Marc wrote:
I'm searching schematics to build a figure eight mic with Pansonic capsules
(omni).

You can't do this. You cannot measure velocity with a device designed
to measure pressure.

What interest me is how to wire the 2 capsules for balanced output and
how to phantom power the capsules (48v) since I know one capsule must be out of
phase and the voltage must be reduced to 9 volts.

If you put two omnis out of phase at the same location, what you get is no
output at all. If you put them in phase a few inches away, what you get is
a comb filter.

You can sort of get a figure-8 mike with digital delay on one channel, and
summing, but it doesn't work very well at all. There were a couple articles
at the Amsterdam AES show on the subject.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:

Marc wrote:
I'm searching schematics to build a figure eight mic with Pansonic capsules
(omni).

You can't do this. You cannot measure velocity with a device designed
to measure pressure.

Actually you can. The time integral of the difference
between two omni's spaced closer than about a quarter of the
smallest wavelength of interest is the velocity component
along the axis connecting the omni's. This is called an
"intensity probe". The problem in doing it in this fashion
is that it greatly amplifies any self noise of the two
capsules in inverse proportion to frequency because of the
integration required. You can live with this if you are
using such a device, for example, to measure impulse
responses because you can average the noise out with slow
sweeps but it makes a lousy method for sound recording.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

Bob Cain wrote:
Scott Dorsey wrote:

Marc wrote:
I'm searching schematics to build a figure eight mic with Pansonic capsules
(omni).

You can't do this. You cannot measure velocity with a device designed
to measure pressure.

Actually you can. The time integral of the difference
between two omni's spaced closer than about a quarter of the
smallest wavelength of interest is the velocity component
along the axis connecting the omni's. This is called an
"intensity probe". The problem in doing it in this fashion
is that it greatly amplifies any self noise of the two
capsules in inverse proportion to frequency because of the
integration required. You can live with this if you are
using such a device, for example, to measure impulse
responses because you can average the noise out with slow
sweeps but it makes a lousy method for sound recording.

There are some folks who have actually tried this for recording, but the
problem is that it's hard to make the thing reasonably wideband. As I
said before, there are a couple papers in the last AES conference proceedings
about doing this and the response plots they show... well, they aren't very
good off-axis, on top of the noise issue.

For measurement stuff, it's a different story, yes.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:

There are some folks who have actually tried this for recording, but the
problem is that it's hard to make the thing reasonably wideband. As I
said before, there are a couple papers in the last AES conference proceedings
about doing this and the response plots they show... well, they aren't very
good off-axis, on top of the noise issue.

Yes, and those last two issues are inversely related. The
configuration is only good off axis to the extent the omni's
are in fact omni in a frequency independant way. This
dictates very small diaphragms which has the unfortunate
consequence of inherently high molecular shot noise which
goes up with the inverse of the diaphragm diameter. There
seems to be some sort of gotcha with mic design no matter
what direction you go in. :-)


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

I know Paia is making an MS mic. This mic use 3 capsules, one for Mid, and 2
for side. Is this mic not a real MS?

Can I use cardioid capsules to make an eigth mic? In phase, out ot phase? (I
will use this eight mic as a Side mic in a MS system)



Two cardioid microphones back to back out of POLARITY (get the terminology
right) will produce a figure 8. Neumann Km 86 does this with two Km 84
capsules.

Matching of the capsules is extremely important.
Richard H. Kuschel
"I canna change the law of physics."-----Scotty