In part 1 of this blog series, I had talked about a patient who had a relatively rare 3000 Hz noise (music) induced hearing loss notch and not the more common one at 4000 Hz or 6000 Hz. The ear canal resonance was at 2000 Hz as compared as opposed to the more common “average” of around 2700 Hz. The lower frequency outer ear resonance was indeed a major reason why there was a 3000 Hz notch rather than in the 4000-6000 Hz region.
There are several theories of why we have a noise induced hearing loss in the 3000-6000 Hz region but one of them is related to having the greatest hearing loss about one half octave higher than the offending frequency. If music (or noise) is enhanced in the 2000 Hz region prior to even getting to the cochlea, then one half octave above this would be 3000 Hz, so this finding is not that surprising (like a G being a half octave higher than the C below it).
What is surprising is finding a 2000 Hz outer ear resonance- this guy must have an ear canal that is 10 feet long!
The question introduced in part 1 of this blog series is what do we do when we want to ensure a flat or uniform attenuation with hearing protection such as Musicians Earplugs. These earplugs “assume” a 2700 Hz resonance in order to create a flat response. Do we just “take the hit” and put up with a 2000 Hz greater attenuation (as a result of the destruction of this person’s natural ear canal resonance) or do we modify the Musicians Earplugs in order to obtain a 2000 Hz resonance which can be off-set by this patient’s low frequency outer ear resonance?
The short answer is “I don’t know” but the slightly longer answer is, let’s find out.
Two pairs of custom silicon earmolds were made for this patient both with the ER-15 Musicians’ Earplug. In the manufacture of these earplugs a “mass meter” is used to ensure that the volume of air in the earplug bore resonates with the known compliance of the ER element. This is known as a Helmholtz resonance- an interplay between volume of air and compliance creates a resonance. (There is another way to do this with a quarter wavelength resonator and this is the idea that is used in the non-custom one-size-fits-all ETY earplug or ER20XS earplug).
But back to the Helmholtz resonance used in the custom earplugs.
One can alter the resonant frequency of these earplugs by changing the dimensions of the bore of the custom made earplug. It can be made narrower or wider, longer or shorter, than prescribed and verified by the mass meter readings.
Specifically, the resonance frequency is proportional to the cross-sectional area and inversely proportional to the length. Increasing the bore width and decreasing the bore length will increase the resonance frequency, but doing the converse (a long, narrow bore) will create an earplug that has a lower resonant frequency (around 2200 Hz).
Subsequently two earplugs were made- one with normal dimensions and another with a longer narrower bore. The longer and narrow bore one would provide a flat response in the in 2000 Hz region and would continue to be smooth up to around 3500 Hz with a roll-off (greater attenuation) above that. The conventional bore dimensions would result in a notch in the 2000 Hz region, a slight bump (less attenuation) in the 3000 Hz region, but otherwise uniform attenuation above that up to about 8000 Hz (my office probe tube microphone doesn’t measure above 8000 Hz).
And the winner is…. That is, the preferred earplug arrangement for this one patient after trying it for several weeks, is…. The longer narrower bore that resulted in a smooth response up to about 3500 Hz even in the 2000 Hz region. He preferred that over having better high frequency acuity but a notch in the 2000 Hz region.
I am not sure that a single subject using a single subject design is definitive, but as stated in part 1 of this blog series, this would make for a really interesting and clinically practical AuD Capstone study. The student could use a simple equalizer to alter the frequency response heard in a number of different ways (including a 2000 Hz notch) and play these audio files to a large number of subjects. And then I would hire that student!