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Normal breathing in close proximity to a microphone can produce unwanted distortion and "wind noise."
Most audio microphones "roll off" response below 20Hz or so, on the theory that lower tones are inaudible. In fact, studio microphones are often used with a windscreen or "pop filter" to mechanically
remove unwanted airflow.
However, there are reasons why one might wish to detect this very-low frequency airflow, and use it to good effect.
Take, for example, Garrison Keillor of NPR fame. In his weekly monologues about Lake Wobegone, he speaks into a microphone positioned very close to his face, and his nasal breathing sounds are quite prominent. Perhaps he likes this effect, but many people find it annoying.
I propose a microphone with "near dc" response, i.e. to about 0.1Hz, and a processor box which would extract a directional "breath" signal. This signal could be used to modulate the 20Hz and up audio signal in several ways, for instance to turn down the gain when "breathing in" is detected. Various other control paths with non-linear processing could be performed.
Mechanical high-pass filter [csea, Aug 06 2009]
for ref. [csea, Aug 06 2009]
||LF decreases dynamic range as all the other frequencies ride on top of the LF like surfers on a wave. This is also relevant on the microphone side as well as the recording and the playing sides.
||[FT] point taken. So, have 2 transducers internal to the mic; one with normal 20-20k response, the other with 0.1-20Hz. Separate outputs, both phantom powered.
||sure but lowcut filters are more like 75-150Hz.
||Lowcut filters are wherever you design them to be. The reason for choosing 20 Hz (which is pretty arbitrary, anyway) is that the signal of interest ("near-dc" wind noise) would be high-pass filtered around there, anyway, while still allowing the audio signal to contain a fair bit of low bass energy for plosives. It might be a good idea to have separate equalization for audio and control paths.