|Two upper rows: made by DST SWISS|
Two lower rows: made by ACS UK
Even though Dr. Craig Kasper of
The peaks are located at 7.4 kHz and 8.5 kHz respectively, which means, if the bore lengths were decreased by 4.6 mm on the left and 3 mm on the right(with the same insertion depth), or the insertion depth was increased by 10 mm on the left and 7 mm on the right would've pushed the peaks to the frequency range above 10 kHz, and simply solved the problem.
There is, however, a way to compensate the insertion-depth induced resonance shift, according to Mr. Don Wilson, the mastermind behind the design of ER-4 (provided by Mark, who is a regular visitor of my blog. Thank you!):
"You can remove the filter from the ER-4 and place it 3mm from the end of the earmold and the response will be good. As long as the hole from the end of the driver to the end of the earmold is 2mm the filter should fit good. Part of the purpose of the filter is to terminate the tube removing any resonances."
According to Dr. Harvey Dillon,
"...Dampers are most effective if they are placed at locations where the resonance causes the largest flow of air particles. For wavelength resonances, particle velocity is least at the end of the tube that joins the receiver..."What we must understand here is that, such notion of acoustic damping is indeed applicable in hearing aid acoustics, of which the required bandwidth is quite limited, but is not applicable in high-fidelity IEM acoustics; The result of Mr. Wilson's damper location adjustment simply cuts away the bandwidth above 8 kHz, leaving ER-4 further crippled in fidelity.
H. Dillon, Hearing Aids, Boomerang Press, Turramurra, Australia, 2001.