In the December 2006 issue of The Hearing Journal (go to www.audiologyonline.com, click on “archives” and select Dec 2006) I gave a quick and easy test to see whether your favorite hearing can handle the louder inputs of music. We all have favorite hearing aids (or favourite for our Canadian and British readers) and typically for speech, it involves some form of WDRC, is multi-band, has good flexibility, is easy to program, and has a good feedback management system. Why should I use a different hearing aid for listening to music? Well, you don’t necessarily need to.
Having a “music program” or a glossy picture of a musician playing music with hearing aids don’t really address the technical questions- just the marketing ones. In last week’s blog I talked about reducing the input to the analog to digital (A/D) converter because that appears to be the weak point with modern digital hearing aids. If music, which is inherently more intense than even loud speech is the input to the hearing aid, is there a simple way to see if your favorite hearing aid can handle the louder elements of music without being overdriven? And the answer is yes.
This only needs to be done once to see how the hearing aid in question performs.
Here’s the recipe: Intense Input + low gain << OSPL90.
Clear? Well let’s try another approach. In a hearing aid test box give the hearing aid in question the most intense input that can be generated by the test system. Even if the specification sheet says 90 dB, moving the test microphone nearer the loudspeaker will essentially give more input. Specifically, one can move it half way to the loudspeaker and get roughly 10 dB more input across the frequency range.
Fooling the stimulus
We can even obtain about a 20 dB more intense input if we moved it even closer however this would no longer be a flat spectral increase. Once we have a sufficiently intense stimulus, merely set your favorite hearing aid to about 5 dB of gain BUT have the maximum possible OSPL90. An intense input (eg. 100 dB SPL) + 5 dB gain = 105 dB SPL output. If this 105 dB SPL output is less than the programmed OSPL90 then any distortion measured during a routine ANSI test is therefore not related to saturation effect (i.e., not the OSPL90) but is related to an input (typically the A/D converter) that cannot handle an intense input such as music. If the distortion is less than 10 % with a 100 dB SPL input then the odds are that your favorite hearing aid can handle the louder inputs, of at least moderate level music. Try this and play with the position of the test microphone in your test box.
