For people with great degrees of loss can understand their dynamic range is less but for mild losses, we don't check for recruitment before prescription. How about a person having good low frequency hearing, how does compression work for them?
Why is wide dynamic range compression (WDRC) used for patients with mild losses? Well, there are a few things to remember. Most non-linear fitting rationales, whether they are independent (DSL I/O, NAL-NL1) or proprietary (ASA2 by Oticon), will attempt to provide to the patient with sensorineural hearing loss access to the same dynamic range of sound input as is available to the normally hearing person. The person with normal hearing can usually hear a range of about 100 dB between threshold and Uncomfortable Loudness Level. In general, it is a goal to try to make this 100 dB input range fit into the remaining dynamic range of the patient. For a patient with mild loss, this range would perhaps be 60 to 70 dB wide. This goal will of course call for the use of a WDRC approach. In cases where UCLs are not entered in the fitting software, these rationales will make predictions based on large sets of data relating thresholds to measured UCLs on hundreds of patients. These predictions have been shown to work well, especially if the hearing loss is less than severe.
First, most non-linear fitting algorithms will provide very little compression for the regions of normal hearing or mild hearing loss. For example, for a sensorineural hearing loss of 30 dB HL at 500 & 1000 Hz. and 40 dB HL at 1, 2 & 4 kHz., compression ratios in the order of 1.5 to 1.8 are apply by the most popular fitting rationales. Given the high quality of modern day compression system to apply compression without any audible side effects, the patient should hear nothing but cleanly processed signals. So although WDRC is being employed, the reality is that very little compression is actually being applied.
Secondly, in all actual hearing aid applications, WDRC is applied with some kneepoint before the hearing aid enters its non-linear range, typically between 35 and 50 dB SPL. (There often is another, very low kneepoint below which expansion is applied, but that will have to wait for another Ask The Expert.) Since the hearing aid needs to be linear below this kneepoint in order to preserve sound quality (the patient does not want too much gain for very soft sounds) and to minimize the likelihood of feedback, a little extra compression is needed above the kneepoint in order to compress the full normal range of inputs into the remaining dynamic range of outputs. In other words, if, theoretically, a patient would need a compression ratio of 1.3 in order to take the full normal range of inputs and place it into his/her remaining dynamic range, then perhaps a compression ratio of 1.7 would be needed to compensate for a fixed kneepoint.
Finally, in order to provide good sound quality through a hearing aid, a little gain is often provided even in the presence of normal or near normal hearing. First of all, placing the aid in the ear will lead to occlusion. That will eventually become less of an issue as more and more fittings become open due to highly effective feedback cancellation systems. However, for cases in which a truly open fitting cannot be provided, gain in the low frequencies is an effective way to minimize the annoyance of occlusion. In addition, a little gain will also just provide a fuller sound quality, especially in the case of normal hearing in the lows and a significant hearing loss in the highs. Finally, a little gain can also make some of the operational noises in hearing aid circuitry and components less noticeable.
Donald J. Schum, Ph.D./CCC-A
Vice President, Audiology & Professional Relations
29 Schoolhouse Rd.
Somerset, NJ 08875
1-800-526-3921, Ext 535
Don Schum currently serves as Vice President for Audiology & Professional Relations for Oticon, Inc. Previous to his position at Oticon in Somerset, Don served as the Director of Audiology for the main Oticon office in Copenhagen Denmark. In addition, he served as the Director of the Hearing Aid Lab at the University of Iowa School of Medicine (1990-1995) and as an Assistant professor at the Medical University of South Carolina (1988-1990). During his professional career, Dr. Schum has been an active researcher in the areas of Hearing Aids, Speech Understanding, and Outcome Measures. ( B.S. in Speech & Hearing Science, University of Illinois; M.A. in Audiology, University of Iowa; Ph.D. in Audiology, Louisiana State University.
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