Multichannel Compression Hearing Aids Research Articles

Individual differences in self-adjustment of hearing aid amplification

Listening in noisy environments remains a common complaint among hearing aid users. Self-adjustment of amplification parameters could improve listener satisfaction in noise. Previous data (Nelson et al., AAS 2015) indicated considerable variability among hearing-impaired listeners when self-adjusting amplification in noisy environments, but the sources of variability are not understood. Data will be presented from 30 adult listeners with mild to moderate hearing loss who self-adjusted amplification parameters in laboratory-simulated restaurant environments. Participants listened using a real-time simulation of a multichannel compression hearing aid and adjusted gain/compression parameters via a simple user interface (EarMachine). On average, listeners selected less gain relative to NAL-NL2 targets, and they reduced gain as noise levels increased. Hearing loss did not predict variation in self-selected amplification. Small but significant gender differences were observed. Female subjects selected more high-frequency gain (2000–8000 Hz) but less low-frequency gain than male subjects. Listeners with prior hearing-aid experience selected more high-frequency gain than listeners with no hearing-aid experience. Younger age was associated with selecting more low-frequency gain. Within-group variability was large compared to mean differences between groups and only a small proportion of variance was explained. These data suggest that self-adjustment of amplification was not easily predicted by listener-specific factors.

Development and current status of the "Cambridge" loudness models.

This article reviews the evolution of a series of models of loudness developed in Cambridge, UK. The first model, applicable to stationary sounds, was based on modifications of the model developed by Zwicker, including the introduction of a filter to allow for the effects of transfer of sound through the outer and middle ear prior to the calculation of an excitation pattern, and changes in the way that the excitation pattern was calculated. Later, modifications were introduced to the assumed middle-ear transfer function and to the way that specific loudness was calculated from excitation level. These modifications led to a finite calculated loudness at absolute threshold, which made it possible to predict accurately the absolute thresholds of broadband and narrowband sounds, based on the assumption that the absolute threshold corresponds to a fixed small loudness. The model was also modified to give predictions of partial loudness—the loudness of one sound in the presence of another. This allowed predictions of masked thresholds based on the assumption that the masked threshold corresponds to a fixed small partial loudness. Versions of the model for time-varying sounds were developed, which allowed prediction of the masked threshold of any sound in a background of any other sound. More recent extensions incorporate binaural processing to account for the summation of loudness across ears. In parallel, versions of the model for predicting loudness for hearing-impaired ears have been developed and have been applied to the development of methods for fitting multichannel compression hearing aids.

Modeling loudness for impaired ears and applications to fitting hearing aids

Models of loudness for impaired ears are based on the assumption that cochlear hearing loss can be partitioned into a component due to outer hair cell dysfunction, resulting in reduced frequency selectivity and a more rapid growth of neural response with increasing level (reduced compression), and a component due to inner hair cell dysfunction, which reduces the neural response at all levels. In the first two models that were developed in Cambridge, the filtering and compression that take place on the basilar membrane were modeled as sequential processes, which is not physiologically realistic. Nevertheless, the models were able to account for many aspects of loudness, and were used to develop methods of fitting multi-channel compression hearing aids that have proven to be effective. More recently, a model of loudness has been developed in which the filtering and compression are modeled using a physiologically plausible nonlinear filter bank. This has also been applied to the fitting of hearing aids. Factors not included in the models include central plasticity resulting from altered auditory input, possible consequences of the operation of the efferent system, and the influence of cognitive factors such as perceived distance of the sound source and perceived vocal effort.

Multichannel compression hearing aids: Effect of channel bandwidth on consonant and vowel identification by hearing-impaired listeners

Aided consonant and vowel identification was measured in 13 listeners with high-frequency sloping hearing losses. To investigate the influence of compression-channel analysis bandwidth on identification performance independent of the number of channels, performance was compared for three 17-channel compression systems that differed only in terms of their channel bandwidths. One compressor had narrow channels, one had widely overlapping channels, and the third had level-dependent channels. Measurements were done in quiet, in speech-shaped noise, and in a three-talker background. The results showed no effect of channel bandwidth, neither on consonant nor on vowel identification scores. This suggests that channel bandwidth per se has little influence on speech intelligibility when individually prescribed, frequency-varying compressive gain is provided.

Evaluation of the CAMEQ2-HF Method for Fitting Hearing Aids With Multichannel Amplitude Compression

A method for fitting multichannel compression hearing aids with an extended high-frequency response, called CAMEQ2-HF, was described by . This study describes an evaluation of the method, using a 16-channel behind the ear hearing aid incorporating slow-acting compression and providing gain for frequencies up to 7500 Hz. Eleven participants with mild to moderate cochlear hearing loss were fitted bilaterally using the gains prescribed by CAMEQ2-HF. The fittings were checked using real-ear measurements with swept sinusoidal signals, and adjustments were made so that the target gains at the center frequency of each channel were achieved with a typical tolerance of +/-3 dB for an input level of 65 dB and with a typical tolerance of +/-5 dB for levels of 50 and 80 dB SPL. Participants were asked to wear the hearing aids as much as possible in their everyday lives and to fill in questionnaires and a structured diary about their experience of loudness and their listening problems in everyday life, both for listening unaided and after a period of use of the aids. Scores obtained using the Profile of Aided Loudness (PAL) indicated that the hearing aids led to a clear increase in loudness (relative to unaided listening) for weak sounds and to smaller increases in loudness for sounds of medium and high intensity. For aided listening, strong sounds were typically rated as "loud, but OK." Satisfaction ratings for loudness obtained using the PAL showed only small differences between unaided and aided listening. Responses obtained via the structured diary (for aided listening only) indicated that target speech was usually judged as "loud enough" in a variety of situations. Clarity judgments ranged from "reasonably clear" to "OK" for most situations, but fell to "not very clear" for a noisy group situation. The loudness of background sounds was mostly judged as "OK," except for the noisy group situation in which the background was judged "bit loud." Results from the Abbreviated Profile of Hearing-Aid Benefit questionnaire indicated fewer problems with "ease of communication," "reverberation," and "background noise" for aided than for unaided listening, but more problems with "aversiveness." Nine of 11 ratings of overall sound quality fell in the categories "satisfied" or "very satisfied." The CAMEQ2-HF fitting method generally led to satisfactory loudness and sound quality in everyday life for people with mild to moderate hearing loss.

The use of a loudness model to derive initial fittings of multi-channel compression hearing aids.

A model of loudness perception applicable to hearing-impaired people has been used to develop three methods for the initial fitting of hearing aids with multi-channel compression, based on the audiometric thresholds. Method one, CAMEQ, has the goals of amplifying speech so that, on average, the specific loudness is the same for all frequencies within the range 500–4000 Hz, over a wide range of overall sound levels, giving about the same overall loudness as “normal” for speech covering a wide range of sound levels. Method two, CAMREST, determines the gains needed give normal specific loudness patterns for speech over a wide range of sound levels. Method three, CAMEQ2-HF, is similar to CAMEQ but differs from it in the following ways: (1) gains are recommended for center frequencies up to 10 kHz (as compared to 6 kHz for CAMEQ), (2) CAMEQ2-HF is based on the assumption that the user may wish to hear sounds from many directions and uses a diffuse-field-to-eardrum transfer function (as opposed to a free-field-to-eardrum transfer function for CAMEQ); (3) CAMEQ2-HF is based on recent wideband measurements of the average spectrum of speech. All of the methods can be applied to any multi-channel compression hearing aid.

Development of a new method for deriving initial fittings for hearing aids with multi-channel compression: CAMEQ2-HF

described a procedure, CAMEQ, for the initial fitting of multi-channel compression hearing aids. The procedure was derived using a model of loudness perception for impaired hearing. We describe here the development of a new fitting method, CAMEQ2-HF, which differs from CAMEQ in the following ways: (1) CAMEQ2-HF gives recommended gains for centre frequencies up to 10 kHz, whereas the upper limit for CAMEQ is 6 kHz; (2) CAMEQ is based on the assumption that the hearing aid user faces the person they wish to hear and uses a free-field-to-eardrum transfer function for frontal incidence. CAMEQ2-HF is based on the assumption that the user may wish to hear sounds from many directions, and uses a diffuse-field-to-eardrum transfer function; (3) CAMEQ2-HF is based on an improved loudness model for impaired hearing; (4) CAMEQ2-HF is based on recent wideband measurements of the average spectrum of speech.Sumario describieron un procedimiento, CAMEQ, para la adaptación inicial de auxiliares auditivos con compresión de múltiples canales. El procedimiento se basó en el uso de un modelo de percepción de la intensidad subjetiva para casos de audición disminuida. Describimos aquí el desarrollo de un nuevo método de adaptación, CAMEQ2-HF, que difiere del CAMEQ en las siguientes formas: (1) CAMEQ2-HF proporciona ganancias recomendadas para las frecuencias centrales hasta 10 kHz mientras que el límite superior para el CAMEQ es 6 kHz; (2) CAMEQ se basa en la suposición de que el usuario de un auxiliar auditivo (se ubica de frente)hace frente a la persona que quiere oír y usa la función de transferencia campo libre-membrana timpánica para una incidencia frontal. CAMEQ2-HF se basa en la suposición de que el usuario quiere oír los sonidos desde muchas direcciones y usa una función de transferencia campodifuso-a-membrana timpánica; (3) CAMEQ2-HF se basa en un modelo de intensidad subjetiva mejorada para la audición disminuida; (4) CAMEQ2-HF se basa en las recientes mediciones de banda ancha del espectro promedio del habla.

Multichannel Compression Hearing Aids: Perceptual Considerations

Multichannel Compression Hearing Aids: Perceptual Considerations

Perceptual Considerations for Multichannel Compression Hearing Aids

Perceptual Considerations for Multichannel Compression Hearing Aids

Effects of Three Amplification Strategies on Speech Perception by Children With Severe and Profound Hearing Loss

Traditionally in the United Kingdom, children with severe and profound hearing loss have been fitted with linear, analog hearing aids. Fast-acting, wide-dynamic-range compression (WDRC) has been shown to give better discrimination of speech than linear amplification for moderately hearing-impaired young adults. For severe and profound hearing losses, higher compression ratios are needed. The resultant distortion of the temporal envelope and reduced modulation depth may offset improvements in audibility offered by WDRC. In this study, speech recognition and discrimination were assessed for severely and profoundly hearing-impaired children, using three different amplification strategies, including WDRC. Fifteen children (ages 7 to 15 yr) with severe and profound hearing loss were fitted bilaterally with high-power, multichannel compression hearing aids, incorporating one of three different amplification strategies: linear with peak clipping, linear with compression limiting, or WDRC. Output responses were matched to Desired Sensation Level (DSL i/o) targets. The children wore hearing aids programmed with each of the amplification strategies in turn, for at least 1 wk, in a counterbalanced order across children. After using a particular amplification strategy for at least 1 wk, speech perception tests were carried out. Speech scores on closed-set testing for the profound group showed significant benefit for WDRC over the other two algorithms. None of the other results showed a statistically significant effect of algorithm on speech performance. WDRC amplification sometimes led to better performance than linear amplification with peak clipping or output limiting, and it never led to poorer performance. Therefore, it appears to be safe to use well-designed WDRC for hearing-impaired children with severe or profound hearing loss.

Comparison of two adaptive procedures for fitting a multi-channel compression hearing aid

We compared two adaptive procedures for fitting a multi-channel compression hearing aid. “Camadapt” uses judgements of the loudness of speech stimuli and the tonal quality of music stimuli. “Eartuner” uses judgements of the loudness and clarity of speech stimuli with differing spectral characteristics. Sixteen new users of hearing aids were fitted unilaterally, using each procedure. The fittings were assigned to Programs 1 and 2 in the aid, in a counter-balanced order. Subjects kept a diary of their experiences with each program in everyday life. Following 2-4 weeks of experience, they filled in the APHAB and other questionnaires and were re-fitted using both procedures. Camadapt generally led to higher low-level gains and lower high-level gains than Eartuner. Gains recommended by the procedures did not change following experience. Eight subjects preferred the Camadapt fitting and eight preferred the Eartuner fitting. Most subjects gave high overall satisfaction ratings for both procedures. Test-retest reliability was better for Eartuner than for Camadapt. Preference for the Camadapt fitting was associated with slightly better speech communication with Camadapt, while preference for the Eartuner fitting was associated with fewer problems with aversion for that procedure.

Comparison of three procedures for initial fitting of compression hearing aids. II. Experienced users, fitted unilaterally

This paper is the second in a series comparing three procedures for the initial fitting of multichannel compression hearing aids. The first paper reported the results for a group of 10 experienced hearing aid users fitted bilaterally. This paper reports the results for a different group of 10 experienced hearing aid users fitted unilaterally. The three procedures were: (1) CAMEQ, which aims to amplify speech so as to give equal loudness per critical band over the frequency range 500-5000 Hz, and to give similar overall loudness to normal over a wide range of speech levels; (2) CAMREST, which aims to amplify speech so as to restore normal specific loudness patterns, over a wide range of speech levels; and (3) DSL [i/o], which aims to map the dynamic range of normalhearing people into the reduced dynamic range of hearingimpaired people, with full restoration of audibility. Each subject was fitted with one Danalogic 163D digital hearing aid, using each of the three fitting procedures in turn; the order was counter-balanced across subjects. Prescribed insertion gains for 55 and 80 dB SPL input levels were verified using real-ear measurements. Immediately after fitting with a given procedure, and 1 week after fitting, the gains were adjusted, when required, by the minimum amount necessary to achieve acceptable fittings. On average, the adjustments were smallest for the CAMREST procedure, slightly larger for the CAMEQ procedure, and largest of all for DSL [i/o]. For the DSL [i/o] the gain changes were mostly negative, especially for high frequencies and the higher input level. After these gain adjustments, users wore the aids for at least 3 weeks before speech reception thresholds (SRTs) for sentences in quiet and in steady and fluctuating background noise were measured. The APHAB questionnaire was also administered. The hearing aids were then refitted with the next procedure. SRTs and APHAB scores did not differ significantly between the three procedures. We conclude that the CAMEQ and CAMREST procedures provide a more appropriate initial fitting than DSL [i/o] for unilaterally experienced hearing aid wearers. Comparison with our earlier study based on bilateral fittings suggests that the preferred gains are similar for unilateral and bilateral fittings.

Evaluation of the noise reduction system in a commercial digital hearing aid: Evaluación del sistema de reducción de ruido en un auxiliar auditivo digital comercial

We evaluated the effectiveness of a noise reduction system implemented in a commercial digital multichannel compression hearing aid. Eight experienced hearing aid wearers with moderate sensorineural hearing loss were fitted bilaterally according to the manufacturer's fitting guidelines. After a 3-month period of regular use of two programs, one with and one without the noise reduction system, speech recognition thresholds (SRTs) were measured in four types of background noise, including steady noise, and noises with spectral and/or temporal dips. SRTs were very similar with and without the noise reduction system; in both cases, SRTs were markedly lower than for unaided listening. SRTs were lower for the noises with dips than for the steady noise, especially for the aided conditions, indicating that amplification can help to 'listen in the dips'. Ratings of sound quality and listening comfort in the aided conditions were uniformly high and very similar with and without the noise reduction system.Sumario: Evaluamos la efectividad del sistema de reducción de ruido en un auxiliar auditivo digital comercial con compresión multicanal. Se adaptaron auxi-liares auditivos bilateralmente a ocho usuarios experimentados con hipoacusias sensorineurales moderadas de acuerdo con las guias de adaptación del fabricante. Después de un periodo de 3 meses de uso regular de dos programas, uno con y uno sin el sistema de reducción de ruido, se midieron los umbrales de reconocimiento del lenguaje (SRT) con cuatro tipos de ruido de fondo, incluyendo ruido estacionario y ruidos con componentes espectrales y/o temporales. Los SRT fueron muy similares con y sin el sistema de reducción de ruido. En ambos casos, los SRT fueron marca-damente más bajos que en condiciones de escucha sin el auxiliar. Los SRT fueron más bajos con los ruidos que tenian componentes, en comparación con el ruido estacionario, especialmente en condiciones de uso del auxiliar, lo que indicó que la amplificación puede ayudar a escuchar en presencia de esos componentes. Los resultados en cuanto a calidad de sonido y de audición agradablc en condiciones de uso del auxiliar auditivo, fueron uniformemente elevados y muy similares con y sin el sistema de reducción de ruido.

Effects of Multi-Channel Compression Time Constants on Subjectively Perceived Sound Quality and Speech Intelligibility

The purpose of this study was to determine the influence of the compression time constants in a multi-channel compression hearing aid on both subjectively assessed speech intelligibility and sound quality in realistic binaural acoustical situations for normal-hearing and hearing-impaired listeners. A nonlinear hearing aid with 15 independent compression channels of approximated critical bandwidth was simulated on a personal computer. Various everyday life situations containing different sounds such as speech and speech in noise were recorded binaurally through original hearing aid microphones placed in BTE hearing aid cases. Two experiments were run with normal hearing and hearing-impaired subjects. For each subject, hearing thresholds were established using in situ audiometry. The static I/O-curve parameters in all channels of the hearing aid were then adjusted so that normal speech received an insertion gain corresponding to the NAL-R formula (Byrne & Dillon, 1986). The compression ratio was kept constant at 2.1:1. In the first experiment with six normal-hearing and six hearing-impaired subjects, the hearing aid was programmed to four different settings by changing only the compression time constants while all the parameters describing the static nonlinear Input/Output-curve were kept constant. The compression threshold was set to a very low value. In the second experiment with seven normal-hearing and eight hearing-impaired subjects, the hearing aid was programmed to four settings by changing the release time constants and the compression threshold while all other remaining parameters were kept constant. Using a complete A/B pair comparison procedure, subjects were presented binaurally with the amplified sounds and asked to subjectively assess their preference for each hearing aid setting with regards to speech intelligibility and sound quality. In Experiment 1, all subjects showed a significant preference for the longest release time (4 sec) over the two shorter release times (400 msec and 40 msec), pertaining to quality and intelligibility. In combination with the long release time, the attack time (1 msec or 100 msec) was of less importance. Larger inter-individual differences were observed among all subjects with regard to the perceived sound quality of musical and nonspeech signals. In Experiment 2 the hearing-impaired subjects showed a significant preference for the hearing aid setting with a long release time (4 sec) and a low compression threshold (20 dB SPL), both with regard to sound quality and speech intelligibility. With a short release time (40 msec), a lower compression threshold (20 dB SPL) was preferred over a higher threshold (50 dB SPL). Among normal-hearing subjects, the same settings were also preferred for speech intelligibility. However, no significant differences were observed in these subjects regarding the assessment of sound quality. Again, more individual variability was found during the assessment of music and nonspeech signals. In sum, the preference of a rather long release time raises the question of whether a compression system with a very short time constant is the optimal means to compensate for the sensory recruitment phenomenon.

Use of a loudness model for hearing aid fitting. IV. Fitting hearing aids with multi-channel compression so as to restore 'normal' loudness for speech at different levels.

Many researchers have proposed that multi-channel compression hearing aids should process sounds so as to restore loudness perception to 'normal'. However, procedures for achieving this have generally been based on measurements or calculations using narrowband stimuli, and these procedures may not be accurate for broadband sounds such as speech. Here, a model for predicting loudness for people with cochlear hearing loss is used to calculate the frequency- and level-dependent gains that would be required to restore loudness perception to 'normal' for speech-like signals. The calculations are based entirely on the pure tone audiogram, and do not require measures of loudness growth. The model was applied to several different hypothetical hearing losses, varying in slope and severity. In each case, the model was used to calculate the insertion gains (IGs) that would be required as a function of frequency so that speech-shaped noise with a level of 65 dB SPL would evoke a specific loudness pattern matching that for a normal ear. A similar procedure was applied using speech-shaped noise with a level of 85 dB SPL (with the spectral characteristics of shouted speech). The results were used to derive functions relating the required IG to hearing loss for each audiometric frequency and each speech-shaped noise level. These functions were used in turn to derive compression ratios and gains for each channel of a multi-channel compression system. The derivations apply to systems with any number of channels. The outcome is a method than can be used for the initial fitting of multichannel compression hearing aids, so as to restore loudness perception to near 'normal' for broadband speech-like signals.

Variables Affecting the Use of Prescriptive Formulae to Fit Modern Nonlinear Hearing Aids

It is routine for audiologists to fit nonlinear hearing aids by using one of several current prescriptive formulae. While such an approach might result in acceptable fittings for single-channel analog compression hearing aids, its direct application to fitting current multichannel compression hearing aids may result in less than an optimal fit. In this paper, the effects of filter bandwidth, number of channels, detector type, compression threshold, attack and release times, and distortion on the final output of a nonlinear compression hearing aid are discussed. The subjective results of such effects and its implications on our clinical practice will be explored as well.

Use of a loudness model for hearing aid fitting: II. Hearing aids with multi-channel compression.

A model for predicting loudness for people with cochlear hearing loss was applied to the problem of the initial fitting of a multi-channel compression hearing aid. The fitting was based on two constraints: (1) The specific loudness pattern evoked by speech of a moderate level (65 dB SPL) should be reasonably flat (equal loudness per critical band), and the overall loudness should be similar to that evoked in a normal listener by 65-dB speech (about 23 sones for binaural listening); (2) Speech with an overall level of 45 dB SPL should just be audible in all frequency bands from 500 Hz up to about 4 kHz, provided that this does not require compression ratios exceeding about 3. These two constraints were used to determine initial values for the gain, compression ratio and compression threshold in each channel of a multi-channel compression system. This initial fitting was based entirely on audiometric thresholds; it does not require suprathreshold loudness measures. The fitting method was evaluated using an experimental fast-acting four-channel compression system. The initial fitting was followed by an adaptive procedure to 'fine tune' the fitting, and the aids were then used in everyday life. Performance was evaluated by use of questionnaires and by measures of speech intelligibility. Although the fine tuning resulted in modest changes in the fitting parameters for some subjects, on average the frequency response shapes and compression ratios were similar before and after the fine tuning. The fittings led to satisfactory loudness impressions in everyday life and to high speech intelligibility over a wide range of levels. It was concluded that the initial fitting method gives reasonable starting values for the fine tuning.

Enhanced speech perception at low signal-to-noise ratios with multichannel compression hearing aids

The effect of the signal-to-noise ratio (S/N) on speech discrimination was measured for two types of hearing-aid amplification, (1) full-range multichannel compression with eight independent frequency bands and (2) frequency-equalized linear amplification. Signal-to-noise ratios from -5 to 15 dB with speech-spectrum noise (at 70 dB SPL) and two voices (male and female) were used. The effect of S/N differed for the two aid types: As the S/N decreased, speech discrimination became relatively better with the multichannel compression hearing aid (MCCHA) in comparison to the linear amplification hearing aid (LAHA). Furthermore, this shift in MCCHA-LAHA performance occurred for every subject, independent of which aid produced better overall performance. Of 16 hearing-impaired subjects, 7 showed significantly better overall speech discrimination with the MCCHA than with the LAHA, 5 showed no difference, and 4 showed significantly better discrimination with the LAHA. Hearing-loss severity and MCCHA performance also were related: Subjects with less severe impairments showed greater improvement with the MCCHA. In a normal-hearing listener, the speech discrimination deficit produced by these MCCHAs was small and not statistically significant in most cases. Taken together, these results indicate that a full-range eight-channel MCCHA, for a mild to moderately severe hearing loss, causes little information degradation and can be of great benefit for speech discrimination in noise, particularly at low S/N.

Multichannel compression hearing aids: effect of number of channels on speech discrimination in noise.

Full-range multichannel compression hearing aids (MCCHAs) with 4, 6, 8, 12, and 16 independent frequency channels were used to determine the effect of the number of channels on the speech discrimination of mild to moderately severe hearing-impaired subjects. Signal-to-noise ratios (S/Ns) from -5 to 15 dB with speech-spectrum noise (70 dB SPL) and two voices (male and female) were used. Average speech discrimination for 16 hearing-impaired subjects increased from 4 to 8 channels but did not change significantly between 8 and 16 channels. The effect of the number of channels did not vary significantly with S/N. Analyses of speech discrimination performance within phonemic categories as well as consonant-confusion analyses revealed response shifts with the number of channels that were consistent with increasing transmission of useful high-frequency speech information as the number of channels increased. These results indicate that a MCCHA with at least 8 (and up to 16) channels provides the mild to moderately severe hearing-impaired subjects with acoustic information that facilitates speech discrimination in speech-band noise.

Fitting multichannel-compression hearing aids.

Multichannel hearing aids have been introduced recently in clinical practice. In this paper, some advantages and disadvantages of multichannel hearing aids are discussed. The advantages include an improved use of compression. However, the number of settings will be larger in multichannel hearing aids and exact knowledge of the goal to be pursued by compression is more necessary than ever. Inexactness of the definitions of these goals may have been a reason for the contradictory results in the studies on multichannel compression. A number of requirements must be fulfilled for a successful application of multichannel compression. One of these is detailed knowledge about the perception of suprathreshold signals. For that reason, an extension of the battery of tests commonly used in rehabilitative audiology is necessary. Another requirement is that the fitting strategy is determined by the purpose of applying compression. The objectives will be different for compression limiting and syllabic compression. A fitting procedure used in clinical practice has to take into account some limitations in the fitting time available. A number of possible fitting tools are discussed. Two strongly different fitting strategies for the same commercially available three-channel compression aid illustrate fundamental differences in fitting philosophy.