Hearing Aid Fitting

William J. Campbell M.S., Audiologist

There appears to be some confusion amongst the hearing aid fitting community concerning the best way to adjust hearing aids for a given individual.  Some suggest a very formal procedure using computer assisted microphones inserted into the ear canal in order to generate ‘real ear’ unaided/aided response curves based upon prescriptive gain target values designed to place the speech signal within the user’s residual dynamic range.  The general formulae are representative of group values and usually need to be “tweaked” on an individual basis.  The newer systems offer speech and noise mixing to potentially enhance measurement usefulness .  This sort of testing provides reliable electro-acoustic  information and quality control  measures, especially useful with infants and children where subject cooperation is minimal.  What’s interesting,  however, is that results obtained with different manufacturers' systems show significant variability.  Which one is correct?  No one knows for sure at this point.

Because hearing depends on multiple complex processes including auditory, working memory, and attention, fitting hearing aids based on audibility alone may not be the most effective approach.

I believe in the final analysis, hearing aids are worn daily under constantly changing conditions that cannot be accurately modeled in the lab with real ear probe microphone systems. The entire auditory system (brain and neural circuitry as well as sense organ) is involved in a perceptually based statistical process.  The internal ‘number crunching’ can adapt to sensory input, allowing for the maintenance of behavioral and perceptual unity even under highly dynamic conditions.  Merely extrapolating from ear canal pressure to upstream perception appears unimpressive given the incredible complexity, let alone the large individual differences associated with most sound related issues.  The dB level measured in an ear canal via a small probe mic has marginal linkage to a given listener's final impression. A variation of the real ear probe microphone test that seems to take a more perceptual approach is called ‘speech mapping’, where, in addition to microphone measures , the patient rates speech as loud , soft or comfortable.  Results are visually displayed and used in adjusting the hearing aid gain to provide an acceptable response.

                                         IMPORTANT

Excitation patterns from the outer ear to the middle ear, to the cochlea, to the lower brain stem ,to the higher brain stem, to the mid brain ,to the auditory cortex and finally to the associational cortical areas are not very well understood .(Sorry for the lengthy walk!!).  The best minds involved in hearing research are baffled by the complexities.  For instance, auditory spatial awareness involves three related disciplines:  physical science (mathematical modeling ); perceptual psychology (subjective attributes) and cultural anthropology (social customs). Formal science is best for measuring things like hearing aid responses in ear canals, but when it comes to subjective perceptions of aural space or sound clarity,  the numbers are less than predictive for anything but the simplest descriptors.  In fact, current research in the area of human auditory perceptual coding seems to emphasize the importance of low cost speech cues that best match the statistics of the signal available for processing.  It‘s as if the brain will make the most of whatever is available for speech communication.  A wonderfully plastic system! 

There is no universal hearing aid fitting method that is best for everyone.  Many adults are capable of directing an interactive fitting process that is driven by personal needs, including judgments relating to pitch balance, loudness ranges , speech quality/ clarity/ understanding, ease of listening, in addition to localization and noise reduction issues.  A professional who has training in electro-acoustics and the associated  psychological correlates can customize a hearing aid fitting for the individual rather than relying upon group averages with their inherent error.   Beware of ideologues who profess to having the only way to fit a hearing aid successfully!!  Hearing is just too complicated to dismiss the relevance of your sound impressions.

Bottom Line:  a hearing aid fitting will be validated by the user during daily communication activities not by magical thinking of well intentioned clinicians.  In a sense, the goal could be viewed as the absence of objectionable acoustic defects.  These defects are quite useful in perceptual based hearing aid assessment, since their unpleasant sound attributes can often be heard by individual patients and potentially reduced by the audiologist.

Suggested Reading

Journal of Acoustical Society of America,  Vol. #119, #2, February 06, pp.  997-1004

Journal of Acoustical Society of America,  Vol. 121, #2, February 07,  pp.  1003-1016

Journal of Acoustical Society of America,  Vol. 121, #3; March 07,  pp.  1576-1587

Journal of Acoustical Society of America, Vol. 121, #6, June 07, pp. 3655-3665

Journal of Acoustical Society of America, Vol. 123, #1, January 2008, pp 500-506

Ear & Hearing, Vol. 29, #2, April 2008, pp 214-227

Spaces Speak, Are You Listening?;  Blesser, Barry & Salter, Linda-Ruth;  MIT Press 2007

Journal of the Academy of Audiology, Vol. 19, #10, Nov.-Dec 2008, pp. 780-798

The Hearing Journal, Volume 62, #8, August 2009, pp 10-17

Journal of Acoustical Society of America, Vol. 126, #3, September 2009, pp 1312-1320

Advance for Audiologists; March-April 2010; pp 46-50

Journal of Acoustical Society of America, Vol. 127, #3, March 2010, pp 1491-1505

Journal of Acoustical Society of America, Vol. 127,#4, April 2010, pp 2509-2520; 2521-2528

Journal of the American Academy of Audiology, Vol. 21, #4, April 2010, pp 249-266

Journal of the American Academy of Audiology, Vol., 20, #4, April 2009, pp 272-281

ASHA Leader, July 6, 2010, pp 14-17