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

In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf
wrote:
In principle you can get two pressure-sensing mics and place them
slightly apart. Then connect so that their outputs 'cancel'. This
produced a setup that outputs the 'differential' in pressure along the
direction between them. So gives a cardiod-ish shape

You've confused me. The easy way to make a cardiod pattern with two mics
is to use pressure and pressure gradient types.

Sorry, I should havbe given more detail. The above jumped over too much.

Two pressure sensors spaced apart will sense the pressures in those two
*different* places. A wave propagating between them takes time to do so.
Hence if you subtract their outputs you can output the spatial differential
of the pressure. However - as per your own comments - this isn't an easy
way to do it. :-)

In fact a 'velocity' sensor is also a type of sensor for the spatial
pressure differential. It works due to the pressures on either side of the
sensor being different. Its advantage is that one bit of material (e.g. a
ribbon) is used to sense both side-pressures so avoids the difficulty of
having to pressure sensors which may have different sensitivities, etc.

So a 1st order pressure pair will give something like a dipolar pattern.
More than three pressure elements can give more like a cardiod. And a
'velocity' sensor can take the place of two pressure elements in the array.
Hence the "pressure and pressure gradient" method.

If, like Phil, you're puzzled by this, have a look at:

Directional Microphones H. F. Olsen
JAES October 1967 (Also in the old 'Microphones' book of reprints by AES)

as an example. See his first figure where he explains 1st order gradient
mics before going on to look at end-fire arrays, etc.

Slainte,

Jim

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. Its main problem is that it has to be a
compromise between cancellation and sensitivity. If the two capsules
are far enough apart to give good differential voltage, they are also
far enough apart to suffer from phase-shift problems - when the
distance is a half wavelength, they will reinforce instead of
subtract.

d