# An extra 4 decibels

An extra 4 decibels does not seem like a lot but it is when it comes to the new hearing aid microphones. Recently modern hearing aid microphones could only transduce up to 115 dB SPL (with 3-4% of distortion). Actually that had been the case since the late 1980s. However, the newest incarnation of microphones can transduce up to 119 dB SPL (with 3-4% of distortion). This is addition of an input headroom of 4 dB.

When it comes to speech, I would agree with everyone out there- “what’s the big deal?” Speech averages around 65 dB SPL RMS at one meter, and even loud speech can be in the low 80 dB SPL range. Taking the peaks into consideration and adding another 12-16 dB (also known as the crest factor), the highest level of loud speech peaks are still less than 100 dB SPL. And even if we measure the level of a hard of hearing person’s own voice at the level of their own hearing aid microphone (less than 6” away), the levels are at most in the low 100 dB SPL range.

So why the big deal when the maximum level that can be transduced by a hearing aid microphone is increased by only 4 dB from 115 dB SPL to 119 dB SPL?

Medium level music (what musicians refer to as mezzo forte or mf) can be on the order of 90 dBA but music, being music, it has a crest factor of 18-25 dB (depending on the window of analysis that is used). 90 dB + 25 dB is 115 dB. Turning our attention to loud music (what those in the performing arts would call forte), levels of 100 dBA are quite common, and with the peaks, they can exceed 115 dB SPL. Another 4 dB or so with minimal additional distortion can provide the hearing aid user with higher fidelity music.

In (many) previous blogs, mention was made of the “peak input limiting level” of hearing aids- the highest sound level that can be transduced effectively by modern analog-to-digital (A/D) converters. With a 16 bit system, the maximum dynamic range was only 96 dB. This is a range from the quietest to the highest level that can be digitized. Many manufacturers set this range from 0 dB SPL to 96 dB SPL, whereas other manufacturers tended to be smarter and may have set this 96 dB range from 11 dB SPL to 106 dB SPL. In any event, the previous “goal” was to increase the 96 dB level to one that would be better for music. But we were limited by the modern microphone.

A website demonstrating that even loud music at 100 dBA can sound distorted with a peak input limiting level of 115 dB SPL can be found here. (100 dB + 18 dB peak > 115 dB).

With this new hearing aid microphone that can reliably transduce 119 dB SPL, we have more headroom before loud music is clipped and distorted. Our previous gold standard of 115 dB SPL for the peak input limiting level is now 119 dB SPL. If you don’t think that an additional 4 dB of microphone capability makes a difference listen to the 100 dBA recording at the above website- you can hear the distortion of loud music (100 dBA audio file) even with a 115 dB SPL peak input limiting level.

At least one manufacturer has already defined their new peak input limiting level as 119 dB SPL to take full advantage of these new hearing aid microphones. As always, contact your local hearing aid manufacturer representative for their knowledge and insight.

Here are two total harmonic distortion curves for the “old microphone” that could reliably transduce up to 115 dB SPL and the newer one that can transduce up to 119 dB SPL.  A comparison is given at 117 dB SPL with the newer (lower distortion) microphone on the left and the older one on the right.