The electroacoustic characteristics of a hearing aid differ considerably from those of a high-fidelity communication system. The hearing aid has a relatively narrow bandwidth, and the frequency response of the hearing aid is almost never flat. Because hearing loss usually increases with increasing frequency, the frequency response of the hearing aid typically provides increased gain as a function of frequency in order to obtain audibility for the hearing aid user. While hearing-impaired listeners may require considerable gain in order to obtain audibility, the output of the hearing aid must be limited in order to prevent the amplified sound from becoming uncomfortably loud when input levels are high.

A number of studies have been carried out to determine the effect of manipulating the electroacoustic characteristics of a hearing aid on sound quality. Sound quality is a multidimensional construct. Gabrielsson and Sjogren (1979) identified eight important “dimensions” related to the manipulation of the frequency response of loudspeakers, headphones, and hearing aids. These dimensions are clarity, fullness, brightness, shrillness, loudness, spaciousness, nearness, and disturbing sounds (distortion). The “overall impression of quality” consists of some weighting of these various dimensions.

The effect of manipulating the frequency response of the hearing aid has been assessed in many studies. The bandwidth, amount of low-frequency versus high-frequency gain, and the smoothness of the frequency response will all affect the sound quality of speech. Several studies have shown that the presence of low-frequency content is a strong determinant of good sound quality for hearing-impaired listeners listening at comfortable listening levels (Punch, 1978; Punch et al., 1980; Punch and Parker, 1981; Tecca and Goldstein, 1984; Punch and Beck, 1986). However, the bandwidth yielding better sound quality changes as a function of the input level to the hearing aid and the amount of amplification provided by the hearing aid. When hearing-impaired subjects listen at higher levels, a frequency response with less low-frequency amplification yields better sound quality (Tecca and Goldstein, 1984).

There is also good agreement among studies that too much high-frequency emphasis degrades sound quality. This type of amplification is characterized by descriptions of sound as shrill, harsh, and tinny (e.g., Gabrielsson and Sjogren, 1979; Gabrielsson, Schenkman, and Hagerman, 1988). Thompson and Lassman (1970), Neuman and Schwander (1987), and Leijon et al. (1991) found that the sound quality of a flat frequency response was preferred to a response with extreme high-frequency emphasis. The results of these studies point to the need for balance between the low-and high-frequency energy for good sound quality. Of course, the optimum balance for any person depends on the way that person's hearing loss varies with frequency. It has also been realized that the frequency response requirements for good sound quality for music differ from those for speech. An extended low-frequency response is more important for music than for speech (e.g., Franks, 1982).

A smoother frequency response has better sound quality than a frequency response with peaks. Davis and Davidson (1996) found that hearing-impaired listeners preferred to listen to speech processed through a hearing aid with a moderate amount of damping that smoothed the large resonant peak in the frequency response of the hearing aid. This preference was true for both male and female voices in quiet and in noise. Smoothing of the frequency response resulted in judgments of greater brightness, clarity, distinctness, fullness, nearness, and openness. Similarly, van Buuren, Festen, and Houtgast (1996) investigated the effect of adding single and multiple peaks to a smooth frequency response. Hearing-impaired subjects rated pleasantness on a five-point rating scale. The smooth frequency response was rated as having better sound quality than any of the frequency responses with peaks. Based on the results of the study, the researchers recommended that peak-to-valley ratios in the frequency response of the hearing aid should not exceed 5 dB.

In spite of the general agreement among studies about the effect of frequency response on sound quality, Preminger and Van Tasell (1995) have shown intersubject differences with regard to their ratings of the various dimensions of sound quality as a function of the manipulation of frequency response. They suggested that measures of speech quality be used to select among alternative frequency responses yielding similar speech intelligibility (close to 100%).

Much of the research described above was carried out with linear hearing aids, with testing carried out at a single input level. However, many current hearing aids are nonlinear, which means that the frequency response characteristics change as a function of the input level. Full evaluation of sound quality would require testing with various signals at multiple input levels and determining optimum sound quality at each level.

The method of output limiting is another hearing aid parameter that has an important effect on sound quality. Output limiting is used in a hearing aid to prevent amplified sounds from becoming uncomfortably loud and to protect the ear from excessively loud sounds that might cause further damage to the hearing. Major methods of output limiting include peak clipping, compression limiting, or wide dynamic range compression.

Peak clipping causes the generation of signals in the output signal that are not in the input signal. Harmonic distortion (integer multiples of the input signal) and intermodulation distortion (combinations of the harmonics caused by sums and differences of the harmonic distortion) are caused by peak clipping. The coherence between the input signal and the output signal is predictive of sound quality. Moderate amounts of peak clipping degrade the sound quality of speech in quiet and in noise, and the sound quality of music (Fortune and Preves, 1992; Palmer et al., 1995; Kozma-Spytek, Kates, and Revoile, 1996). There is also an interaction between the frequency response shaping of the hearing aid and the clipping level. This interaction is subject-dependent (Kozma-Spytek, Kates, and Revoile, 1996). Fortune and Preves (1992) found specifically that hearing aids having less distortion (higher coherence) were perceived as having better clarity and brightness, as producing less discomfort, and as yielding better overall sound quality.

Compression is a nonlinear form of amplification in which the gain of the amplifier is decreased as the input to the amplifier increases. Compression amplification may be used to limit output (compression limiting) or to fit a wide range of signals into the listener's dynamic range (wide dynamic range compression). In general, compression limiting preserves sound quality better than peak clipping. Compression limiting does not generate harmonic and intermodulation distortion and yields higher coherence values. Hawkins and Naidoo (1993) compared the effect of asymmetrical peak clipping and compression limiting on sound quality and clarity of speech in quiet, speech in noise, and music. For both sound quality and clarity, and for all three types of stimuli, compression limiting was preferred to peak clipping under conditions in which the hearing aid input was high enough to cause limiting.

For hearing aids utilizing wide dynamic range compression, compression ratio, attack, and release time all affect the sound quality of the processed signal. The effect of compression variables also depends on whether the signal of interest occurs in quiet or in noise. Several studies have shown that high compression ratios have a negative effect on sound quality (Neuman et al., 1994, 1998; Boike and Souza, 2000). Neuman and colleagues (1994, 1998) found that compression ratios higher than 3:1 significantly degraded the sound quality of a single-band-compression hearing aid. Compression ratios that did not significantly degrade sound quality in quiet, degraded sound quality in noise. The sound quality of linear amplification (no compression) was preferred when background noise levels were high. Boike and Souza (2000) also found that speech quality ratings decreased with increasing compression ratio for speech in noise. Compression ratio did not significantly degrade quality for speech in quiet. Research to determine the effect of compression on specific dimensions of sound quality revealed that clarity, pleasantness, background noise, loudness, and overall impression all showed negative effects of increasing compression ratio (Neuman et al., 1998).

Release time also affects sound quality. If short release times are used, low-level noise is amplified in the pauses between words. This amplification of low-level noise has been found to have a negative effect on the perceived sound quality. Neuman and colleagues (1998) found that hearing-impaired listeners' ratings of the clarity, pleasantness, and overall quality of speech in quiet and speech in noise all decreased as release time was decreased from 1000 ms to 60 ms (single-band-compression hearing aid). Ratings of the amount of background noise increased as release time decreased.

It is clear that the electroacoustic characteristics of a hearing aid have a significant effect on sound quality. Sound quality has been recognized as an important factor in the acceptability of a hearing aid to the user, and because of individual differences among listeners, it has been suggested that sound quality should be considered a factor in hearing aid fitting (e.g., Gabrielsson and Sjogren, 1979; Kuk and Tyler, 1990; Preminger and Van Tasell, 1995; Lunner et al., 1997). Past research has shown that characteristics of the listeners, characteristics of the signal being amplified, and characteristics of the amplification system all affect sound quality. Application of sound quality judgments to the fitting of nonlinear and multimemory hearing aids should be helpful in determining the appropriate settings for these devices (e.g., Keidser, Dillon, and Byrne, 1995).

See also hearing aid fitting: evaluation of outcomes; hearing aids: prescriptive fitting.