On Sat, 23 Mar 2013 12:57:16 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Sat, 23 Mar 2013 12:08:26 +0000 (GMT), Jim Lesurf
wrote:

In article , Dave Plowman (News)
wrote:

I think what you are describing in the first part is the "distance
cancelling" microphone. It responds to the pressure difference between
two capsules, hence responds strongly to close sources, and very little
to more distant ones.

That's a different effect to the one I (and Olsen, etc) was referring to.
he goes on to use this to explain the construction of highly directional
end-fire mic arrays. I was just pointing out you can do this, but getting
it right isn't easy.

The point is there is a time delay required for a sound wave to propagate
from one sensor element to the other. That leads to a pressure difference
given a finite wavelength, even for a plane wave which has the same peak
amplitude in both locations.

In effect, it is just like using a phased array in RF to sculpt a required
pattern of directionality.

Slainte,

Jim

Ah, OK, I see what you are talikng about. Sennheiser use a tube in
front of the mic with a slot down the side. The tube is lightly damped
so it doesn't resonate. Anyway, the idea is that sound coming from
boresight enters the tube all the way down the slot in phase
(travelling, but in phase) and reinforces at the capsule. Any other
angle and there is sufficient length of tube for cancelling wavefronts
to enter, and reduce the level at the capsule. they manage a pretty
good shotgun this way,.

d