@half_dog I wonder how some high-end, shallow insertion models tone down that specific "8k" peak so well...
Do you reckon it's general tuning or specifically preventing the resonating factor in the ear canal by physical design?
I mean, is it even possible to prevent the resonance, or will it always have to be resolved by reducing the overall level of that frequency response to compensate for it?
IEMs aren't my thing, so there's only so much I can offer on this topic. Most of half_dog's comments seem to be right on point though.
The ear canal is shaped essentially like a tube, or long funnel. So it follows more or less the acoustic and harmonic principles of a tube or cylinder as described here...
en.wikipedia.org
The precise resonant frequencies of a human ear canal are determined by its
length. Which, in the case of an IEM, would also be effected by its insertion depth.
An
unblocked ear canal of average length will resonate at harmonic frequencies of about 3k, 8-9k and 15k Hz, as shown below. And this resonant behavior is a major contributor to the peak or ear gain that occurs in the upper mids/lower treble when doing in-ear measurements of a headphone (or other sound source) at the ear drum reference point, or DRP...
The other major contributor to that ear gain region is the concha, which is the small bowl-shaped part of the pinna at the entrance to the ear canal...
The first, second and third harmonic resonances of the ear canal can sometimes be seen fairly distinctly in the raw in-ear measurements of better sounding headphones, made on measurement rigs that do a good job of simulating the acoustics of a typical human ear. Some recent examples of this made on the new HBK 5128 HATS rig...
You can even see the same three resonant peaks in the HBK 5128's measured response to a completely diffused external sound field (sans headphones), which is represented by the dashed purple curve on the Sennheiser HD-650 plot above.
So yes, these resonances are actually
in the measurement rig! And are a perfectly normal part of its acoustic behavior, with certain kinds of sound sources... Even ones which are
highly diffused. Go figure.
When you use an IEM though, most of this normal acoustic behavior of the ear goes
out the door. Because there's no gain from the concha (or other areas of the pinna), and the normal harmonic resonances of the ear canal are changed, because the canal is blocked at both ends, and has been shortened in length by the IEM's insertion depth (as half_dog explained above). This means that you may
lose some resonances that you want or need for normal sound reproduction inside the ear. And you may
gain some others that are unwanted, which might unnaturally distort or color that normal sound.
It seems to me that an IEM manufacturer has basically two or three choices in this situation. If they want their earphones to have a neutral response that approximates a good pair of headphones or loudspeakers in a room, then they have to try tune their IEMs to match the measured response of those other sound sources at the ear drum, presumably by artificially boosting some frequencies and damping some others.
Another option is just to leave things as is, and give the user the flexibility to make those adjustments and tune the earphones how they want. Option number three would be some combination of the above.