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diaphragm output & phase: HC selectivity ... and the lateral organ of the fish

Thanks Steve. I didn't mention ribbon mics, and I was referencing the signal post diaphragm and before preamplification inside the microphone case.

The old AKG 414 didn't have DC decoupling as far as I could tell. (One of the selling points of the Neumann U-87 was that it was transformer decoupled, blocking DC, and adding low frequency phase distortion to the output signal, I was told.)

The phase of the output of a condenser is 90 degrees from that of a dynamic, one being velocity, one being displacement. This would imply that it would be possible to measure displacement of the diaphragm, and therefor possible to measure (near) DC values

Somehow I haven't found out about the phase relationship of the driving force of the stapes and the output of the OC hair cells. The piezo is based on bending forces changing the resistance of the crystal, so a DC signal could probably be measured.

I thought I had read that the cochlea was an evolution of the lateral organ of the fish which was filled with a non-compressible liquid. Underwater, a large object, such as a whale, would have produced a very long wavelength 'pressure zone' wave (displacement), and this would 'bend' the lateral organ, and the fish would "hear" the presence of this enormous enemy. When we moved from water to air, the system added an impedance matching interface (eardrum and middle ear), but, if I understand partly from this discussion, the inner ear is still a salt water pressure organ.

I've been on the tips of my toes in the cochleal model discussion and thank all of those who have shared their knowledge and points of view.

Best

Kevin




Date:    Fri, 5 Oct 2007 13:24:25 +0100
From:    Steve Beet <steve.beet@xxxxxxxx>
Subject: Re: HC selectivity ...

Hi Kevin,

It's not really a question of dynamic vs. condenser (or electret), it's a question of velocity vs. pressure microphones. Ribbon microphones (as an
example) fall into the category of dynamic microphones, and their output =
is proportional to the velocity of the pressure wave, as you were implying.
However, "normal" dynamic microphones (i.e. those with a diaphragm, enclosed
on one side) produce voltages proportional to the pressure itself, at least
over the design frequency range. Thus they're essentially the same as
condenser/electret microphones (it's just that condenser microphones work
over a more extended frequency range and have a better phase response).

Also, if you do get DC out of a condenser microphone, it's only because it's broken! In fact, most condenser microphones require a decoupling capacitor
in the audio path to get rid of the DC bias applied to the "plates" of the
capacitor. This limits the low-frequency response, quite apart from any other factors.

Steve Beet