Every headphone frequency response curve that I know of starts wobbling frantically above 2,000 Hz. Witness these four:
Why is that? Is it desirable? If a headphone design could get rid of it, would it be better? Or is that wobble what gives each headphone its "signature" or unique voice?
For a while, I thought that perhaps the wobbles were as a result of the vibration modes of the earphone diaphragm. I convinced myself that for a typically large 50 mm membrane at the right tension and spring constant, the various mode patterns would cause peaks at funny multiples of a fundamental around 4,000 Hz.
But then I saw similar behavior for the frequency response curves of IEMs, which of course have diaphragms that are an order of magnitude smaller. The green and orange curves above are for IEMs, while the red and blue are for on-ear headphones... both have that wiggle structure in a similar place, and all have a peak at about 9,000 Hz.
So then I wondered... is it a result of the head-related transfer function (HRTF), which affects the frequency response measured by microphones inside a surrogate head, and perhaps could impose this pattern? Here is the HRTF of a particular head used for measurements...
Sort of looks like that wiggling in the headphone frequency response, but inverted. Even has a dip at 9,000 Hz where the headphone frequency response has a peak.
As I understand it, raw measurements made by microphones in the ears of an artificial head used for headphone measurements are compensated for the HRTF before being plotted. Is that what is happening?
Edited by ruthieandjohn - 5/16/14 at 9:56pm