Evidence for the importance of sound quality in hearing aids
Hearing aid manufacturers desire to maintain and increase sound quality through advanced technology and innovation. Recently, spatiality and intelligent binaural processing have become commercially available and have further increased the ability of hearing instruments to achieve high-quality outcomes.
Sound is often characterized by its quality. The perceived quality may determine if we will listen to it and, if so, for how long. It’s quite the same when individuals with hearing impairment wear hearing devices for the first time. The quality of amplified sound is likely to be perceived as “different” from the unamplified sound they’re accustomed to hearing through compromised auditory systems.
Initial quality perceptions are often used by new hearing instrument wearers to form opinions and make decisions regarding perceived benefit of amplification and may impact the continued use of their hearing instruments.1 Therefore, the sound quality perceived by the hearing instrument wearer must be maximal.
|Ravi Sockalingam, PhD, is senior audiologist, and Joel Beilin, MScEE, is director of audiology at Oticon A/S in Smørum, Denmark; Douglas L. Beck, AuD, is director of professional relations at Oticon Inc, Somerset, NJ.|
Defining sound quality is difficult. Arguably, in the final analysis, “quality” is a subjective measure which may or may not include objective parameters. For example, psychologists may refer to sound quality as a perception, engineers and physicists might define it in terms of replicable physical attributes, musicians might refer to sound quality with respect to timbre and overtones. Audiologists may address sound quality in terms of loudness, background noise, internal instrument noise, bandwidth, total harmonic distortion, etc. However, wearers of hearing instruments define sound quality based on their own terms, their past listening experience, their sound perceptions and preferences.
Amplified and Unamplified Sound Quality
Unamplified sound is typically altered by unintentional factors and variables based on the contents and shape of the local environment (ie, room size, reverberation, background noise, damping materials, loudness, duration, signal-to-noise ratio, etc). As unamplified sound enters the hearing instrument, it is intentionally altered through signal processing and the resultant sound quality is a key consideration for manufacturers. Indeed, a key concern for hearing instrument engineers is that optimal sound quality must be achieved.
One qualitative measure of sound quality is the concept of “acoustic transparency.” Sandlin2 stated acoustic transparency is achieved when hearing-impaired listeners perceive the sound from the hearing aid as the same (or nearly the same) as the sound heard without the hearing instruments.
For our purposes, we consider acoustic transparency to mean the output of the hearing aid must be “true” to the original sound source while facilitating audibility for speech and other sounds, which the wearer would not hear without hearing aids. In other words, an acoustically transparent hearing aid should deliver the original sound source, while simultaneously compensating for the hearing loss the wearer presents without audible artifacts.
Evaluating Sound Quality
Questionnaires, interviews, and rating scales have been used in controlled laboratory environments and in the real world to investigate sound quality. To date, consensus as to how to evaluate sound quality does not exist.
Dillon et al3 used six stimuli to compare the sound quality of five advanced hearing instruments. The six stimuli included subject’s own voice, a female voice, a male voice, and a male voice in impulse noise. Subjects compared the sound quality of two instruments at a time and rated one instrument as better by indicating “much better, moderately better, or slightly better.”
Munro and Lutman4 and Keidser et al5 used paired comparisons to evaluate sound quality. Hearing instrument wearers rated two different hearing instruments or two different features of a particular instrument across a range of situations for clarity, comfort (in quiet and in noise), music listening experience, and naturalness. Paired-comparison methods have the inherent potential to minimize subjective influence in sound quality judgments.
Gabrielsson, Schenkman, and Hagerman6 developed the Judgement of Sound Quality (JSQ) which employs eight dimensions. Seven dimensions relate to various qualities of sound and one relates to the overall impression. The seven sound quality dimensions are softness, brightness, clarity, fullness, nearness, loudness, and spaciousness. Though this scale was developed to assess the performance of loud speakers,7 it has been used for earphones, earmolds and hearing instruments.8,9
Narendran and Humes10 studied the potential of the JSQ to use sound quality as an outcome measure for a group of elderly hearing aid wearers. The authors concluded the JSQ was potentially a useful measure of hearing aid outcome with particular regard to groups, but was less applicable to individuals with respect to analysis of hearing aid quality.
Looi et al11 evaluated timbre of different music samples in a group of cochlear implant users using a dichotomous scale along 6 dimensions. Individuals were asked if their listening experience for a particular music piece was unpleasant or pleasant, unnatural or natural, empty or full, tinny or rich, dull or sharp, and rough or smooth.
Scientists at Delta Senselab (Denmark) are developing an evaluation protocol to delineate subtle differences in sound quality between hearing instruments. Investigations are underway to formulate the optimal combination of sound samples, sound quality dimensions, and trained listeners to differentiate hearing instruments based on subjective sound quality judgements in complex listening situations.
Sound Quality and Speech Intelligibility
Previous research has shown that, when listeners were asked to select a sound system based on sound quality, they often selected systems that did not provide maximum speech intelligibility. Generally, listeners prefer frequency responses with greater low frequency amplification, even though greater high frequency amplification provides better speech intelligibility.12
Although speech intelligibility and sound quality are two distinct entities, Killion13-15 contends they tend to go hand in hand. He notes that hearing instruments that provide better speech intelligibility in noise are the ones with good sound quality. Boike and Souza16 also reported a high correlation between speech intelligibility scores and sound quality ratings for single-channel wide dynamic range compressed speech.
Therefore, the relationship between sound quality and speech intelligibility is far from clear and warrants systematic research.
|FIGURE 1. A flow diagram showing the relationship between sound quality, ease of listening, speech intelligibility, performance of hearing aids, and user satisfaction.|
Sound Quality and Adaptation
Munro and Lutman4 studied elderly hearing instrument users who used linear programmable hearing devices for at least 4 hours daily. Sound quality was reported along dimensions of clarity, comfort, and overall preference while listening to running speech in quiet, steady noise, and speech babble. Sound quality evaluations did not change significantly over a 24-week period. This dramatic finding holds an important take-home message. Specifically, sound quality reports early in the fitting process appear to reflect personal sound quality precepts which are unlikely to change in the short term.
Bilateral Advantage and Sound Quality
There is little doubt that better sound quality is experienced by exploiting binaural cues in bilateral hearing aid fittings. The advantage of listening with two instruments has been reported along several dimensions of sound quality, such as clarity, fullness, spaciousness, and overall quality.17
The just noticeable difference (JND) for intensity and spectral features is also smaller when listening with two ears, thus indicating wearers can make finer judgements with two instruments than with one.18,19 Providing the brain access to spatial cues by having the compression settings of the two instruments co-ordinated wirelessly allows for enhanced natural perception of spaciousness and externalization of sound.
Sound Quality and Hearing Instrument User Satisfaction
Most hearing aid satisfaction ratings have examined extrinsic and intrinsic factors. Intrinsic factors include: age, gender, demographics, hearing loss, self-perceived disability and handicap, hearing aid experience, expectations of hearing instruments, attitude and personality, and hours of aid usage. Extrinsic factors are associated with sound quality, types of hearing instrument, listening situations, benefit, fitting-related problems, and counseling.20
Several studies20 have documented positive relationships between sound quality and hearing aid user satisfaction (Figure 1). Of the top-10 factors most highly associated with overall hearing instrument satisfaction, Kockkin21 noted three factors related to issues of sound quality (#2 “clarity of sound”; #5 “natural sounding”; and #7 “richness or fidelity of sound”).
Bentler and colleagues22 found some 20% of variance in hearing aid satisfaction can be attributed to sound quality. Naturalness, music quality, clarity of voice, and loudness have been reported to improve satisfaction.23,24
Improving Sound Quality
In 1979, Killion noted the importance of broader bandwidths and less distortion.25 Many hearing instruments (in 2009) have bandwidths that extend to 10 kHz and beyond.
It has been noted hearing instruments with the latest technology often give the best customer satisfaction ratings.20 Kochkin26,27 found that individuals using instruments less than a year old were more satisfied than users of older instruments. Hansen28 reported better sound quality and speech perception in complex listening environments when using high performance wireless hearing instruments.
Other studies have demonstrated a positive relationship between sound quality associated with bilateral fittings and user satisfaction. Kochkin29 found an overall improvement in satisfaction for bilateral users of hearing devices. Sinclair and Goldstein30 and Kochkin31 also suggested higher satisfaction among bilateral users of hearing instruments.
Signal Processing Technology and Sound Quality
Ricketts and Hornsby32 found the sound quality of speech in noise to be better with digital noise reduction than without it. Ricketts and colleagues33 noted subjects with mild to moderate hearing loss rated speech amplified in the higher frequencies (up to 12 kHz) as higher quality than speech amplified only up to 6 kHz. Beck and Olsen34 reviewed a number of publications that noted extended bandwidths offer improved sound quality for speech and music.
Spatial hearing and understanding Speech in complex environments, by Tobias Neher, PhD; Thomas Behrens, MSc; and Douglas Beck, AuD. November 2008 HR./p>
Myths that discourage improvements in hearing aid design, by Mead C. Killion, PhD, January 2004 HR.
Neuman et al35 and Souza et al36 reported that compression impacts sound quality, too. They noted subjects preferred lower compression ratios, longer release times, and fewer compression channels.
With the advent of extended bandwidth and binaural processing, synchronization and coordination between hearing instruments, the concept of spatial fidelity37 has assumed extraordinary importance. In other words, sounds originating from different directions and distances have become paramount considerations with respect to quality. For the first time in subjective sound quality evaluations, quality dimensions of spatial hearing such as “nearness,” “fullness,” and “spaciousness” (see Gabrielsson6) have been incorporated in the product verification process for advanced wireless broadband hearing instruments.
Hearing aid manufacturers desire to maintain and increase sound quality through advanced technology and innovation. Recently, spatiality and intelligent binaural processing have become commercially available and have further increased the ability of hearing instruments to achieve high-quality outcomes. Manufacturers strive to optimize sound quality while achieving the best speech intelligibility in quiet and noisy listening environments.
- Ovegård A, Lundberg G, Hagerman B, Gabrielson A, Bengtsson M, Bråndstrom U. Sound quality judgements during acclimatization of hearing aid. Scand Audiol. 1997;26:43-51.
- Sandlin R. Observations and future considerations. In: Sandlin R, ed. Textbook of Hearing Aid Amplification: Technical and Clinical Considerations. 2nd ed. Florence, Ky: Thomas Delmar Learning; 2000:729.
- Dillon H, Keidser G, O’Brien A, Silberstein H. Sound quality comparisons of advanced hearing aids. Hear Jour. 2003;56(4):30-40.
- Munro KJ, Lutman ME. Sound quality judgements of new hearing instrument users over a 24-week post-fitting period. Intl J Audiol. 2005;44(2):92-101.
- Keidser G, Dillon H, Dyrlund O, Carter L, Hartley D. Preferred compression ratios in the low and high frequencies by the moderately severe to severe-profound population. J Am Acad Audiol. 2007;18(1):17-33.
- Gabrielsson A, Schenkman BN, Hagerman B. The effects of frequency responses on sound quality judgements and speech intelligibility. J Sp Hear Res. 1988;31:166-177.
- Gabrielsson A, Sjogran H. Perceived sound quality of hearing aids. Scand Audiol. 1979;8(3):159-169.
- Gabrielsson A, Hagerman B, Bech-Kristensen T, Lundberg G. Perceived sound quality of reproductions with different frequency responses and sound levels. J Acoust Soc Am. 1990;88:1359-1366.
- Lundberg G, Ovegård A, Hagerman B, Gabrielsson A, Brändström U. Perceived sound quality in a hearing aid with vented and closed earmould equalized in frequency response. Scand Audiol. 1992;21(2):87-92.
- Narendran M, Humes L. Reliability and validity of judgements of sound quality in elderly hearing aid wearers. Ear Hear. 2003;24:4-11.
- Looi V, McDermott HJ, McKay CM, Hickson LM. Comparisons of quality ratings for music by cochlear implant and hearing aid users. Ear Hear. 2007;28(2 Supplement):59S-61S.
- Souza PE. Effects of compression on speech acoustics, intelligibility, and sound quality. Trends Amplif. 2002;6:131-165.
- Killion MC. Myths that discourage improvements in hearing aid design. Hearing Review. 2004;11(1):32-40. Available at: /issues/articles/2004-01_03.asp.
- Killion MC. Hearing aids: past, present, future: moving toward normal conversations in noise. Brit J Audiol. 1997;21:141-148.
- Killion M. Research and clinical implications for high-fidelity hearing aids. In: Hearing Care for Adults: International Conference Proceedings. Chapter 14. 2006:181-191.
- Boike KT, Souza PE. Effect of compression ratio on speech recognition and speech-quality ratings with wide dynamic range compression amplification. J Sp Lang Hear Res. 2000;43:456-468.
- Balfour P, Hawkins D. A comparison of sound quality judgments for monaural and binaural hearing aid processed stimuli. Ear Hear. 1992;13(5):331-339.
- Hall J, Fernandes M. Temporal integration, frequency resolution, and off-frequency listening in normal-hearing and cochlear-impaired listeners. J Acoust Soc Am. 1983;74(4):1172-1177.
- Hall J, Tyler R, Fernandes M. Factors influencing the masking level difference in cochlear hearing-impaired and normal-hearing listeners. J Sp Hear Res. 1984;27:145-15.
- Wong LLN, Hickson L, McPherson B. Hearing aid satisfaction: what does research from the past 20 years say? Trends Amplif. 2003;7(4):117-161.
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- Bentler RA, Niebuhour DP, Getta JP, Anderson CV. Longitudinal study of hearing aid effectiveness II: Subjective measures. J Sp Hear Res. 1993;36:820-831.
- Stock A, Fichtl E, Heller O. Comparing determinants of hearing instrument satisfaction in Germany and the United States. In: Kochkin S, Strom KE, eds. High Performance Hearing Solutions, Vol 2. Hearing Review. 1997;[Suppl]4(11):40-46.
- Spitzer JB. Factors predictive of patient satifaction with hearing aids. Hear Jour. 1998;51(3):31-42.
- Killion M. Design and evaluation of high fidelity hearing aids [PhD dissertation]. Evanston, Ill: Northwestern University. Ann Arbor, Mich: University Microfilms No. 7917816; 1979.
- Kochkin S. MarkeTrak IV: 10 year trends in the hearing aid market—has anything changed? Hearing Jour. 1996;49(1):23-34.
- Kochkin S. Subjective measures of satisfaction and benefit: establishing norms. Sem Hear. 1997;18:37-48.
- Hansen LB. Epoq study measures. Hear Jour. 2008;61(9):47-49.
- Kochkin S. MarkeTrak V: Consumer satisfaction revisited. Hear Jour. 2000;53(1):40-55.
- Sinclair JS, Goldstein JL. Long-term benefit, satisfaction, and use of amplification among military retirees. J Acad Rehab Audiol. 1991;24:55-64.
- Kochkin S. MarkeTrak III: Higher hearing aid sales don’t signal better market penetration. Hear Jour. 1992;45(7):47-54.
- Ricketts T, Hornsby BW. Sound quality measures for speech in noise through a commercial hearing aid implementing digital noise reduction. J Am Acad Audiol. 2005;16(5):270-277.
- Ricketts T, Dittberner AB, Johnson E. High-frequency amplification and sound quality in listeners with normal through moderate hearing loss. J Sp Lang Hear Res. 2008;51:160-172.
- Beck DL, Olsen J. Extended bandwidths in hearing aids. Hearing Review. 2008;15(11):22-26. Available at: /issues/articles/2008-10_02.asp.
- Neuman AC, Bakke MH, Mackersie C, Hellman S, Levitt H. The effect of compression ratio and release time on the categorical rating of sound quality. J Acoust Soc Am. 1998;103:2273-2281.
- Souza PE, Yeuh B, Sarubbi M, Loovis CF. Fitting hearing aids with the Articulation Index: impact on effectiveness. J Rehab Res Dev. 2000;37:473-481.
- Neher T, Behrens T, Beck DL. Spatial hearing and understanding speech in complex environments. Hearing Review. 2008;15(12):22-25. Available at: /issues/articles/2008-11_03.asp.
Correspondence can be addressed to HR at or Ravi Sockalingam, PhD, at .
Citation for this article:
Sockalingam R, Beilin J, Beck DL. Sound quality considerations of hearing instruments. Hearing Review. 2009; 16(3):22-28.