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Basic acoustics on earmolds (dampers, ports, diameters)?

post #1 of 2
Thread Starter 

[I tried this in another forum, but got no interest.  If this belongs elsewhere, please let me know.]



I apologize if this is a FAQ; honestly I did search for a while....


I would like to learn more about basic acoustics of earmolds, and the effects of adding dampers and ports.  I have tried some basic experiments with knowles dampers and drilling ports, but I really don't know what I'm doing.


My specific issue today is a custom silicone earmold for my bluetooth headset.  The headset was not designed to have such a tight acoustic seal, and so it is both loud and a bit bass heavy (basic occlusion effect).  I have drilled a small port, which helps with the exaggerated  bass effect, and have added a knowles damper, which brought down the volume.  But I am basically just stabbing in the dark, trial by error.  A bit more knowledge would be very helpful.


More generally, I have custom silicone earmolds for a few different IEM I own, and, if I become more knowledgeable, may try to tweak them.


So can anyone help?  I am an electrical engineer and knowledgeable about basic sound and acoustics.  But would rather avoid solving differential equations, if it can be avoided :-)

post #2 of 2

Note that a dynamic driver is mostly unipolar. The vent makes a closed system into a resonant one - the relative volumes of the vent and the interior sounding chamber control the resonant frequency, strength and width of the resonance.


You will also need some driver parameters: https://en.wikipedia.org/wiki/Thiele/Small


There's also another effect in play, diffraction... let's not go into that yet.


Lack of any vents increases general stiffness of the driver, damping it and usually reducing both ringing and high frequency loudness. In your case, the midbass remained while the highs got cut. (Result is as if the compliance in Thiele/Small was reduced.)


Another effect on top of that is the remaining back wave (since the speaker is only mostly unipolar) reflected from the casing.


A damper will decrease the effective compliance of the driver without introducing any resonant effects - almost the inverse of venting.

This is sometimes specified as impedance in Ohms, but should be some other unit. (Pa/s = pressure drift)

Edited by AstralStorm - 10/27/12 at 4:15am
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