In part I of this blog we will overview the basis behind the use of the -6 dB/octave microphone. In part II we will talk about whether it should not only be used for music, but perhaps for both speech and music. Should the – 6 dB/octave microphone be the standard of care and a broad band microphone be the exception?
A – 6 dB/octave microphone, as the name suggests, is less sensitive to sound below 1000 Hz by 6 dB at 500 Hz and 12 dB at 250 Hz. These have been commercially available for decades and are certainly not new. However, it may be time to bring them back in force. The – 6 dB/octave microphone has been a useful “low tech” alteration to hearing aids in order to improve the fidelity and quality of music for years.
As discussed in previous blogs, modern digital hearing aids have a weak point and this is the analog to digital (A/D) converter. Due to their design, the various engineering decisions that need to be made, and their limited 16 bit architecture, the A/D converter at the front end of the hearing aid can be over driven with inputs in excess of signals in the 90- 95 dB SPL region. This has nothing to do with the hearing aid software or any other component that occurs later in the hearing aid. A distorting A/D converter at the front of the hearing aid results in poor fidelity and high distortion and no software changes that occur later in the system will be able to remove this front-end distortion.
Here is the short story: the most intense components of speech (i.e., the lower frequency vowels) do not typically exceed 85 dB SPL, so modern hearing aids can easily handle speech. The same cannot be said of music however, where even quiet music can be in excess of 95 dB SPL. It is these more intense components of music that overdrives the A/D converter.
There are several innovations in the field to counter this problem, one of which is to simply exchange the normal broadband microphone that is in common use with a microphone that is less sensitive (by -6 dB/octave) for the lower frequency inputs of music (which are quite intense). Low frequency sound still reaches the ear, but typically directly through the vent or unoccluded ear canal. If low frequency amplification is still required, then with software adjustments, this missing low frequency sound energy can be re-established. This is without distortion since the low frequency additional gain is generated after the A/D converter.
This low-tech innovation is useful even for quieter music with levels around 80 dB SPL since the crest factor of music adds an additional 20 dB or so (80 dB + 20 dB >> 95 dB). The crest factor is the difference between the average or RMS of the signal and the peak of the signal. The waveform of music is “peakier” relative to speech because of the lower level of damping inherent in many musical instruments- the crest factor for music is typically 18-20 dB whereas that for speech is only about 12 dB.
However, maybe we are wrong and the crest factor for speech is also much greater than 12 dB. The hard of hearing person’s own voice at the level of their hearing aid, with the higher SPL and perhaps higher crest factor, will also overdrive modern A/D converters. Perhaps we are doing a disservice to hard of hearing people? But, that is part II… stay tuned.
