Weird AKG K1000 Isolation Measurements
Frankly, I thought I must have just done something stupid and there was something wrong with the measurements (though I did double-check at the time). So, I remeasured again this weekend...and basically got the same result!
In the plot at the top of the page, the black trace is with no headphones on my measurements head with pink noise playing in the measurement chamber. The plots is somewhat noisy, but it's flat +/-5dB.
The red plot is with the AKG K1000 on the measurement head, and with the ear speakers in the closed, closest to the ear, position. As you can see there's a huge notch at 60Hz indicating about 25dB of isolation. With the ear speakers in ever more open positions, we see ever decreasing amounts of isolation in the notch.
My guess is that were seeing a natural resonance at 60Hz of the K1000 earspeaker, either in part or as a whole, which is somehow anti-resonant with the incoming sound and canceling it out at the ear. You can also see in the plot some very specific peak features shared by all the K1000 curves at about, 120Hz, 170Hz, 210Hz, 270Hz, 310Hz, and a few more. I'll take a shot and guess this may be the grills singing a bit...or, it's some sort of standing wave cancelation between the parallel grills. (Note the effect doesn't lessen as much with angle as the 60Hz notch, which indicates the artifacts are likely from different mechanisms.)
I have my doubts, that's for sure...I'd lay maybe even money on my guesses. Anybody else what to take a stab at it?