The -6 dB / octave microphone is less sensitive for the lower frequencies of speech and music. And many of the lower frequency components are also the intense ones. The net result is that a lower sound level reaches the analog-to-digital (A/D) converter. The net benefit is that the A/D converter is not as easily over driven. There is less distortion from music than would otherwise be the case with a broadband microphone that has similar sensitivities to low frequency sound as it does to mid and higher frequency sounds.
The above explanation is the reason in a “nut shell” why less may be more when it comes to more intense inputs to a hearing aid such as music. There are other technical innovations but this is the essence. “How can we reduce the sound level so that it won’t cause the A/D converter to distort?”
A -6 dB/octave microphone has been in use and has specifically been used for music, but maybe it should also be used for all hearing aids where the hard of hearing consumer only requires 30 dB or less gain for the lower frequency region. If a person requires significant low frequency gain then it can be easily replaced by software adjustments after the A/D converter. One just needs to add 6 dB to the required gain at 500 Hz and 12 dB to the required gain at 250 Hz.
A person’s own voice is about 65 dB SPL at 1 meter (3 feet for our American friends) but about 85 dB SPL at the level of the person’s own ear. The hearing aid microphone doesn’t know whether the sound is intense from a large distance, or “average” from about 6 inches. All of the vowels in a person’s speech have their first formant at or below 500 Hz. High vowels have a very low frequency first formant (<250 Hz) and low vowels have their first formant at 400-500 Hz.
And as discussed in last week’s blog, even speech can have instantaneous peaks of about 20 dB above their RMS or average value, if the analysis window is short enough. Well, add 80 – 85 dB SPL to an instantaneous peak that is 20 dB greater and we have levels in excess of 100 dB SPL… more than enough to overdrive the A/D converter. It’s not just music that is intense, it’s also a person’s own voice at the level of their own hearing aid that is intense as well. Hard of hearing people just cannot hear their own voice with fidelity that they should be receiving.
So, why aren’t hearing aid manufacturers flocking to install a -6 dB/octave microphone in all of their hearing aids? The figure below shows the internal noise for three conditions. The red line is the internal noise spectrum with a broadband microphone, and the violet one is the spectrum with a -6 dB/octave microphone. This increase in internal noise level has erroneously scared off the hearing aid industry. The black line shows a significantly reduced internal noise spectrum of a -6 dB/octave microphone if expansion is set to maximum. There are no negative effects of expansion, and all it does is come to the rescue. These noise spectra are courtesy of Mark Schmidt (Unitron Hearing).
The internal noise spectrum of a broadband microphone and a -6 dB/octave microphpone. Also, shown is the improved noise spectrum with expansion implemented in the hearing aid. Figure courtesy of Mark Schmidt (Unitron Hearing)
So, the criticism of the – 6 dB/octave microphone is erroneous.
The benefits of all hearing aid microphones (for those who require 30 dB or less gain in the lower frequency region) are evident. These hard of hearing consumers will still obtain all of the amplification they require (where any missing low frequency amplification can be replaced by software adjustments after the A/D converter), will have an increased head-room for the lower frequency vowel (and other sonorant) components of their own speech, and will be able to have increased appreciation for the music that they listen to and play.
For those people with greater than a low frequency moderate hearing loss, a broadband microphone may be useful since they do require substantial gain that may not be able to be replaced by software adjustments, at least not without incurring a significant reduction in battery life.
However for those with better than a 60 dB HL hearing loss at 250-500 Hz, a -6 dB/octave microphone seems to be the way to go.
