Seems to be a very common thing. Anyone know why? Why not 6 or 14kHz? Is it diaphragm material or plastic that resonates at that frequency?
I remember Tyll explaining the peaks/valleys in that region as being resonances between the driver and the ear, the concha, and inside the ear canal itself. While the driver is could be putting out a flat frequency, the resonances make the measurements appear as if it isn't. Apparently minor position shifts on the dummy head can cause quite a bit of variability in those frequencies as the position of the ear relative to the driver has shifted, thus altering the resonance.
I think all this info is in one of the innerfidelity articles if you look at the back catalog.
My guess is that it gives a little bit of sizzle at a frequency that young and old can hear. If you had most people compare a flat response headphone to one with a bump at 10k I bet most would think the bumped one sounded better. Consumer speakers often have the same thing.
No idea why, but that has been one of my biggest pet peeves since I got into headphones. Glad somebody else noticed, haha. I always have to EQ those spikes down if they are there.
Also, I've noticed that the spike is usually centered around 9kHz.
They're already spaced properly. The graphs account for musical octaves. As for why not 6khz, I thought it was a common practice among the audiophile headphones sennheiser offers to peak at 6khz-- right in the region of sibilance :S