Starkey Research & Clinical Blog

Understanding the benefits of bilateral hearing aids

A Prospective Multi-Centre Study of the Benefits of Bilateral Hearing Aids

Boymans, M., Goverts, S.T., Kramer, S.E., Festen, J.M. & Dreschler, W.A. (2008). A prospective multi-centre study of the benefits of bilateral hearing aids. Ear and Hearing 29(6), 930-941.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

The benefits of binaural amplification are generally well established and include improved speech discrimination in noise (Hawkins and Yacullo, 1984; Kobler & Rosenhall, 2002), improved localization of sound sources (Dreschler & Boymans, 1994; Punch et al, 1991) perception of balanced hearing, improved speech clarity (Chung & Stephens, 1986; Erdman & Sedge, 1981) and reduced listening  effort (Noble, 2006). However, some studies have shown either little subjective difference between unilateral and bilateral amplification (Andersson et al, 1996) or even a subjective preference for unilateral hearing aids, especially in noise (Walden & Walden, 2005; Schreurs & Olsen, 1985).

The authors of the current study sought to confirm subjective evaluations of binaural hearing aids with objective, functional tests of localization and speech discrimination in noise. They also examined three diagnostic measures to determine their potential as predictors of binaural success.

Two hundred fourteen hearing-impaired subjects were recruited from eight audiology clinics in the Netherlands. Participant inclusion criteria were limited only to  participants who were native Dutch speakers and were physically able to complete the test procedures, with no contraindications for binaural hearing aid fitting. Therefore, individual characteristics varied widely with regard to prior hearing aid use, hearing aid style and circuitry, age and degree of hearing loss. Ten participants with normal hearing were also tested for reference purposes.

Prior to hearing aid fitting, in addition to basic diagnostic audiometry, participants completed three tests that were chosen as potential predictors of binaural benefit:

1. Interaural time differences.

2. Binaural masking level differences.

3. Speech reception thresholds in background noise.

Following the hearing aid fittings, functional binaural benefit was evaluated and questionnaires were administered to obtain subjective responses to unilateral and bilateral fittings. Three assessment tools were used:

1. Speech intelligibility in background noise with spatial separation of speech and noise.

2. Horizontal localization of everyday sounds.

3. Subjective questionnaires to examine differences between unaided, unilateral, and bilateral conditions for detection of sounds, discrimination of sounds, speech intelligibility in quiet and noise, localization, and comfort of loud sounds.

Not surprisingly, on all three diagnostic measures, normal hearing participants performed significantly better than hearing-impaired participants. There was a great deal of inter-participant variability within the hearing-impaired group.

On the functional test of speech intelligibility with spatially separated speech and noise, bilateral hearing aid users performed significantly better than unilateral hearing aid users. Improvements were noted for conditions in which competing sounds were presented ipsilateral and contralateral to the speech stimulus.  On the localization test, bilateral hearing instrument wearers again performed significantly better than unilateral hearing aid wearers.  Subjective questionnaires showed that unilateral hearing aid use was favored over unaided conditions for all categories except comfort of loud sounds. Similarly, bilateral hearing aid use was favored over unilateral for all categories except comfort of loud sounds.  This finding is in agreement with previous work by the lead author of the current study (Boymans, 2003).

Participants were asked to provide reasons why they preferred one or two hearing aids. The most common reason for preferring a unilateral fitting was that the user’s own voice was more pleasant with one hearing aid. For preferred bilateral fittings, the most common reasons were, intelligibility on both sides, better localization, better sound quality, and better balance.  Following completion of the study, 93% of the participants chose to purchase bilateral hearing aids, whereas 7% chose to purchase only one hearing aid.

One primary goal of the study was to determine if subjective benefit could be supported with objective test results. There was a significant positive correlation between bilateral benefit for speech perception and subjective satisfaction ratings, but other evaluated factors did not show this relationship. Therefore, the authors determined that functional test results could not distinguish between groups who preferred unilateral or bilateral fittings. Overall, however, the vast majority of participants preferred bilateral hearing aid fittings and the functional test results support a strong binaural benefit.

The second goal of the study was to evaluate potential predictive measures of binaural benefit. The results did not show strong correlations between bilateral hearing aid performance and interaural time difference, binaural masking level difference or speech reception threshold measures.  Therefore, these measures were not determined to have particular predictive value for determining binaural hearing aid success.  In fact, the strongest correlation between bilateral benefit and any other diagnostic measure was found for traditional audiometric measures of pure tone average and maximum speech recognition.

Binaural benefit was also examined with regard to other subject variables. The authors found greater binaural benefit for users with more severe hearing loss and for those with more symmetrical hearing loss. There were no significant differences between subjects who had previously been fitted with unilateral hearing aids and those who had been previously fitted bilaterally. Participants without prior hearing aid experience demonstrated slightly less binaural benefit and less satisfaction than those with previous experience. The authors point out that this finding is confounded by the fact that previous users tended to have significantly greater degrees of hearing loss than first-time users.

The bilateral benefit for localization was higher for in-the-ear hearing aid users than for behind-the-ear hearing aid users. The authors surmised that this could be related to pinna effects, but pinna effects generally aid vertical localization and front/back localization (Blauert, 1997), whereas the localization measures in the current study were strictly horizontal. Still, it is possible that preservation of pinna-related spectral cues in combination with binaural cues could have had an additive effect for the in-the-ear hearing aid users in the present study.

It is interesting to note that despite the highly variable subject population in this study, significant binaural benefit for speech intelligibility and localization was found across participants, and participants overwhelmingly preferred the use of binaural hearing aids over monaural. Variables such as microphone mode, noise reduction technology, and circuit quality were not specifically addressed or controlled. It is reasonable to surmise that performance in the one category in which subjects preferred unilateral hearing aids, comfort for loud sounds, could be improved by adjustments to noise reduction settings, MPO or gain settings, or use of adaptive directionality.  Therefore, the study as a whole offers strong support for binaural hearing aid recommendations and indicates that the only negative effect, that of loudness discomfort, could probably be easily corrected with current technology.

Participants in this study were all willing to consider binaural hearing aid use and therefore had relatively symmetrical hearing losses. The binaural benefits measured here can probably be reasonably extrapolated to individuals with asymmetrical hearing losses, but this issue might benefit from further study.  Also, it is likely that similar binaural benefits may also apply to potential hearing aid users who are unwilling or reluctant to consider binaural hearing aid use, but these clients will require more thorough counseling with regard to expectations and acclimatization.  The primary reason given for unilateral hearing aid preference was related to occlusion and the sound quality of one’s own voice. A reluctant user of new binaural hearing aids will need to understand that this is a common, but often short-lived, outcome of binaural hearing aid use.

Because of the poor predictive value of diagnostic tests for binaural hearing aid success, the authors advise that it is probably best for hearing aid users to determine binaural benefit individually, during their initial trial period. This is appropriate advice and may be in line with what most clinicians are already recommending to their patients. Because an individual’s work, home, and social activities are important determinants of their perceived hearing handicap, binaural hearing aids should always be tested thoroughly in these situations to evaluate benefit.  There is little financial risk involved, as most clinics offer at least a 30-day trial period with new instruments and many offer a 45- or 60-day trial. Should a client determine that the benefit of a second hearing aid does not outweigh the financial burden, they would be able to return the aid for a refund, losing only the cost of a custom earmold and/or a trial period fee.

The current study shows strong evidence for functional improvements as well as perceived advantages in binaural hearing aid users. However, the authors were unable to identify a diagnostic tool to effectively predict binaural success.  This raises an important question about the value of such a predictive measure.  The significant improvements enjoyed by binaural users and the overwhelming preference for two hearing aids over one suggest that binaural fittings should be the recommendation of choice for all clients with bilateral, aidable hearing loss.  Granted, there are some audiometric findings that preclude a binaural recommendation, such as profound hearing loss in one ear, normal hearing in one ear, or exceptionally poor word recognition ability in one ear. But these are obvious, well-known, and relatively uncommon clinical contraindications to binaural hearing aid use. It seems reasonable, as the authors eventually suggest, to forego predictive measures and allow clients to experience binaural benefits individually and determine the proper decision for themselves during their trial period.


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Blauert, J. (1997). Spatial Hearing: The Psychophysics of Human Sound Localization. Cambridge: MIT Press.

Boymans, M. (2003). Intelligent processing to optimize the benefits of hearing aids. Ph.D. thesis, University of Amsterdam.

Boymans, M., Goverts, S.T., Kramer, S.E., Festen, J.M. & Dreschler, W.A. (2008). A prospective multi-centre study of the benefits of bilateral hearing aids. Ear and Hearing 29(6), 930-941.

Chung, S.M. & Stephens, S.D. (1986).  Factors influencing binaural hearing aid use. British Journal of Audiology 20, 129-140.

Dreschler, W.A. & Boymans, M. (1994). Clinical evaluation on the advantage of binaural hearing aid fittings. Audiologische Akustik 5, 12-23.

Erdman, S.A.  & Sedge, R.K. (1981). Subjective comparisons of binaural versus monaural amplification. Ear and Hearing 2, 225-229.

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Kobler, S. & Rosenhall, U. (2002). Horizontal localization and speech intelligibility with bilateral and unilateral hearing aid amplification. International Journal of Audiology 41, 395-400.

Noble, W. (2006). Bilateral hearing aids: a review of self-reports of benefit in comparison with unilateral fitting. International Journal of Audiology 45, 63-71.

Punch, J.L., Jenison, R.L. & Alan, J. (1991). Evaluation of three strategies for fitting hearing aids binaurally. Ear and Hearing 12, 205-215.

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Walden, T.C. & Walden, B.E. (2005). Unilateral versus bilateral amplification for adults with impaired hearing. Journal of the American Academy of Audiology 16, 574-584.