--- In Walter Knapp <>
wrote (in part):
>
> If your description were true think about how a omni mic picks up
sound
> from it's own shadow. If it could only pick up sound that impacted
the
> mic diaphragm directly due to a line of sight between source and
> diaphragm it would not have a omni pattern. It picks up sound from
> behind because pressure is equal in all directions.
Yes, think about it some more. The microphone is immersed in the
acoustic medium (air), such that a passing acoustic pressure wave
effectively wraps completely around it almost as if the microphone
were not there. One of the great challenges of microphone design is
to make that "almost" as complete as possible across the entire
spectrum of interest. This is especially difficult at wavelengths
short compared to microphone dimensions, and various types of side-
and rear-porting are typically used to flatten performance as a
function of direction and wavelength.
This discussion pertains to pressure gradient microphones, which are
typically what we all use in the field. (I apologize to any users of
velocity mikes who may feel left out.) The small volume of air
trapped between the diaphragm and the backplate rests at ambient
static pressure. As an acoustic wave surrounds the mike, the
instantaneous pressure on the exposed side of the diaphragm
oscillates around this static pressure at the frequencies of the
sound and at amplitude proportional to the loudness of the sound.
The diaphragm is faced momentarily with unequal pressure on its two
sides and, being compliant, moves in or out to compress or expand the
air volume behind the diaphragm to equalize the instantaneous
pressure. This induced motion of the diaphragm, of course, is what
the microphone transduces to an electrical analog of the acoustic
pressure.
At all wavelengths much longer than the diaphragm's diameter,
regardless of direction of travel, the entire diaphragm
effectively "feels" the same oscillations of acoustic pressure, and
certainly moves in unison. No problem. However, at wavelengths
short compared to the diaphragm diameter, any off-axis excitation
will sweep across the diaphragm, rather than driving it all at once.
The exact response is dependent on the mike's porting, the lateral
compliance of its diaphragm, and probably many other arcane effects.
Actual measurements confirm the leveling-off of gain in the
neighborhood of 10 KHz for most of our reflector systems.
Mathematical "proof" of cause is elusive - not because we can't
calculate the acoustic performance of a parabolic reflector or of a
particular microphone, but because we must model BOTH elements at
once. That's a real challenge!
Good recording,
Randy
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