Up until very recently, our collective amplified heads were submerged in a fog of front end distortion. Sounds, including music and that of the voices of hard of hearing people who wear hearing aids, caused their own hearing aids to be overdriven even before the transmission reached the stage of the brilliantly designed software programming. Even the best of algorithms would function sub-optimally if provided with an initial signal that was less-than-perfect. For more information on this go here.
In the last several months the hearing aid industry has (finally) responded with digital hearing aid analog-to-digital (A/D) converters that can digitize speech and music without distortion. Many manufacturers are now starting to use 24 bit A/D converters rather than the old industry standard 16 bit systems. Other manufacturers have implemented some ingenious methods to circumvent the problem. Some use an electronic transformer method, another uses an analog compressor prior to the A/D converter with digital expansion after digitization, and yet another uses a change in the digital register to increase the limited dynamic range to one that is optimized for both speech and music.
Whatever the innovation or the ingenious method of implementing an innovation, for perhaps the first time, sound can get in to a hearing aid without distortion. And with the newer hearing aid microphones that can transduce up to 119 dB SPL (as opposed to last year’s model that could only transduce up to 115 dB SPL) all important stimuli, at virtually all sound levels, can reach the software algorithm and programming stage without distortion. This includes all music, even Rock, and perhaps just as importantly, the hard of hearing person’s own voice at the level of their own hearing aids.
I would argue that much of the previous research by hearing aid manufacturers into the development of “new and improved” algorithms has been limited by this front end digitization problem. Algorithms or technologies that have seen limited real-world benefits in the past may now yield significant clinical success.
This reminds me of the history of wide dynamic range compression (WDRC). I am pretty sure that this was from an article that I had read by Mead Killion back in the early 1990s (because I recall reading it while waiting for my young daughter’s soccer practice to end). I have looked everywhere but haven’t been able to find it. However, (and let’s assume that it was from Mead Killion) WDRC was proposed and implemented in the 1970s but because of the limited bandwidth of hearing aids of that era, it didn’t work- I believe that 1970s WDRC just turned down everything- the manufacturer was Computer Hearing Aids. With the advent of true wideband receivers in the late 1980s, WDRC was again tried, but this time with great success. This was the 1988 K-AMP designed by Mead Killion and his colleagues. (Incidentally this hearing aid circuit is still available from www.GeneralHearing.com and also in the Bean PSAP from www.etymoticresearch.com).
This is a thinly veiled request to revisit many of the hearing aid algorithms of the recent past but with this newer post-16 bit technology that is now available. Throughout this blog I have made several implicit assumptions that may come back to drown me. One is that each of the newer innovations and newer platforms are equal in their function. I wouldn’t be surprised if indeed this is the case, but that is still yet to be determined. Another assumption is that the various engineering and marketing decisions make by any one manufacturer will not negate the benefits of these new approaches. And the third is that the technical information that has been released to the consuming audiology public is valid- is a 24 bit platform, truly a 24 bit system? Are the instructions only 16 bit?
Clinically we have all run in to situations where an innovation should work, but doesn’t. Clinically, it doesn’t take long to dismiss something (especially if there is a flood for return for credit of hearing aids). Perhaps it’s time for us clinical audiologists and other hearing health care professionals to retry what didn’t work last year?
