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.
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.
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:
- Can the occlusion effect be beneficial, and if so, to what extent?
- Can amplification from a narrow inertance related resonance do the same thing?
- 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?
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…
