I received this recent reply to my “The -6 dB rule” blog entry and thought that I would reply in some semi-specific terms…
Comment:
I’m a musician (flutist), about to purchase a new set of hearing aids. I’ve read several articles by Dr. Chasin and others that tell me that hearing aids are made for speech, not music, and that the peak input level should be high enough for musical sounds. But NOBODY can tell me which hearing aids, of the hundreds of available brands and models would be best for me, a musician with a mild to moderate hearing loss. I’m about to spend $5000 on equipment with no information as to what would work best for me as a musician. My audiologist (not a musician) controls the software, so it’s hit or miss on the adjustments. As far as I can see, the software is not available to me to do my own adjustments. Please tell me exactly which brand(s) and model(s) of hearing aids would be best for me.
I actually receive emails and phone calls such as this on a weekly basis, and I am hesitant to give explicit answers. An exact hearing aid prescription is a complex endeavour and is not just a listing of electro-acoustic features. There is the entire realm of auditory training, aural rehabilitation, and use of assistive listening devices, not to mention the personal interaction with the hearing health care professional. An exact hearing aid prescription through the internet would do an end-run around these important aspects. I usually respond by saying that here are some general approaches that work well with music, and that if there is interest, I would be happy to work with your local hearing health care professional.
Having said this, here are several semi-specific approaches that have been shown to work well with musicians and people who like to listen to music. These two approaches are based on ensuring that the more intense components of music do not overdrive (or distort) the front end of the hearing aid. This typically refers to ensuring that the analog to digital (A/D) converter is not overdriven since most A/D converters cannot handle inputs in excess of 96 dB SPL. This is equivalent to, as our reader states, “the peak input level should be high enough for musical sounds”. In some sense, analog hearing aids of the 1990s such as the K-AMP, were (and still are) much better for music than most of the modern digital hearing aids.
There are essentially two technical routes.
- Reduce the sensitivity of the low frequency region of the hearing aid microphone. This can be implemented in a wide range of hearing aids regardless of the manufacturer. A low cut (or -6 dB/octave) microphone is less sensitive to the intense low frequency components of the music, such that intense low frequency fundamental musical energy is reduced at the level of the A/D converter. In turn, this intense low frequency energy enters the ear canal directly, by-passing the hearing aid completely. Understandably this is best for those who do not require significant amounts of gain and output in the lower frequency region, and these clients are typically fit with a non-occluding ear mold. It is a low-tech innovation that preconditions music such that we can reduce the probability of front-end related distortion.
- Alter the operating range of the A/D converter. There are currently two approaches to accomplish this. One is to “auto-range” the front end which means that the operating range of the A/D converter keeps changing depending on what is entering the hearing aid. This technology derives from a third party manufacturer of IC circuits and is called HRX, or “head room extension”. This is a trademark of a gem of a company who sells their components to virtually every hearing aid manufacturer in the world. Up until recently, it was called Gennum, then Sound Design, and has since been purchased by On Semi Conductors. This HRX technology serves as the basis behind the modern version of the K-AMP, called Digi K, as well as many other manufacturer’s products. Another “alter the operating range of the A/D converter” approach is a modification of the front-end that simply allows inputs of up to 115 dB SPL to get through the hearing aid undistorted. Most 16 bit hearing aids have a maximum capability to handle inputs of 96 dB SPL, but actually the “true” science is that 16 bit hearing aids have a 96 dB “dynamic range”. Nobody said that this range needed to go from 0 dB – 96 dB SPL- just that the range between the quietest and the most intense had to be 96 dB. This alternative implementation allows inputs from 15 dB SPL to 111 dB SPL- everything is shifted up by 15 dB…. Still a 96 dB dynamic range, but the range is now more appropriate for music.
Feel free to share this blog with your hearing health care professional. These two approaches are very useful for music but there is more to a hearing aid than these technical front end innovations. And I would be happy to discuss the specifics of which hearing aids have these technologies with the hearing health care professionals involved in your care.
