Marshall Chasin, Hearing Health Matters HEAR THE MUSIC

Marshall Chasin, Editor
"This is the best of blogs; this is the worst of blogs'. To paraphrase Dickens’s A Tale of Two Cities, it is the two-headed nature of music and hearing aids that a hearing aid can be great for speech yet useless for music, and, conversely, great for music yet less than optimal for speech. What are some tricks that can be used to improve a hearing aid for music? How can we prevent hearing loss from loud music? This blog asks these questions, and with your input, even more. Comment Policy


Tedpa

In parts one and two of this blog series, the creation of a vent-associated resonance, called an inertance, was discussed. Venting can reduce the echoey, back pressure sensation with hearing protection for musicians, especially for brass players, some reeded woodwinds, and for vocalists.  However, the mass of air in the vent can oscillate and create a potentially unwanted low frequency resonance according to the formula F = 5500 (area/LoVe)1/2.  In this formula, a long, narrow vent would create a very low frequency inertance, whereas a short, wide vent, would create a higher frequency one.

Effect of a vent, "followed" by a small resonance called an inertance where the mass of air in the vent oscillates as a single unit. Courtesy of HearingReview.com

Effect of a vent, “followed” by a small resonance called an inertance where the mass of air in the vent oscillates as a single unit. Courtesy of HearingReview.com

The question arises whether this vent would ever be useful, or is it just one of those things to be avoided? Many vocalists report that their vented musicians’ earplugs actually allow them to hear themselves slightly better in the background of the music while singing.  And others report that a completely occluding musicians’ earplug that is shorter, thereby allowing a significant occlusion effect to occur, can be better for monitoring of one’s own voice.

Courtesy of bdeatech.org

Courtesy of bdeatech.org

Here we have an experiment ready to be run, and actually it would make for an interesting Capstone paper for an AuD student. To improve monitoring:

  1. Can the occlusion effect be beneficial, and if so, to what extent?
  2. Can amplification from a narrow inertance related resonance do the same thing?
  3. Which is better, and for whom?

In other words, even though we do know that some low frequency enhancement can positively improve monitoring, what is the nature of that enhancement?

Low frequency wide band enhancement caused by the occlusion effect. Courtesy of earmolds.info

Low frequency wide band enhancement caused by the occlusion effect. Courtesy of earmolds.info

The occlusion effect is well-understood and can easily be measured in the clinic using a probe tube microphone device. It results in a low frequency enhancement as the vocal sound is transduced through the bony medial portion of the outer ear canal.  This low frequency enhancement will only be heard if the ear canal is sufficiently occluded (to trap the longer wavelengths in the ear canal). Much of this is explained in Patty Johnson’s excellent guest blog.

An acoustic inertance results in a single resonance somewhere in the lower frequency region but is not as widespread as is the occlusion effect. Would an inertance be just as good as a monitoring tool and the wider spread occlusion effect?  If a musician can get away with a small resonance rather than a widespread low frequency enhancement, this may improve comfort with less downside.

There are more questions than answers here, but the results of such a proposed capstone will have direct clinical usage, and of course make the lucky AuD student rich and famous… or at least famous…

 

In part one of this blog series, the benefits of having a small 1.4 mm (1/16”) vent drilled into a musicians’ earplug were discussed. Apparently this question was first addressed by Shakespeare himself, but later addressed by more modern Bards.  In the original form, he wrote “To vent, or not to vent/That is the question…”, but of course his editor later changed it to the more boring existential question, “To be, or not to be…”

Shakespeare was the firsts to talk about vents

Shakespeare was the firsts to talk about vents.  True story!

A musician, who was fit with a musicians’ earplug and a 1.4 mm diameter vent, felt that there was an unacceptable low frequency resonance, and he wondered if this was due to the presence of the vent. In my usual clinical genius, I simply said “I have no idea”, but did use some Kleenex to fully plug up the vent.  I asked him to walk around, go back to work, and call me if this solved the issue.  An hour later he called and said it was perfect. He returned the molds to me and I have sent them back to the lab to have something more “professional” and more permanent than Kleenex installed.  (Toilet paper would also work, but I happened to have a box of Kleenex sitting right there).

So what was happening?

I immediately went to my acoustics dictionary and looked up the word “inertance”. (I do this whenever I can’t figure something out, and usually just say that “it’s probably related to an inertance”.  You should try saying that- it will make you sound smart!)

Inertance

In acoustics, an inertance is a mass of air that moves together as a single (or lumped element) mass. We see this in Helmholtz resonators all of the time, and an inertance is formed in a pop bottle by the mass of air vibrating in the neck of the bottle as you blow over it.  Inertances are always low frequency since above 2500 Hz air tends to become compressible and it cannot move together as a single unit.  That’s why in speech acoustics, Helmholtz resonances are only found for the lowest two formants, F1 and F2.  Any higher formants need to be wavelength related.

But back to earmold acoustics or earplugs with vents. The mass of air in the vent acts like a single vibrating mass; in earmold acoustics we used to call this a “vent associated resonance”.  This inertance actually can create a resonance which can be noticeable by some listeners, especially if they have good low frequency hearing thresholds.

Effect of a vent, "followed" by a small resonance called an inertance where the mass of air in the vent oscillates as a single unit. Courtesy of HearingReview.com

Effect of a vent, “followed” by a small resonance called an inertance where the mass of air in the vent oscillates as a single unit. Courtesy of HearingReview.com

Robyn Cox was the first to introduce me to this word in her 1979 monograph on earmold acoustics. This is an amazing monograph and there is nothing better than sitting down with your favorite wine or beer and spending an hour or so reading through it.  Dr. Cox also came out with a simple formula to calculate the resonant frequency of the vent associated inertance.

Resonant Frequency = 5500 Hz ( cross sectional area/L0Ve)1/2

Actually, in the equation, it didn’t really say “LoVe” but its equivalent.  The Ve is the equivalent volume of air that is trapped between the end of the earplug and the eardrum, and L0 is the Length of the vent…. I just put the subscript “o” in there because I am in a romantic mood, and it makes it easier to memorize.

Dr. Robyn Cox, who among many other things, wrote an amazing monograph in 1979 called "Acoustic Aspects of Hearing Aid-Ear Canal Coupling Systems".

Dr. Robyn Cox, who among many other things, wrote an amazing monograph in 1979 called “Acoustic Aspects of Hearing Aid-Ear Canal Coupling Systems”.

In any event, this tells us that the wider the vent is (the top of the equation), the higher the resonant frequency or inertance.  And, the longer the vent (bottom of the equation), the lower the resonant frequency or inertance.  So, to make a very low frequency inertance, one needs a narrow, long vent.  Believe it or not, this can be very useful to help a vocalist hear their own voice in a noisy musical background.

So, back to the musician that I talked about in part one of this blog series; the vent that I had drilled created a low frequency resonance that was just too much for him.  Plugging the vent, in his case, resolved his problems and like all happy clients, he vowed to name his first born son after me.