J Am Acad Audiol 13 : 14-24 (2002)

Benefits of Syllabic Input Compression for Users of Cochlear Implants Hugh J. McDermott*t Katherine R. Henshall* Colette M. McKay*

Abstract

Ten users of multielectrode cochlear implants participated in an evaluation of the perceptual effects of input-signal compression . A syllabic compressor was introduced into the microphone circuit of Spectra-22 or SPrint sound processors . The post-compression gain was adjusted to provide sim-ilar loudness for speech at an average level of 65 dBA with compression either enabled or disabled . Sentence recognition was measured at three levels . Averaged across all listeners, statistically sig-nificant score increases were obtained at each level with compression enabled (45 dBA : 19 .6 percentage points, p < .0001 ; 55 dBA : 16 .6 percentage points, p < .0001 ; 70 dBA : 3 .1 percent-age points, p = .031) . A test of speech intelligibility in noise showed no significant effect of compression . Generally, participants in the trial reported improved perception of low-level sounds with compression, although a few disliked the increased loudness of some background noises . Some participants suggested that the ability to enable or disable compression with a manual switch would be helpful . Overall, the results show that input compression can improve the performance of these sound processors for users of cochlear implants, especially when listening to speech at low levels .

Key Words: Cochlear implants, compression, speech perception

Abbreviations : AGC = automatic gain control, BTE = behind the ear, CI = cochlear implant, C-level = highest (comfortable) stimulation level, CT = compression threshold, SIT = Speech Intelligibility Test, SNR = signal-to-noise ratio, T -level = threshold (lowest audible) stimulation level

Sumario

Diez sujetos con implantes cocleares de multi-electrodo participaron en una investigacion sobre los efectos perceptuales de la compresion de senales de entrada (input-signal . Un compresor de silabas fue introducido en el circuito del microfono de los procesadores de sonido de un Spectra-22 o un SPrint . La ganancia post-compresion fue ajustada para aportar una sonoridad (loudness) similar para el lenguaje, a un nivel promedio de 65 dBA, con la compresion activada o desacti-vada . El reconocimiento de frases fue medido en tres niveles . Promediado para todos los sujetos, se obtuvieron incrementos estadisticamente significativos de los puntajes en cada nivel activado de compresi6n (45 dBA:19.6 puntos porcentuales, p < .0001 ; 55 dBA:16 .6 puntos porcentuales, p < .0001 ; 70 dBA : 3 .1 puntos porcentuales, p = .031) . La realizacion de una prueba de discrim-inacion del lenguaje en medio ruidoso no mostro un efecto significativo de la compresi6n . En general, los participantes del estudio reportaron mejoria en la percepcion de sonidos de bajo nivel con la compresion, aunque a unos pocos les desagrado la sonoridad incrementada para algunos ruidos de fondo. Algunos participantes sugirieron que la posibilidad de activar o desactivar la com-presion con un interruptor manual seria 6til . Globalmente, los resultados muestran que la compresion de entrada puede mejorar, para usuarios de implante coclear, el rendimiento de estos proce-sadores de sonido, especialmente cuando se trata de escuchar el lenguaje a bajo nivel de intensidad .

Palabras Clave : compresi6n, implante coclear, percepcion del lenguaje

Abreviaturas : AGC = control automatico de ganancia, CI = implante coclear (IC), C-level = nivel mas alto (confortable) de estimulacion, SIT = Prueba de Inteligibilidad del Lenguaje, SRN = tasa o relacion sepal/ruido, Tlevel = nivel umbral (audible minimo) de estimulacion

*The University of Melbourne, East Melbourne, Australia, and the tCo-operative Research Centre for Cochlear Implant and Hearing Aid Innovation, East Melbourne, Australia

Reprint requests : Hugh J. McDermott, Co-operative Research Center for Cochlear Implant and Hearing Aid Innovation, 384-388 Albert Street, East Melbourne 3002, Australia

14

F

or many people with a sensorineural hearing impairment of such severity that conventional acoustic hearing aids are of

minimal benefit, the cochlear implant (CI) is now established as a safe and relatively effec-tive means of restoring hearing. However, despite the rapid evolution of CI designs, particularly during the last two decades, no existing Cl can restore hearing to normal . Although some users of multichannel CIs can understand about as much speech without visual cues as people with normal hearing in ideal listening situations, performance with a CI is usually severely degraded in less favorable conditions . For exam-ple, when listening to speech with a competing noise, users of' CIs generally require a larger signal-to-noise ratio (SNR) than do listeners with normal hearing to obtain the same intelli-gibility. Even in listening conditions without background noise, intelligibility may be greatly affected by the level at which speech is picked up by the microphone of the CI unless the sen-sitivity of the sound processor is adjusted to maintain optimum audibility.

A manual sensitivity control is provided on most sound processors that are available for use with existing CIs. This control directly varies the gain applied to the electrical signals obtained from the microphone before they are conveyed to the analysis and stimulus generation parts of the processor. In some sound processors, an automatic gain control (AGC) is also provided . The design of the AGC differs among processor types, but the main reason for the inclusion of an AGC is to reduce the overall range of levels presented to the signal analysis stages of the processor.

At present, a range of multielectrode CI systems are manufactured by several companies, including Cochlear Limited . Two sound proces-sors currently supplied by Cochlear are the Spectra-22 (for use with the CI22 implant) and the Sprint (for use with the more recent C124 implant) . Both of these processors are worn on the body and may be programmed with a num-ber of distinct sound processing algorithms. However, in both devices, the input signals from the microphone are preprocessed (prior to analy-sis and stimulus generation) in essentially the same way (Fig . 1) . The preprocessing comprises electrical gain that can be adjusted by the user via a sensitivity control and a fast-acting com-pression limiter that acts to prevent distortion in the following stages of the sound processor when the intensity of input signals is excessive . For input levels below the level at which limit-

Benefits of Syllabic Input Compression/McDermott et al

ing occurs, the operation of the preprocessor is linear (i .e ., the gain is not affected by the signal level) .

The range of input signal levels that is audi-ble to users of these sound processors is restricted by comparison with the range of levels that is audible (and comfortable) to listeners with nor-mal hearing . The restriction arises from the way in which acoustic signal levels are con-verted to the levels of the electric stimulation delivered by the intracochlear electrode array (see Fig . 1) . For example, in the SPEAK pro-cessing scheme, which may be programmed into either the Spectra-22 or Sprint processors, elec-trical signals obtained from the microphone are analyzed by a set of up to 20 bandpass filters (Skinner et al, 1994 ; Seligman and McDermott, 1995) . The filters have partially overlapping frequency responses encompassing at least the range of frequencies assumed to carry most information about speech . The amplitude enve-lope of the signal passed by each filter is esti-mated . At periodic times, these amplitudes are scanned, and the largest 6 to 10 are identified . The active electrodes in the CI are assigned to the filters such that the lowest input frequen-cies activate the most apical electrode and the highest frequencies activate the most basal elec-trode . When the filters with the largest ampli-

tudes are identified, electric pulses are delivered by the corresponding electrode to stimulate audi-tory neurons . The levels of the pulses are related to the amplitudes by an instantaneous, nonlin-ear function . The range of output stimulation lev-els produced by this function extends from the T-level (the lowest audible level) to the C-level (the highest comfortable level) . The T-level and the C-level are determined for each electrode independently in each user of a Cl prior to fit-ting the sound processor. The amplitude con-version function is designed to provide an appropriate growth of stimulation levels between the T-level and the C-level for an increase in acoustic sound pressure level over approximately 30 dB.

The sound pressure level at the microphone that produces stimulation at the T-level and is thus just audible to the CI user depends on two factors . The first factor is the frequency response

of the microphone . The microphones typically used with the Spectra-22 and Sprint processors are directional, two-port microphones . Such microphones inherently have a frequency response that rises at approximately 6 dB/octave for frequencies up to about 4 kHz. At higher frequencies, the sensitivity falls steeply. Thus,

15

Journal of the American Academy of Audiology/Volume 13, Number 1, January 2002

Sensitivity control & compression limiter

Spectral analysis

(bandpass filter-bank)

0

Selection of active

electrodes (from, filter outputs)

Conversion of acoustic to electric levels

H Transmission of

electrode and level data to implant

Figure 1 Schematic diagram showing the main functional blocks in the sound processor. In the experiment, sig-nals from the microphone (at left) passed through either a syllabic compressor (upper path) or a linear preamplifier (lower path), depending on whether compression was enabled or disabled. The signals then passed via a sensitivity control and a compression limiter to a spectrum analyzer. The spectral estimates were then converted into levels of electric stimulation on appropriate electrodes by means of a fixed transfer function . Finally, digital data specifying the active electrodes and the current levels to be generated were transmitted sequentially to the listener's cochlear implant. All blocks to the right of the dotted line are components of the Spectra-22 or SPrint sound processor.

audibility for the user of a CI varies as a func-tion of acoustic frequency and generally improves with increasing input frequency up to 4 kHz. As indicated in Figure 2, the microphone's fre-quency response compensates partially for the shape of the speech spectrum, in which the aver-age levels tend to decrease at frequencies above about 500 Hz. The second factor affecting audi-bility is the electrical gain applied to micro-phone signals in the preprocessor. As mentioned above, this gain is affected by the manual sen-sitivity control. By adjusting the sensitivity, the CI user can select the sound levels that are just audible and therefore also the sound levels that will produce electric stimulation at the maximum possible level (i .e ., the C-level) on each elec-trode. However, the shape of these levels as a function of frequency cannot be changed by the user.

Figure 2 shows the just-audible sound lev-els for users of the Spectra-22 and SPrint proces-sors for typical settings of the sensitivity control and pure-tone input signals (solid line) . Note that these levels do not vary among CI users as a function of the users' T-levels, provided that the T-levels are set to produce just-audible elec-tric stimulation. Audibility can be varied during programming of the sound processor by adjust-ment of certain parameters that affect the ampli-tude conversion function, but those parameters were not altered in the present experiment and are not discussed further here . The minimum audible levels shown as the bottom line in Fig-ure 2 are representative of the just-audible sound levels for typical settings of the sensitiv-ity control and the other relevant processing parameters . On average, those levels are some 30 dB higher (worse) than the pure-tone thresh-olds of people with normal hearing for sounds

presented in free-field conditions (ISO, 1996). Also shown in Figure 2 is the sound level that first produces stimulation at the C-level on the electrode corresponding to the frequency of the input signal (dotted line). (Note that, although increases in sound intensity beyond this level will not increase the stimulation level on that elec-trode, the overall loudness may still be per-ceived to increase because the levels on nearby electrodes, which are partly responsive to the same input frequency, will continue to increase .) As explained above, these two curves are sepa-rated in the vertical direction by 30 dB .

The hatched area in Figure 2 indicates the approximate range of levels associated with speech signals when received at the microphone at an average level of 60 dBA (Byrne et al, 1994). The range of 30 dB at each frequency is assumed to include all of the speech components that are necessary for full intelligibility, at least for lis-teners with normal hearing (ANSI, 1997). The observation that this range matches approxi-mately the range of levels that is audible for typ-ical users of Spectra-22 and SPrint processors helps to explain why many such users of CIS obtain excellent speech perception when speech is presented (in quiet) at moderate to high lev-els. However, large variations in the overall level of speech signals may occur for different speakers and when speakers vary their vocal effort, as well as in different acoustic conditions .

Further inspection of Figure 2 leads to a prediction that intelligibility will decrease if speech is presented at lower levels . For exam-ple, if the average speech level was reduced by 5 dB (to 55 dBA), a larger part of the hatched area would fall below the minimum audible levels (solid line). This would result in some important components of the speech signal

16

Benefits of Syllabic Input Compression/McDermott et al

100 . . . . . Maximum input

------ Compression threshold Minimum input (linear)

80 --- -° Minimum input (compression) . . .. . .. . -- Normal threshold

60 m

40 J

n

20

0

0.25 0.5 1 2 4

Frequency (kHz)

Figure 2 From bottom to top, the curves show average pure-tone threshold levels for normal hearing (ISO, 1996), approximate thresholds of audibility for users of cochlear

implants with compression enabled, approximate thresh-

olds for cochlear implants with compression disabled,

the compression threshold (i .e ., level above which com-

pression was applied), and input levels above which elec-

tric stimulation occurs at the C-level. The hatched area

shows the approximate range of speech spectra (levels in

1/-octave bands) at a moderate overall level (60 dBA) .

becoming inaudible, and, therefore, speech intel-ligibility would be reduced . This hypothesis is consistent with the findings of a study that measured the ability of 10 adult users of the

Spectra-22 processor and SPEAK scheme to

recognize speech at different overall levels (Skin-ner et al, 1997) .

The objective of the experiments reported

below was to measure the effect on intelligibil-ity of the average level of speech and to inves-

tigate whether compression of input signals would partially compensate for the effects of

varying speech levels, without requiring changes to any of the parameters in the sound processors .

It was expected that input compression would

be beneficial because it would result in a reduc-tion of the sound levels that were just audible

to the CI users . However, it was also antici-

pated that the expected improvement in audi-

bility might be accompanied by poorer tolerance

of low-level background noises that are softer or

inaudible to CI users when listening via their

existing sound processors without input com-pression . This possible side effect was investi-

gated by measuring speech perception in

competing noise with and without compression

and by asking the participants in the experiment

to report on their experiences using compression in everyday listening situations away from the laboratory.

For many years, compression has been

applied in acoustic hearing aids (for a review of

various compression systems and their perfor-

mance, see Dillon [1996]) . Some systems com-

press sound signals over a relatively large dynamic range and thus reduce the range of

levels that is produced at the output of the hear-ing aid . These systems can reduce the natural differences in intensity among phonemes (such

as the differences between weak consonants and intense vowels) if the gain changes occur

rapidly enough . Fast-acting (or "syllabic") com-pressors typically have attack and release times of about 5 and 50 msec, respectively. (By con-

vention JEC, 19831, these values specify the

time taken for the circuit to complete a gain change in response to an abrupt change in input level of 25 dB.) Compression systems can also

compensate partially for the variations in inten-sity of speech signals that occur over longer

times . If that is the primary objective of the compression, the attack and release times may

be much longer (up to several seconds) . Evi-dence from evaluations of various compression systems in hearing aids suggests that speech

intelligibility can be improved (relative to that for hearing aids without compression) when speech is received at lower than optimum lev-

els, provided that listeners do not increase the gain of the aid manually (Dillon, 1996) . Either

syllabic or slow-acting compression (or a com-

bination) can be effective at maintaining ade-quate audibility, and therefore good intelligibility,

of speech in spite of level variations and with-

out the need for frequent adjustment of the

hearing aid by the listener. There appear to be only a few published

reports of the effects of input compression in sound processors for Cls. As outlined above, sound processors usually contain a nonlinear function that converts amplitudes of selected components of the incoming acoustic signal into levels of electric stimulation that are appropri-ate for each electrode in each user of a CI (Zeng and Galvin, 1999). However, it is important to realize that the operation of this amplitude con-

version function is separate from the operation of an input AGC circuit (if one is included in the sound processor) . In particular, the amplitude conversion function is designed to suit the loud-

ness growth characteristics of each electrode independently, and it is applied to the signal amplitudes with zero attack and release times.

17

Journal of the American Academy of Audiology/Volume 13, Number 1, January 2002

Table 1 Relevant Information about the Participants

Years of Years of Number of Sound Age Profound Implant Electrode Electrodes Device Processing Participant (yr) Sex Deafness Experience Etiology Configuration in Use Type Scheme

P1 50 F 5 4 Unknown, BP + 1 15 C122/ progressive Spectra-22 SPEAK

P2 63 M 28 11 Otosclerosis CG 17 C122/ Spectra-22 SPEAK

P3 65 M 10 8 Genetic, BP + 1 20 C122/ progressive Spectra-22 SPEAK

P4 25 F 17 8 Unknown, BP + 2 19 C122/ progressive Spectra-22 SPEAK

P5 59 M 15 9 Otosclerosis BP + 1 14 C122/ Spectra-22 SPEAK

P6 48 M 10 5 Genetic, Mono 20 progressive C124/SPrint ACE

P7 72 M 29 14* Meningitis Mono 8 C124/SPrint CIS P8 78 M 6 2 Unknown, Mono 20

progressive C124/SPrint SPEAK P9 47 F 20 3 Otosclerosis Mono 18 C124/SPrint SPEAK P10 69 M 23 22* Trauma Mono 20 C124/SPrint SPEAK

*Participants P7 and P10 used earlier versions of the cochlear implant before they had them replaced by the C124 device in 1998 . Electrode configurations : BP + n = bipolar stimulation, in which the active electrodes are separated by n inactive electrodes ; CG =

common ground stimulation, in which the stimulus passes between the active electrode and all other intracochlear electrodes ; Mono = monopolar stimulation, in which the stimulus passes between the active electrode and two extracochlear electrodes . Details of the device type and sound processing scheme are provided in the text .

In contrast, input compression systems apply AGC to the whole input signal, before frequency analysis and other processing, and apply gain changes with finite response times.

Syllabic input compression was employed in the Spectral Maxima Sound Processor (SMSP), which was developed in 1989, primarily for use with the CI22 implant (McDermott et al, 1992, 1993). Although comparative studies showed that the SMSP provided generally better per-ceptual performance than the Mini Speech Processor (MSP), which at the time was supplied commercially to users of the C122 device (McKay and McDermott, 1993), no tests were conducted to evaluate the specific function of the input compressor . Subsequently, when the SPEAK sound processing scheme was developed based on the SMSP, input compression was not included for technical reasons (Skinner et al, 1994) . The functional principles of the SPEAK scheme were outlined above.

In a recent study, the effects on speech per-ception of a number of different input compres-sion systems were investigated with experienced users of the Med-El Combi-40 multichannel CI (Stobich et al, 1999). The attack and release times, compression ratios, and compression thresholds (CTs) varied among the experimen-tal systems, but the parameters of the instan-

taneous amplitude conversion function in the sound processor were held constant . The results of measurements of speech recognition at aver-age levels of 55, 70, and 85 dB SPL showed that compression was effective at maintaining speech intelligibility but that there were only minor differences in performance among the com-pression systems tested .

The effects of syllabic input compression, when applied to the Cochlear Limited Spectra-22 and SPrint sound processors, have not been studied or reported previously. This study was designed to determine such effects.

METHOD

Subjects

Ten adults participated in the experiments. Relevant background information about them is provided in Table 1. Five participants (Pl-P5) were users of the CI22 implant, whereas the other five (P6-P10) were users of the CI24 implant. The participants with the CI22 implant were all tested while using a Spectra-22 sound processor programmed with the SPEAK strat-egy (Skinner et al, 1994 ; Seligman and McDer-mott, 1995). Three of the participants with the CI24 implant (P8-P10) also used the SPEAK

18

Benefits of Syllabic Input Compression/McDermott et al

strategy, whereas P6 used the advanced combi-nation encoder (ACE) strategy (which is similar to SPEAK, but with a higher stimulation rate) and P7 used the continuous interleaved sampling (CIS) strategy (Wilson et al, 1991). Each of P6-P10 used the SPrint sound processor in the experiments.

No special criteria were applied in selecting people to take part in the study. All participants volunteered their time and received no payment other than reimbursement of expenses such as travel costs .

Signal Processing

The microphone typically used with a Spectra-22 or SPrint sound processor is pack-aged in an enclosure that is physically similar to a conventional behind-the-ear (BTE) hearing aid . To provide the input compression function in the experiments, this microphone was replaced by a modified BTE hearing aid (Berna-fon SB13) . This hearing aid has a directional,

two-port microphone with frequency response nearly identical to that of the microphone nor-mally used with the Spectra-22 or SPrint proces-

sor. The microphone signal is processed by a compressor that has programmable CT and gain . The modification to the hearing aid enabled the processed signal to be delivered to the input of the Spectra-22 and SPrint proces-

sors in place of the normal microphone signal . An important objective of the experimental

procedure was to avoid any changes to the pro-gramming of each participant's sound processor or to the settings of user controls (such as the

sensitivity control) . By meeting this objective, the trial succeeded in evaluating the effects of input compression without many potentially con-founding factors . Accordingly, the modified SB 13 was adjusted initially with the compressor disabled to provide signals similar to those from

the standard microphone. Subsequently, it was adjusted to provide similar loudness with com-pression enabled for each participant with all of the settings of the Spectra-22 or SPrint proces-

sor held constant . The details of this fitting pro-

cedure are described below. Initially, a suitable setting was selected for

the CT, which is programmable in the SB13. When input signals exceed the CT, the com-pression ratio applied by the SB13 is 2:1 . For example, when the input signal level exceeds the CT and changes by 10 dB, the output signal level changes by 5 dB . Below the CT, the gain is constant (i .e ., an input level change of 10 dB

results in an output level change of 1OdB) . Set-ting the CT involves selecting a compromise between the range of levels over which com-pression is applied and the listener's tolerance

of low-level background noise . The compromise arises because the overall gain applied after the input signal has been compressed must be higher

than the gain required to obtain similar loudness without compression for signals of moderate

level . Thus, lower CTs result in larger gain increases and generally louder background noises . A recent study of listeners' preferences for each of two CT settings, in which the SB13

hearing aid was used by a group of adults with hearing impairment, reported that 60 percent of participants preferred a CT of approximately 66 dB SPL and 31 percent preferred a CT of approximately 50 dB SPL (Dillon et al, 1998) .

Based on this finding and on preliminary trials

with several users of CIs, a nominal CT of 55 dB SPL was selected for the experiments reported below. As shown in Figure 2 (long-dashed line), the actual CT varies as a function of frequency.

The overall input-output characteristic of the

compressor at 1 kHz is illustrated in Figure 3 . With compression disabled, the gain of the

SB13 was adjusted so that the loudness of con-tinuous speech played from a recording at an average level of 65 dBA was judged similar to the loudness perceived with the standard micro-phone . This adjustment was carried out ini-tially with three users of the Spectra-22 (P2, P4, and P5) and three users of the SPrint processor (P6, P7, and P8) . Because these listeners' adjust-ments were consistent, the same gains were used for the remaining participants in the test

condition with compression disabled . This pro-

cedure ensured that the SB13 could be substi-tuted directly for the standard microphones without requiring any changes to the sound processors .

Subsequently, all 10 participants individually adjusted the gain with compression enabled to obtain the same loudness as was perceived with compression disabled when listening to the speech material described above. The results were very consistent among participants . The average gain with compression enabled exceeded that for com-pression disabled by 6.0 dB (see Fig. 3) . The gains selected in this fitting procedure were used in all of the speech perception tests reported below. Note that it was necessary to determine these gain settings using speech at the moderate-to-high average level of 65 dBA to ensure that there would be an effective gain difference between the compression and no-compression conditions

19

Journal of the American Academy of Audiology/Volume 13, Number 1, January 2002

30 40 50 60 70 80

Input Level (dB SPL)

Figure 3 The output level versus input level charac-teristic at 1 kHz of the compressor used in the experiments (solid line), showing linear gain at levels below the com-pression threshold and 2:1 compression at higher levels . The horizontal dotted line indicates the range of input lev-els that produced stimulation at the C-level (see text for details) . For comparison, the input-output characteristic of the preamplifier without compression is also shown (dashed line).

for input signals at lower levels, at which improve-ments in speech intelligibility were anticipated .

Procedure

To evaluate the expected benefits of input compression, speech perception was assessed for each listener with sentences presented in quiet at average levels of 45, 55, and 70 dBA. Sentence material was selected for all of the speech perception experiments (rather than alternatives, such as monosyllabic words) because it contains phonemes of varying dura-tion with natural level fluctuations and few pro-longed silences . The sentences were taken from the Speech Intelligibility Test (SIT; Magner, 1972). This material comprises 40 lists, each containing 15 sentences of varying length . The material was presented to individual listeners from audio recordings in a medium-sized sound-attenuating booth. The recordings were played through a loudspeaker located approximately 1.5 meters from the listener. At the conclusion of each sentence, participants were asked to repeat as many words as they could identify, and the proportion of key words correctly recognized was recorded (of a total of 80 key words per list). No list was repeated for any participant throughout the experiments.

Participants attended three test sessions, each of about 1 hour's duration, which were scheduled at weekly intervals. In each session, four tests of speech perception were carried out.

These tests measured perception of speech in quiet at three levels and perception of speech in noise. For each test, scores were obtained both with compression enabled and with compres-sion disabled. Immediately after the compression was enabled or disabled within a test session, participants engaged briefly in general conver-sation with the experimenter before the test material was presented, but no formal procedure was followed to provide additional listening experience with the changed signal processing condition. Participants were not able to use the compression system before or between these three test sessions .

For each of the three speech levels, tests were conducted in an order designed to minimize the possible confounding effects of practice on the task . Half a list of SIT sentences was presented in one compression condition immediately before one full list was presented in the other com-pression condition. This was followed by the remaining half of the first list of sentences in the first condition. The order in which the two con-ditions were presented (i .e ., with compression enabled and with compression disabled) was alternated across test sessions .

A test of speech perception with competing noise was also carried out in each session. Because participants had a wide range of lis-tening abilities with their CIs, an adaptive procedure was used to establish the signal-to-noise ratio (SNR), which caused a similar decrease in each participant's speech percep-tion score relative to the score they obtained in quiet. The target score for the procedure was 70 percent of the score obtained by each lis-tener in the test with speech at 70 dBA in the noncompression condition. The noise was a steady random noise with an average spec-trum shaped like the average spectrum of speech . In the test procedure, the level of this noise was decreased or increased at the com-pletion of each sentence according to whether the listener's score was greater or less than the target score. The level was changed in steps of 5 dB until two turning points were obtained . The step size was then reduced to 3 dB, and a further six turning points were obtained . The final SNR for each test was calculated by com-paring the noise levels averaged across these six turning points with the mean speech level, which was held constant at 65 dBA. The order in which the two compression conditions were applied in this procedure was alternated in the same way as for the tests of speech per-ception in quiet.

20

Finally, to determine more generally whether users of Cls would choose to have input compression enabled for normal use of their sound processors, six participants were asked to wear the microphone with compression in place of the standard microphone for a period of approximately 2 weeks. These participants were three Spectra-22 users (Pl-P3) and three SPrint users (P6-P8). At the conclusion of the trial period, they were asked to report specifi-cally on their experiences with the compres-sion enabled in the variety of listening situations they encountered.

RESULTS

A Ithough the 10 users of Cls who partici-pated in this study used either the Spectra- 22 (Pl-P5) or the SPrint (P6-P10) processor, as explained previously, the input signal process-ing is very similar in these two devices . For example, the sound levels at the microphone that produce stimulation on one electrode at the T-level (see Fig . 2) for typical settings of the sensitivity control (and other parameters) are very close for both types of processor at all fre-quencies . Furthermore, the pattern of results obtained from each of the four speech percep-tion tests was very similar for the two groups of listeners (P1-P5 and P6-P10) . Preliminary statistical analyses confirmed that there were no significant differences between the scores that were attributable to the type of sound processor used by each participant . Therefore, the scores of all listeners were considered together in the statistical analyses, which are reported below.

As described above, in the experimental procedure with each participant, scores were obtained within each session for the two pro-cessing conditions (with and without compres-

sion) using an alternating sequence of tests that was intended to minimize the possibly con-founding effects of practice on the speech recog-nition task . This procedure produced 30 scores (10 participants x 3 sessions) for each of the two processing conditions that were applied in each of the four perceptual tests (speech presented at three levels and speech with competing noise) .

For each test, the 30 pairs of scores were ana-lyzed using a paired t-test.

The results of the tests at an average level of 45 dBA are shown in Figure 4 . Many listen-ers performed poorly on the test without com-pression, with six scoring less than 10 percent key words correct . All participants except P1

Benefits of Syllabic Input Compression/McDermott et al

Figure 4 Results of each participant for recognition of key words in Speech Intelligibility Test sentences pre-sented at an average level of 45 dBA. Filled columns: without compression. Open columns: with compression. Error bars show 1 SD . Results averaged across the 10 par-

ticipants are shown on the right .

obtained a higher score with compression. The mean score without compression (11.4%) increased with compression by nearly 20 per-centage points (to 31 .0%) . This increase was highly significant (t =-5.88, df = 29, p < .0001) .

A similar pattern of results was obtained from the tests with speech at 55 dBA (Fig. 5) . The compression produced an increase in score

for all participants but one (P7) . The mean score without compression (51 .5%) increased with compression by 16.6 percentage points (to 68.1%) . This increase was highly significant (t = -6.05, df = 29, p < .0001) .

The effect of compression with an average speech level of 70 dBA was much smaller than at the two lower levels (Fig . 6) . Without com-pression, the mean score was 76.8 percent, and with compression, the mean score was 79.9 per-cent . Although the score increase was small (3.1 percentage points), it was statistically significant

(t = -2 .27, df = 29, p = .031) . The results of the test of speech perception

with a competing noise are shown in Figure 7. For most listeners, the effect of compression was small. On average, the SNR required to reduce speech recognition to 70 percent of the score obtained in quiet was 11.5 dB without compres-sion . With compression, the required SNR increased by 1 .6 dB (to 13.1 dB), but this effect was not quite statistically significant (t = 2 .02, df = 29, p = .052). Note that the direction of this change is consistent with the expectation that compression might worsen speech intelligibility in noisy listening conditions . Possible explanations for this pattern of results are discussed below.

21

Journal of the American Academy of Audiology/Volume 13, Number 1, January 2002

100

90

80

U 70 - 2 00 60 - U

a 50 ~ 0 3 40 - T e Y 30 -

20 -

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TI

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T z z

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P, P2 P3 P4 P5 P6 P7 P8 P9 P10 Mean

Participant

Figure 5 Results of each participant for recognition of key words in Speech Intelligibility Test sentences pre-sented at an average level of 55 dBA. Filled columns: without compression. Open columns: with compression. Error bars show 1 SD. Results averaged across the 10 par-ticipants are shown on the right.

DISCUSSION

T he results of this trial suggest strongly that syllabic input compression can provide prac- tical benefits to users of CIs . The improvement in perception was largest for speech presented at an average level of 45 dBA, which is about 10 to 15 dB lower than typical levels of conversa-tional speech (Skinner et al, 1997). This improve-ment is almost certainly a consequence of the increased sensitivity of the microphone with the compression enabled. As shown in Figure 2, the input sensitivity of the CI sound processor was increased by 6 .0 dB when the input com-pressor was enabled and listeners adjusted the

TIC TI i~ TI

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P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 Mean

Participant

Figure 6 Results of each participant for recognition of key words in Speech Intelligibility Test sentences pre-sented at an average level of 70 dBA. Filled columns: with-out compression. Open columns: with compression. Error bars show 1 SD . Results averaged across the 10 partici-pants are shown on the right.

m a 25

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P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 Mean

Participant

Figure 7 Results of each participant for the Speech Intelligibility Test sentence test presented at an average level of 65 dBA in noise. The results show the signal-to-noise ratio required to reduce each listener's score to 70% of the score obtained in quiet. Therefore, taller columns indicate poorer performance. Filled columns: without compression. Open columns: with compression. Error bars show 1 SD . Results averaged across the 10 par-ticipants are shown on the right.

gain applied to the microphone signal to equal-ize the loudness of speech (presented at an aver-age level of 65 dBA) to the loudness of the same speech material when processed without com-pression . That listeners selected a higher gain with compression enabled is not surprising : enabling the compression without increasing the gain would have reduced the electric stim-ulation levels corresponding to the higher-level components of the acoustic speech signal, and, therefore, a lower average loudness would have been perceived.

The improved intelligibility of the speech presented at 45 dBA presumably resulted from the increase in the range of levels in the information-carrying parts of the spectrum that were audible with the compression enabled. This can be deduced from Figure 2 by imagining the hatched area shifted down by 15 dB and then comparing the proportion of the speech spectrum that lies in the audible range with compression enabled to the proportion that is audible with compression disabled . The same reasoning provides an explanation for the improvements in speech recognition that were obtained with compression enabled when the average speech level was 55 dBA. Those improvements were only slightly less than the improvements obtained when the speech level was 45 dBA.

It seems possible that similar improvements in the intelligibility of low-level speech might

22

Benefits of Syllabic Input Compression/McDermott et al

have been obtained, even without input com-pression, simply by increasing the sensitivity con-trols on the sound processors to settings higher than those normally selected by these implant users . This would have improved audibility of the lower-level parts of the speech spectrum but would also have led to higher loudness levels, rel-ative to the loudness perceived with normal sen-sitivity settings . In theory, the loudness could then have been reduced to more acceptable lev-els by decreasing the range of stimulation lev-els programmed into the sound processors . A decrease in the C-levels on all electrodes, or a decrease in both the T-levels and the C-levels, would result in lower loudness being perceived for a particular input signal . However, there is evidence from the present study that such an adjustment of sound processors would be unlikely to provide the benefits reported above using input compression applied to unmodified sound processors . As shown in Figure 6, small but statistically significant increases in speech recognition scores were obtained with com-pression enabled when the average speech level was 70 dBA. The data in Figure 2 suggest that most of the speech spectrum should have been audible to listeners in this condition even with compression disabled . It is probable that the level of the speech would have varied naturally within and across sentences during the experi-ment, even though the average signal level was held constant . Therefore, it seems possible (though unlikely) that some improvement in intelligibility did result from improved audibil-ity of low-level signals, as for the lower speech levels . Nevertheless, a different effect may also have contributed to the significant score improve-ment at the 70-dBA level with compression enabled . The postulated effect is a reduction in limiting of electric stimulation levels for high-level components of the acoustic speech signal . As shown in Figure 2, peaks in the spectrum of speech received at an average level of 70 dBA (without compression) exceed the acoustic level that is converted in the sound processor to stim-ulation at the C-level on corresponding elec-trodes . Thus, when such peaks are processed, the resulting pattern of stimulation levels across electrodes would be flatter than the short-term spectrum of the acoustic input. It is plausible that the flattening of the stimulation pattern when spectral peaks are processed could reduce intel-ligibility. With input compression enabled, these peaks are reduced in level before reaching the sound processor because they exceed the CT (see Fig . 2) . The operation of the compressor

ensures that the acoustic short-term spectral shape is unchanged in this condition . Conse-quently, the electric stimulation pattern would not be flattened as often by the limiting of some stimulation levels to the C-levels of the corre-sponding electrodes . Therefore, it is likely that the use of input compression, rather than an increase in sound processor sensitivity accom-panied by compensatory reprogramming of the stimulation levels, is necessary to achieve improvements in intelligibility of speech received at high and low levels .

Although the experimental results confirm that input compression is generally beneficial for understanding speech at various levels in quiet conditions, the data in Figure 7 suggest that compression may have an adverse perceptual effect when speech is mixed with a competing noise . On average, the effect on scores was small and statistically insignificant. However, input compression of the type studied improves the audibility of all low-level signals without dis-criminating between speech sounds and unwanted noises . In the fitting procedure, participants adjusted the post-compression gain to perceive the same loudness for speech at an average level of 65 dBA whether compression was enabled or dis-abled . As explained above, enabling the com-pression would have resulted in greater loudness being perceived for a given level of the back-ground noise used in the test that provided the data in Figure 7 . Although the operation of the compressor would have ensured that the SNR at its output would always have equaled the SNR at its input, the loudness of the noise would have increased during silences in the speech . This may have been distracting to some listeners without significantly reducing their average speech recog-nition scores on the test .

This notion is supported by the comments made at the conclusion of the trial by the six par-ticipants who used the input compression sys-tem away from the laboratory. All of these participants commented that compression increased the loudness of background noise in some situations . Many reported that they dis-liked the louder noise, although three stated that they also found most low-level environ-mental sounds easier to hear and, consequently, easier to identify. Overall, all but one of these six participants decided that the benefits of the compression system outweighed any disadvan-tages. Three of the participants suggested that they would prefer to be able to switch the com-pression on or off with a manual control to suit the ambient listening conditions .

23

Journal of the American Academy of Audiology/Volume 13, Number 1, January 2002

Taken together, the findings of this study show that perception of low-level sounds, espe-cially speech sounds, can be improved substan-tially with minimal adverse side effects, and with no need to reprogram existing sound proces-sors, by applying syllabic compression to input signals. As a result of these findings, a similar compression circuit has been introduced into a new version of a BTE sound processor manu-factured by Cochlear Limited.

Acknowledgments. We would like to thank the 10 vol-unteers who participated so willingly in the experiments. Many colleagues assisted with this work, particularly Nick Kydas, Dr. Peter Seligman, Justin Zakis, and Prof. Graeme Clark. Financial support was provided by the Garnett Passe and Rodney Williams Memorial Foundation and the Co-operative Research Centre for Cochlear Implant and Hearing Aid Innovation .

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