Musician earplugs use some very creative methods to ensure that the attenuation that a musician receives is as flat as possible. The lower frequency fundamental notes and the higher frequency harmonics are all treated identically- the spectrum of the music remains the same, albeit at a slightly lower level. Technologies to accomplish this typically involve passive acoustic networks that utilize either a Helmholtz resonators (e.g. ER-15 and ER-25; as well as the Dynamic Ear Company DM series of “filtors”) or a wavelength resonator that uses a “folded acoustic horn” to enhance the missing higher frequency sounds (e.g. ETY plugs).
Because of the rules that sound waves live their lives by, higher frequency sounds (having shorter wavelengths) are more easily obstructed by hearing protection than are the lower frequency (longer wavelength) sounds. Acoustic resonance or other approaches (such as an acoustic horn) are necessary to help offset the degree of higher frequency attenuation.
But these earplugs not only serve to replace some of the higher frequency energy that is “over attenuated” but also serve to replace the ear canal natural resonance that is lost upon earplug insertion. Like placing your hand in a French horn that removes some of the natural wavelength associated resonances, insertion of an earplug destroys the 17 dB resonance at about 2700 Hz.
What if an individual’s ear canal resonance is higher or lower than this median value? How do we handle our “short or long eared” clients? Like all things in life, there is a formula! This is true whether its potato salad or love.
Here’s the formula: F α L/C
Every young audiologist-to-be learns this formula by the time they are in their teens. The resonance frequency (F) of the system is proportional to the inductance (L) and inversely proportional to the square root of the compliance (C). This gets really interesting when one realizes that the inductance L is proportional to the “length of the bore” / “cross sectional area of the bore”. If we lengthen the bore, and/or make it narrower, the resonant frequency of the system will increase.
So, the resonance frequency of a hearing protector with a hole running down the center (such as the ER-15 musician earplug) is proportional to the length and inversely proportional to the cross sectional area (assuming a constant compliance).
Or… even clearer, a long, narrow bore has a higher resonant frequency (than 2700 Hz) and a short, fat bore has a lower resonant frequency.
If we see a client that has a real ear unaided response of about 2400 Hz (such as a big football playing tuba player) then we should modify our musician earplug to have a custom bore that is shorter and fatter than what the ear mold laboratory typically manufacturers for us. Conversely, if we see a small, piccolo playing person with a real ear unaided responses of about 3000 Hz, then the ear mold laboratory should be instructed to make the musician earplug bore longer and narrower than they would normally make it.
Clinically someone may come in with a pair of musician earplugs already (from somewhere else who perhaps didn’t fully understand that F α L/C) we may be able to use this formula to our clinical advantage.
If real ear measurement shows that the musician earplugs do not have uniform attenuation because that person has a 3100 Hz ear canal resonance, then mold “inserts” may be the way to go,…, just as shoe inserts may be the way to go for some short people. Inserting a small piece of #13 tubing into the bore of the musician earplugs (or even going a bit crazy and inserting a piece of #16 tubing into a piece of #13 tubing, into the bore) would increase the resonance frequency of the system. This may increase it from the “standard” 2700 Hz to a magical 3100 Hz which would be ideal for this person- ideal means that the real ear measurement would indicate a much flatter (uniform) attenuation.
But like the smell of cauliflower on a hot summer’s day, all is not perfect. Once we start to play with the dimensions of the musician earplug we lose some of the uniform attenuation character. Lowering the resonant frequency below 2700 Hz by decreasing the inductance (e.g. shortening the bore length) will result in a slight increase in the attenuation in the higher frequency region. And the opposite is true as well- narrowing the bore dimensions with #13 or #16 tubing inserts will minimize the higher frequency attenuation.