Comparison of Hearing Aid Fitting Algorithms

by King Chung, Ph. D., Project Director at Lexington RERC

Advances in technology and new understandings of cochlear mechanics have revolutionized the design of hearing aids.  Since the late 1980’s, hearing aids with multichannel compression, multi-band frequency shaping and multiple adjustable options have flourished in the forms of digitally programmable hearing aids and later in 100% digital hearing aids.  These high performance hearing aids can provide very flexible adjustment to a person’s amplification needs.  They can also perform various functions which were not possible in the conventional linear hearing aids or simple compression hearing aids. 

 

One of the advantages of the high performance hearing aids is automatic volume control through wide dynamic range compression.  People with hearing loss often experience recruitment.  They need more amplification for soft sounds and less amplification for loud sounds.  Users of the conventional and simple compression hearing aids often need to adjust the volume control because their hearing aids provide the same amount of amplification for soft sounds and loud sounds.  The digitally programmable and the 100% digital hearing aids, on the contrary, can automatically provide more amplification for soft sounds and less amplification for loud sounds.  This feature not only enhances the audibility of many soft speech components (e.g., f, s, sh), but also reduces the need to adjust volume control.  In general, the high performance hearing aids have received better customer satisfaction, sound quality and speech understanding ratings. 

 

As the complexity of hearing aid designs increases, our concept of hearing aid fitting has also changed.  Most of the manufacturers of high performance hearing aids provide proprietary fitting algorithms to fit their hearing aids to people with different degrees and configurations of hearing loss.  Many researchers have also proposed different fitting algorithms.  Our research question was to find out which hearing aid fitting algorithm would enhance hearing aid users’ speech understanding ability, listening comfort and perceived sound quality. 

 

We are currently conducting a research study to explore the proficiency of two hearing aid fitting algorithms, namely, the Visual Input-Output Locator Algorithm (VIOLA) developed by a group of researchers in the U.S. and the National Acoustic Lab fitting algorithm for non-linear hearing aids (NAL-NL1) developed recently in Australia.  In our study, research participants would wear a pair of 100% digital hearing aids programmed to either VIOLA or NAL-NL1 for 1 month.  During this period, they fill in diaries to document their hearing aid performance in quiet, in noise, in one-to-one conversations, in lecture halls, in religious services, in group settings, in traffic noises, in construction noises…  Then they come to the Lexington center to rate their speech understanding ability, the amount of effort required to understand speech in quiet and in noise, the loudness of speech and overall sound quality of the hearing aids when they listen to speech at different presentation levels.  Their speech understanding ability was also assessed by repeating words or sentences in quiet or in noise.  After they have finished with all the testing for the first algorithm, their hearing aids are programmed for the second fitting algorithm and the whole process is repeated. 

 

The research subjects are compensated for their time spent on testing and their travel expenses.  They can also choose to purchase the 100% digital hearing aids at 50% off the manufacturer’s suggested retail prices at the end of the study. 

The results of this study will shed light on the effectiveness of the two hearing aid fitting algorithms in improving hearing aid users’ speech understanding ability, listening comfort and perceived sound quality.  They may also help clinicians to better serve the hearing-impaired population.