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I am surprised you could get acoustic resonance down to 700Hz , maybe quarter wavelength for an open type headphone? That would still have to make it about 12 cm from the base of the earpad to the open end of the earcup, quite a big headphone! Felt and other porous materials effectively lower that resonance (make the earcup feel bigger in terms of acoustic waves propagating, but 700Hz still seems quite low...
I am not an expert here, so please excuse me if I am missing something, but I do not believe that mechanical resonances are only made because of air standing waves in headphones. I cannot really understand the physics behind ortho operation, but every mechanical structure has some sort of resonance. In an ortho headphone there may be no resonances directly from the plastic membrane that produces the sound, but there are other potential sources, like the magnet structure, the ear cups, even the headband.
Anyway, I made an interesting experiment. I plug my HE-500 in my laptop
mic input, using it like a huge microphone. In a quite room I singly hit in various places the ear cups and the metallic grill with a metal knife. I recorded the output with audacity (see attached image). I repeated this many times.
In every hit there is a residual resonance which fades away rather quickly. This resonance has a period of 136 samples. With 44100 samples per second, this makes a frequency of around 325Hz.
With this, rather crude, way I captured the
vibrations of the plastic membrane of the ortho transducer when I hit the ear cup and the metallic grill. Of course this doesn’t necessarily mean that this same resonance exists when HE-500 is normaly used, as a headphone, but it is not unlikely.
So, a relative low frequency resonance probably exists. Maybe this is one of the reasons for the euphonic ‘creamy’ mids of HE-500.