First measurements in the box.
I think it was Dread who asked about the effect of the chamber walls on open headphone acoustics, where reflections might get back into the cans and they certainly do. Just not a lot.
Here's the FR of Senn HD800 in on the head in different positions in the box:
Here's the graph.
Here's a day later with the acoustic treatment in the box.
Looks like there's a bit less bumps and wiggles between 500Hz and 6kHz
Here's FR measurements of four headphones.
The rise in FR between 2kHz and 5-8kHz is due to the Head Related Transfer Function of the head. By the time flat sound in the outside world reaches you eardrums, there's a peakyness to it, just like the above.
We're gonna talk about this frequency response curve a LOT.
I also looked at pink noise generated in the box both with and without the acoustic treatment.
The foam creates some extra bumps and wiggle from selective absorption and mixing up the modes in the chamber a bit.
With the foam in, I moved the head around the box without any headphones on. Quite a bit of variation.
Then I measured with pink noise in the box and the head in one position with sealed headphones on; then with the headphones off; then with the headphones back on again to see if it was fairly repeatable with replacing the headphones.
You can see the light blue and red trace follow each other fairly well after replacing the headphones on the head.
Otay, let's see if we get low enough noise floor to see the blips in the harmonic distortion spectra. Here it is with HD800s and the door open and closed.
You can see it would have been hard to see the harmonics if there had not been a chamber.
Here's the HD800 against the HD201 --- which of course has much more distortion.
Think the rise between 2.2 and 3.7kHz is from exiting vibrations in the plastic housing.
Looks like the noise floor is low enough to see the needed details.