[Scott Dorsey]

Roy W. Rising wrote:

I think this oversimplifies the pressure vs. velocity subject. I've just
reviewed Howard M. Tremaine's discussion of mics in his Audio Cyclopedia.
In *every* case he refers to ribbon-velocity mics as contrasted to pressure
responding condenser and dynamic types. However, he acknowledges there
certainly are bi-directional condenser mics ... without explanation.

Nope, it's not oversimplified at all. There are bidirectional ribbon
microphones, and they are velocity microphones. Imagine a microphone with
two diaphragms, one on either side of the cartridge, and a perforated
stator between them. Air blows in one direction, and one diaphragm is
pushed toward the stator, while another is pulled away from the stator.
The degree depends on how fast the air is moving and is independant of
the air pressure at the capsule. If the air pressure at the capsule
changes, both diaphragms move the same amount with respect to the stator.

Generally, directional patterns are accomplished by controlling the phase
of sound pressure reaching the back of the diaphragm. The Altec 639 summed
the outputs of forward facing dynamic and ribbon elements in one of the
very few attempts to use the 90 degree phase difference between pressure
and velocity.

Right, although again you can look at dual-diaphragm capsules as being
something different... they can be a pressure capsule if you look at
the charge between diaphragms, or a figure-8 capsule if you look at the
difference between the charge between each diaphragm and the stator.

You can use them as a cardioid by combining the two.

On another note, the EV RE15 and RE20 are examples of directional mics with
*very* little frequency response change with respect to direction.

They both employ some really ingenious tricks. Really, really ingenious.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."