This article focuses on the effects of increasing the number of channels and bands in a hearing aid. For a comprehensive review of topics related to compression, the reader is referred to Souza (2002) or Kates (2008).
Most modern hearing aid manufacturers offer products at multiple technology levels. To a patient, a difference in technology level often means a difference in cost. To an audiologist, a difference in technology level may mean a difference in patient outcome. A feature commonly used by hearing aid manufacturers and audiologists to differentiate one technology level from another is the number of channels and bands available within a device. But, the audiologist and the patient both may question the value of a greater number of channels and bands. This article reviews available literature on the effects of number of channels and bands. Considerations related to hearing aid development, audibility, speech understanding, and digital signal processing are discussed.
Increasing Channels and Bands: Development Considerations
For the purpose of this article, a hearing aid channel will be defined as a range of frequencies that is created by a digital filter or series of digital filters within the hearing aid. In addition to expansion and compression, most signal processing features such as digital noise reduction, feedback suppression, and multichannel directionality operate on a channel-by-channel basis. Bands will be defined simply as the number of adjustment "handles" provided in the programming software for gain manipulation.
Increasing channels in a hearing aid has design implications that must be considered. For example, as the number of channels increases, the processing complexity increases, which increases the amount of time required for a hearing aid to process sounds. This processing delay can be quantified as the group delay. For the purpose of this article, the group delay is referenced as the time required for a sound to be processed by a hearing aid, effectively delaying amplified sound path relative to the direct sound path. The signal processing implemented in modern hearing aids typically requires a few milliseconds (msec). For example, in open-canal hearing aid fittings, unamplified sound will pass around the hearing aid, entering a patient's ear canal without delay (the direct path); whereas sound processed by the hearing aid will be slightly delayed in comparison. A significant delay between the direct sound and the amplified sound may be perceived as degraded sound quality or the perception of an echo. Stone and Moore (2008) suggest that delays as short as 3 to 6 msec may be noticeable to the average listener in an open-canal hearing aid fitting; increasing this delay to 15-18 msec results in decreased sound quality that the patient may find to be unacceptable.
The type of digital filter used in hearing aids affects these delays, resulting in frequency-specific group delays that are greatest in the low frequencies and decrease with increasing frequency. Figure 1 shows the group delay of current hearing aids available from six different manufacturers. Group delay as a function of frequency is apparent for most manufacturers. Those hearing aids with greatest numbers of channels show the longest processing delays. However, all of the hearing aids shown in Figure 1 fall within the acceptable limits suggested by Stone and Moore (2008).
Figure 1. Manufacturer-specific group delay (msec) as a function of frequency (Hz).
The Effect of Channels and Bands on Audibility
The number of bands affects an audiologist's ability to shape a hearing aid's frequency response and, in turn, may affect restoration of audibility. The number of bands may also affect the gain adjustments that can be made for the purpose of feedback management without deviation from prescribed targets.
Aahz and Moore (2007) investigated the impact of number of bands on fitting accuracy and ability to control feedback. They defined an accurate fitting as one in which frequency response was within 10 dB of NAL-NL1 targets. At default settings, 64% of 42 hearing aid fittings fell outside of this defined range. After manual adjustments, the number of prescriptively appropriate fittings increased to 83%. For flat or slightly sloping hearing loss, 4 bands provided adequate flexibility for frequency shaping; however, for steeply sloping losses, a greater number of adjustment bands was required. Specifically, increasing the number of adjustment bands from four to seven improved match to target while preventing the occurrence of feedback.
From Bands to Channels
Thus far, this article has discussed the bands available in a manufacturer's programming software. Beyond the use of bands to manually shape the frequency response of the hearing aid, the number of compression channels plays an important role in the restoration of audibility. Figure 2a shows the time-averaged frequency response of a classical music piece. Overlaid on this frequency response are a series of boxes that represent hearing aid channels. In this example, the hearing aid is a three-channel device. In a hearing aid using wide dynamic-range compression, more gain is applied to soft sounds than louder sounds. In this example, more gain would be applied to channel three than channels two or one. Within channel three, the level of the music sample falls by 30 dB between 2200 Hz and 5000 Hz (as illustrated by the arrows in Figure 2a). However, because these frequencies are contained within the same channel, the same amount of gain will be applied at both frequencies.
Figure 2b displays the same frequency response of a classical music piece, but with boxes representing eight hearing aid channels. In this eight channel hearing aid, the range between 2200 Hz and 5000 Hz contains three separate channels. With a finer channel resolution, the hearing aid will estimate input level in each of the eight channels. Because input in the frequency range of channel eight is lower than the input level for channel six, more gain will be applied to the frequency range encompassed by channel eight, possibly improving audibility for the range of frequencies within that channel while retaining comfort for the signal contribution from channels 6 and 7. In this regard, increasing the number of hearing aid channels may improve the ability of the hearing aid to selectively amplify frequencies and may improve the restoration of audibility. However, there may be an optimal point at which a given number of channels will offer sufficient frequency resolution, and additional channels would not provide further benefit and may possibly degrade performance.
Figure 2a. Level (dB) is plotted as a function of frequency (Hz) for a classical music sample. The illustrated boxes represent the frequency distribution of a three-channel hearing aid. Red numbers call attention to channel numbers, one, two, and three, and also to frequencies of interest.
Figure 2b. Level (dB) is plotted as a function of frequency (Hz) for a classical music sample. The illustrated boxes represent the frequency distribution of an eight-channel hearing aid. Red numbers call attention to channel numbers and also to frequencies of interest.
Woods and colleagues (2006) completed a retrospective analysis that evaluated the number of hearing aid channels required to maximize predicted speech intelligibility for 957 audiograms. Their findings suggest that the ideal number of bands will change with audiometric configuration, but a hearing aid featuring nine channels should accommodate the majority of audiograms. Kates (2010) used computer modeling to predict the effect of various compression parameters on speech quality and intelligibility. He observed that increasing the number of channels from 3 to 18 significantly improved speech audibility for a steeply sloping hearing loss. In the case of flat or mildly sloping hearing loss, a benefit of increasing the number of channels from 3 to 18 was not observed.
Increasing the number of hearing aid channels will increase the ability to restore frequency-specific audibility, particularly for steeply sloping audiograms. Using predictive metrics, researchers have completed systematic investigations of this relationship. It appears that frequency-specific audibility can be restored for flat or mildly sloping hearing loss with a three channel hearing aid; for patients with a steeply sloping hearing loss, the ideal number of channels required for restoration of audibility may fall between 9 and 18 channels.
The Effect of Channel Number on Speech Recognition
Compression is regarded as a necessary technology in hearing aids, as it facilitates restoration of audibility and comfortable listening levels for patients with a reduced dynamic range. While this fundamental assumption is true, it may come as a surprise that years of research on compression have not proven that improved clinical outcomes will universally be observed with the use of compression. It seems that for every study showing improved speech recognition ability with the use of compression, there is a study documenting how compression degrades speech recognition ability.
The potential benefit of improved audibility with an increasing number of channels has been discussed in this article. However, it is important to note the potential detrimental effects associated with greater numbers of compression channels as well. As the number of compression channels increase, the level differences of conversational speech are minimized. These level differences are important cues for both normal hearing and hearing-impaired listeners (Moore & Glasburg, 1986). These cues are most valuable for the perception of vowel sounds. In this regard, some studies have documented improved audibility for vowels and improved understanding of vowel sounds with a greater number of compression channels (Stelmachowicz, Kopun, Mace, & Lewis, 1995), while other studies have shown poorer vowel recognition as a result of the distorted level differences that occur with a greater number of compression channels (Franck, van Kreveld-Bos, & Dreschler, 1999). This reduction of level difference across the speech frequency spectrum is an important topic; however, the focus of this discussion will remain on the beneficial impact of increasing compression channels.
Yund and Buckles (1995) completed one of the most robust behavioral investigations into the effects of increasing compression channels. Sixteen subjects with sensorineural hearing loss of varying degrees and etiologies participated in their experiment. The authors included participants with a broad range of hearing losses in an effort to examine any interactions between compression characteristics and hearing loss configuration or severity. Test materials were based on the Nonsense Syllable Test (NST; Resnick et al., 1975). Test stimuli, male and female talkers, were CV or VC monosyllables that varied according to voicing (voiced or voiceless), consonant position (initial or final) and vowel context (/a/, /i/, or /u/).
The authors found a significant interaction between number of channels and voice; increasing the number of channels produced a greater improvement in discrimination for the male than it did for the female voice. As noted above, speech-weighted noise has more energy in low frequencies, which could be more effective at masking the male voice. Because gain reductions in a multi-channel hearing aid are based on the level within a given channel, an instrument with fewer channels would be expected to lose more low-frequency speech information in its attempts to reduce gains applied to high-level, low-frequency noise. Instruments with a higher number of processing channels would therefore be expected to have less of a detrimental effect on low-frequency speech information.
Most importantly, there was a significant effect overall for the number of processing channels. Speech discrimination clearly improved as the number of channels increased from 4 to 8 channels and remained consistent above 8 channels. There was no significant interaction between number of channels and signal-to-noise ratio; increases in the number of channels did not yield further improvement or decrement for performance at poor signal-to-noise ratios.
Moore, Peters, and Stone (1999) investigated the interaction between number of compression channels and the effect of background noise. The authors hypothesized that a fast-acting, multi-channel hearing aid may be able to adapt to the temporal and spectral modulations of speech, amplifying the "dips" within the speech signal, thus improving speech recognition for hearing impaired listeners. For this reason, they proposed that increasing the number of channels may be beneficial as it would allow for more discrete frequency ranges to be amplified. However, in general, the observed benefits of compression were modest. The best performance was observed in conditions using a background noise with spectral gaps and an eight-channel hearing aid.
The expectation of improved speech understanding with multi-channel compression is difficult to substantiate. The literature in this area is clouded by a wide range of variables (e.g. differences in study design, test material, and the compression behavior itself) (Hickson, 1994). Increasing the number of compression channels may have positive benefits such as improved speech audibility for high-frequency speech sounds (i.e. consonants; Braida, Durlach, DeGennaro, Peterson, & Bustamente, 1982). In contrast, valuable cues such as spectral relationships among vowel sounds may be reduced (Franck et al., 1999). With a focus on the improvement of speech recognition, it appears that eight compression channels will optimize audibility of high-frequency sounds and may also provide some benefits for the understanding of speech in various backgrounds of noise (Yund & Buckles, 1995; Moore, et al., 1999).
The Effect of Channel Number on Hearing Aid Features
Clinical audiologists appropriately focus on improving speech understanding and audibility as a treatment goal. After all, restoration of speech audibility is one primary goal of every hearing aid fitting. However, modern hearing aids provide a wide range of features and functionality that operate within hearing aid channels but fall outside the discussion of compression. Examples of these features include feedback suppression, digital noise reduction, and directional microphones.
The spectral and temporal cues required by humans for speech communication and understanding are well-documented. In modern hearing aids, however, a number of signal processing techniques operate on non-speech information. Feedback suppression, for example, is a technique for minimizing the whistling that may occur in a hearing aid fitting that involves a sophisticated analysis of the hearing aid's acoustic characteristics (Galster & Galster, 2010). The more accurately the hearing aid can estimate the acoustic characteristics of a hearing aid's feedback path, the more effectively the hearing aid will be able to suppress the occurrence of feedback or whistling (Kates, 2001). It stands to reason, then, that increasing the number of channels will improve the performance of a feedback canceller. This exact relationship has not been investigated, however. Similar speculation can be made for digital noise reduction: increasing channels should improve the ability of the hearing aid to reduce noise within frequency regions in which little speech is present. But this relationship has not been investigated.
In terms of multi-channel directional microphone technology, the effect of increased channel numbers on predicted speech recognition has been investigated. Woods and colleagues (2010) used hearing aid microphones to collect recordings representative of everyday listening environments around the city of San Francisco. The data analysis was performed on a desktop personal computer, simulating 16, 32, and 256 channels. The findings of this study suggest that directional processing in 256 channels may provide significant speech recognition advantages when compared to 16 or 32 channels. While these data support dramatically increasing the number of channels, modern hearing aids would not be capable of the processing demands that 256 channels would require.
It is a reasonable expectation that increasing the number of channels in a hearing aid should improve the hearing aid's signal processing performance. However, at the time of this publication, the authors are unaware of studies that support this speculation within the technological constraints of modern hearing aids.
Summary
The perceived value of many modern hearing aids is often associated with the number of channels and bands. However, the variables that factor into patient outcomes related to increasing the number of bands and channels are many, and the conclusions drawn by studies that have investigated the effect of band and channel numbers on patient outcomes are, at best, equivocal. Because increasing numbers of channels and bands is an accepted facet of more advanced technology, it is worthwhile to focus on what the research suggests at this time.
Increasing the number of channels will increase processing load. This results in a concomitant increase in processing delay. Considering that many hearing aids are fit in an open-canal configuration, a delay at or below 5 msec is preferred; fortunately, this is the case for most available hearing aids. Research in the area of bands, or the adjustment handles available in a hearing aid's programming software, suggests that that seven bands are sufficient to shape a frequency response to match prescribed targets for most configurations of hearing loss. In addition to number of bands, a range of studies have investigated the impact that increasing the number of compression channels has on restoration of audibility and speech recognition ability. These studies suggest that hearing aids with as few as eight channels offer sufficient frequency resolution to restore audibility, even in listening conditions with background noise. Hearing aids offering fewer than eight channels may not offer sufficient frequency resolution to restore audibility of speech in the cases of particular hearing loss configurations and stimulus types.
The possibility remains that signal processing techniques in hearing aids (e.g. feedback suppression, digital noise reduction, directional microphones) may benefit from increasing the number of processing channels. However, there does not appear to be a sufficient body of evidence that supports or discounts this hypothesis within the limitations of modern hearing aid technology.
As hearing aid technology continues to advance it is likely that new hearing aids will be introduced with increasing numbers of available processing channels. While the exact benefits of this signal processing strategy may not be readily apparent, there remains opportunity for research studies to document the interaction between the number of available channels in a hearing aid and a wide variety of patient outcomes.
About the Authors
Jason Galster, Ph.D., is Manager of Clinical Comparative Research with Starkey Laboratories. He investigates the clinical outcomes of modern hearing aid features while ensuring that product claims are accurate and backed by supporting evidence. Dr. Galster has held a clinical position as a pediatric audiologist and worked as a research audiologist on topics that include digital signal processing, physical room acoustics, and amplification in hearing-impaired pediatric populations.
Elizabeth Galster, Au.D., is a Research Audiologist with the Clinical Product Research team at Starkey Laboratories, Inc., conducting clinical research trials on emerging technology and fitting processes. Previous research experience focused on evaluation of signal processing algorithms, directional microphones, and speech understanding in reverberation. She has also worked clinically with the Veterans Administration.References
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