significant subjective preference was found for WDRC for loudness appropriate-ness to signals at 50 and 80 dB SPL and for pleasantness to signals at 80 dB SPL (p < .05) . No significant preference for clarity was found in any test condition.

DISCUSSION

Speech Intelligibility in Quiet

The SRSs obtained in the monosyllabic word test with WDRC were significantly better than those with linear amplification at 50 dB SPL in

Figure 4 Mean SRS (%) obtained in quiet with WDRC and linear amplification.

quiet. That is, WDRC yielded better word recog-nition in quiet, at least to low-level signals. One possible reason for this finding is the better audibility ensured by WDRC. WDRC provided significantly more gain to input at 50 dB SPL than did linear amplification . As the signal level increased from 50 to 65 dB SPL, the gain pro-vided by WDRC decreased to an amount approx-imately equivalent to that provided by linear amplification, accounting for equivalent per-formance between programs . When the signals were presented at a high level (80 dB SPL), the gain provided by WDRC decreased further. For linear amplification, the gain also decreased due to activation of compression limiting, but to a lesser extent than WDRC. Although signals were louder, speech intelligibility did not improve significantly with linear amplification. At such a high presentation level, audibility is no longer a dominating factor for speech intelligibility. Other factors, such as distortion in the auditory system or the hearing aid, contribute to the dif-ficulties of speech perception experienced by those with moderate or greater cochlear losses (Moore, 1996).

For sentence material, the SRT obtained in quiet with WDRC was significantly lower than

90

a bo = Mean+SD o. ~ 50 ------------ ------------------------° . Mean-SD

------------1----------------------- --- ° Mean

40

,~ 30 -------------------

- ---------------- 2 to --------- - ----- " ---- - ---------------------

o 10

a; o

~ -10

a ~ -20

' -- ..

.. .{. . .------~--- ..--;. . . .. . . . . T. .--_-- . ; 13

WDRC In Quiet Linear In Quiet WDRC N Nom Lirrear N Noise

Test Condition

Figure 5 Mean and SD of SRTs obtained in quiet and in noise with both programs .

452

Comparison of "RC and Linear Amplification/Kam and Wong

Table 6 Matched-Pair t-test Results for Speech Recognition Scores Obtained in Quiet with Both Programs

Program

Presentation Level (dB SPL) Mean

WDRC

SD

Linear

Mean SD df t

50 69.50 19.53 54 .50 23 .11 19 4.54*

65 88.75 10.87 83.50 11 .01 19 1 .64

80 91 .75 9.36 90.50 6.67 19 0.63

WDRC = wide dynamic range compression *p < .01 ,

that obtained with linear amplification . That is, subjects were better able to repeat sentences

at lower level in quiet with WDRC. The effect is congruent with increased gain provided by

WDRC to low-level signals . Another possible reason is the increase in consonant-vowel-ratio

(CVR) caused by WDRC, which ensures that

the more intense vowel sounds receive less gain

than the less intense consonant sounds (Hick-son and Byrne, 1995; Kuk, 1996) . Speech intel-ligibility may be improved by increased CVR.

Increasing the consonant level should serve to

enhance the audibility of acoustic cues necessary

for perception . At the same time, decreasing the

vowel level should decrease the masking effects

of the stronger components of speech on the

weaker ones, thus also enhancing the audibil-ity of consonant acoustic cues .

The findings in quiet were consistent with those obtained in some other studies. For exam-ple, Dreschler (1988a) found better phoneme identification in quiet with a compression aid compared with a linear instrument in a group of 16 hearing-impaired subjects . Using a modi-fied rhyme test, Nabelek (1983) showed better

word recognition in quiet with WDRC in eight

628

OSRS osingWDRC

" SRS uvng Linear

--- Flt Ilne for WDRC

. . Fttlinefo,Linear

q 6 3 0 -3 -6

Signal-to-poise Ran. (SNR) (dB SPL I

Figure 6 Mean speech SRS (%) obtained in noise with

WDRC and linear amplification .

hearing-impaired subjects . Due to the large vari-

ation in methodology used in different studies,

it is impossible to make any fair comparison between studies . However, as suggested by Kuk

(1996), one common conclusion could be drawn

from those studies that reported supportive evi-

dence for WDRC : positive effect was observed in

tests when the stimulus was presented in quiet

at a fixed low level . Results from the present

study give further support to this conclusion.

Speech Intelligibility in Noise

The overall SRSs obtained in the monosyl-labic word test and the SRT obtained using HINT with WDRC were not significantly dif-

ferent from those obtained with linear amplifi-cation . In other words, WDRC did not provide significant improvement in speech intelligibil-ity in noise. This finding is not uncommon . Many previous studies, which also employed a con-stant background noise in testing, reported sim-

ilar results . For example, no significant difference in SRT obtained in noise in 12 hear-

ing-impaired listeners was reported by Dreschler et al (1984) . Tyler and Kuk (1989) also failed to

Table 7 Analysis of Variance for Speech Recognition Scores Obtained in Noise with

Both Programs

Source df Effect MS Effect df Error MS Error F

Program 1 232.23 19 167 .76 1 .38 SNR 6 29162.38 114 262 .98 110.89* Program x SNR 6 620.36 114 152.16 4.08*

MS = mean square SNR = signal-to-noise ratio . *p < .01 .

453


Table 8 Matched-Pair t-test Results for Speech Recognition Scores Obtained in Noise with Both Programs

Program

SNR (dB) Mean

WDRC

SD Mean

Linear

SD df t

+9 79 .25 13.79 77.50 12 .41 19 0.50 +6 75.50 15.30 66.75 13 .79 19 2 .49* +3 68.50 19 .27 63.50 20.72 19 0 .98 0 50.00 19 .74 62.75 18.95 19 2.88* -3 39.00 24.47 47.00 21 .97 19 1 .51 -6 15 .00 16.86 23.00 21 .67 19 3.24** -9 7.50 13.91 7 .00 12.92 19 0.31

SNR = signal-to-noise ratio, WDRC = wide dynamic range compression . *p< .05;**p< .01 .

demonstrate any significant improvement in consonant recognition with WDRC over linear amplification in a multitalker babble background noise for 11 listeners.

Reasons for poor performance with WDRC in noise were not well understood and have not been verified. There are some proposed expla-nations. Tyler and Kuk (1989) suggested that the temporal information contained in the speech sig-nal may be disrupted by the dynamic amplifi-cation. Another possible reason is that compression amplification may increase the level of the background noise in the gaps of the speech signal (if the noise is present at a lower level than the speech) and hence cause masking of the speech signal (Hickson et al, 1995).

The relationship between circuit type and speech intelligibility requires further clarifica-

tion . Although WDRC did not improve speech intelligibility in noise, it did not degrade the performance in comparison to linear amplifica-tion. In the monosyllabic word test, at favorable SNRs, the mean SRSs obtained with WDRC were better than those with linear amplification. In more adverse listening conditions, linear amplification provided slightly better speech intelligibility. For the sentence test, the SRT obtained with WDRC was lower (i .e ., better speech intelligibility) than that with linear amplification, although the amount was not sta-tistically significant .

The findings in the present study do not support the notion that WDRC has a negative effect on speech intelligibility in noise when compared to linear amplification. However, WDRC may be slightly more vulnerable to noise

Table 9 Subjective Preference for Program in Various Test Conditions (N = 20)

Quality Signal Level Number of Subjects Number of Subjects (dB SPL) Preferring WDRC Preferring

Linear Amplification

Loudness 80 in quiet 15* 5 65 in quiet 11 g 50 in quiet 15* 5 In noise at SNR = +6 11 9

Clarity 80 in quiet 7 13 65 in quiet 10 10 50 in quiet 11 9 In noise at SNR = +6 9 11

Pleasantness 80 in quiet 15* 5 65 in quiet 13 7 50 in quiet 12 8 In noise at SNR = +6 8 12

SNR = signal-to-noise ratio, WDRC = wide dynamic range compression. *Sign test significant, p < .05 .

454


IS M

O,

0 WDRC moderately better

p WDRC slightly better

M

z

0

WDRC much better

Linear much better

Linear moderately better

0 Linear slightly better

a- I

80 dB SK In Quiet 65 dB SPL U Quiet 50 dB SPL hi Quiet

Test Condition

In Noise

Figure 7 Strength of subjective preference for "loudness" in various test conditions .

than linear amplification . From Figure 6, it can be seen that the regression line for WDRC is

steeper than for linear amplification . That is, a

shift from a higher SRS to a lower SRS takes a

smaller change in SNR for WDRC than for the

linear program . This finding could not be com-

pared to related studies in the area as other

studies seldom varied the SNR. The implica-

tion of this observation is that when perfor-

mance comparison between WDRC and linear

amplification is made, the use of a single SNR

is undesirable . A range of SNR should be included to give a fair evaluation of both systems .

This issue had been discussed in Yund and Buck-

les's (1995) study, which evaluated the perfor-

IS M WDRC much better

WDRC moderately better

F71 WDRC slightly better

I

Linear much better

Linear moderately better

Linear slightly better

80 dB SPL In Quiet 65 dB SPL In Quiet 50 dB SPL In Quiet Test Condition

In Noise

Figure S Strength of subjective preference for clarity

in various test conditions .

15

~, t0 U

w O

N

E S z

80 dB SPL In Quiet 65 dB SPL In Quiet 50 dB SPL In Quiet

Test Condition

In Noise

Figure 9 Strength of subjective preference antness in various test conditions .

for pleas-

mance of multichannel systems instead of sin-gle-channel compression.

Subjective Preference

Significant subjective preference was found for WDRC for loudness appropriateness to sig-nals at 50 and 80 dB SPL and for pleasantness to signals at 80 dB SPL. The explanations for these findings are straightforward . At 50 dB

SPL, the gain provided by WDRC was signifi-

cantly higher than that provided by linear pro-gram. Better audibility gained more votes for

better loudness . In fact, most subjects complained that the sound with the linear program was too soft . Better audibility also resulted in signifi-

cantly better speech intelligibility to 50 dB SPL signals. At 80 dB SPL, significantly more gain was provided by linear amplification. However,

the extra gain seemed to be too much for most

subjects in this study Although the stimuli at 80

dB SPL was not so pleasant, subjects' word recog-nition scores, compared to WDRC condition, were not degraded . It seems that although the signal was not pleasant, it was not distorted enough to reduce speech intelligibility.

Interestingly, subjects did not show a sig-

nificant preference for clarity in all test condi-tions. One possible reason is that once the stimuli

were audible or clear enough using either pro-

gram, it was difficult for the subjects to tell which program sounded clearer. Preference of

clarity does not seem to be affected by preference of pleasantness .

A large intersubject variability was observed in the preference judgment . From the strength

M WDRC much better E3 WDRC moderately better

p WDRC slightly better

Linear much better 0 0 Linear moderately better

M Linear slightly better

455


of preference shown in Figures 7, 8, and 9, it can be seen that in one condition, different subjects preferred different programs . Even when voting for the same program, some subjects found it much better while some found it only slightly bet-ter than the other program.

Clinical Implications

The results obtained in this study showed better speech intelligibility and listening com-fort using WDRC in certain situations . This means that there is at least some support for the selection of single-band WDRC for clients with moderate to moderately severe flat sensorineural hearing loss . However, the usefulness of WDRC in noisy situations, where it is often advertised as being of great value, was not found.

In this study, the performance of hearing aid programs varied with stimulus presentation levels and SNRs . This indicates the need to employ multiple testing levels when fitting hear-ing aids, especially nonlinear instruments. We recommend that, in quiet, the hearing instru-ments should be evaluated with at least three stimulus levels : high, average, and low. In noise, SNRs of +3 and +6 dB are recommended as the test conditions . It has been found that the aver-age SNR of conversational speech is about +4.8 dB in noisy environments and substantially less in automobiles (Teder, 1990).

Subjective preference for clarity did not yield significant findings in this study. In clinical prac-tice, subjective evaluation of clarity may be omit-ted. However, the inclusion of subjective judgments of other dimensions such as loudness appropriateness and pleasantness may provide some valuable information for circuit selection. A simple paired-comparison procedure may be adequate and easy to administer in the clinical evaluation of hearing aid performance.

It is interesting to note that results using materials in Cantonese, a tone language with its own phonologic system and language structure, yielded similar results to those obtained using English materials . It would be interesting to compare the results to studies using other lan-guage materials to determine whether general-izations can be made. This would have significant implications for audiologists serving multicul-tural populations .

for their support. Thanks also to Anthony Yuan, Polly Lau, and Kevin Yuen for their assistance in preparing the multitalker babble . We are indebted to the subjects whose cooperative spirit made the research possible and enjoyable . Appreciation is extended to Dr. Bradley McPherson for his helpful suggestions during the prepa-ration of this manuscript.

Kam ACS, Wong LLN. (April 4, 1998). Comparison of Performance with Wide Dynamic Range Compression and Linear Amplification. Poster presentation at the 10th Annual Convention of the American Academy of Audiology [Abstract p. 1191, Los Angeles, CA .

This study was submitted by the first author for the degree of Master of Science in Audiology at the University of Hong Kong in May 1998.

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