The effect of acoustic dampers on IEMs

I thank Knowles electronics for the support.

On most of balanced-armature transducer driven IEMs available in the market, there is a tiny screen implemented at the opening of the acoustic output port. This is an acoustic damper, and it serves two purposes:

1. The filter blocks off external materials from getting into the transducer, thus preventing damages to occur deriving from such causes.
2. The filter has an unique resistive value, and acts as a passive acoustic filter mainly reducing the first peak resulted by the mechanical resonance at f0. 

"The bump at 2 kHz is primarily caused by the mechanical resonance of the receiver: the mass of the diaphragm and springiness, or compliance, of the diaphragm combined wit the compliance of the air inside the receiver." H. Dillon, Hearing Aids, Boomerang Press, Turramurra, Australia, 2001. 

Their rated performances have been demonstrated by one of the manufacturers, but there has not been a single instance of which an actual IEM has been utilized. So Etymotic Research ER-4B is used to find out how the filters help shape the frequency response of an IEM.

With stock acoustic dampers:

With conventional acoustic dampers:

The first two peaks are definitely being controlled, and especially when the acoustic resistance reaches 2.2k ohm.  After this experiment, I changed the stock green filters (1.5 k ohms) on my ER-4B to orange ones, which are 3.3 k ohms. It's only flatter, with increased high-frequency bandwidth!

And here is the conclusive analysis of the experiment with ER-4, relative level differences plotted all at the same time. Click the graph, and it will get bigger.

Since the damper can turn ER-4B into ER-4S with broader frequency bandwidth, maybe vice versa, or even evolving ER-4P into other two might be possible without using an electrical filter. And that will be the topic of my next article, it seems!