How would one go about making a condenser mike with a bidirectional pattern
using only one diaphragm?

Norm Strong

[Don Pearce]

On Tue, 6 Feb 2007 09:28:52 -0800, wrote:

How would one go about making a condenser mike with a bidirectional pattern
using only one diaphragm?

Norm Strong


You make it with a porous backplate, so that there is a limited amount
of sound transmission through it. That will give it a partial response
to pressure, and a partial response to velocity. You can't get the two
to match though, so you need to spend your development money bolting
on tubes and cavities to try and get an equal presure/velocity
response over as wide a range of frequencies as possible.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Don Pearce]

On Tue, 06 Feb 2007 17:38:14 GMT, (Don Pearce)
wrote:

On Tue, 6 Feb 2007 09:28:52 -0800, wrote:

How would one go about making a condenser mike with a bidirectional pattern
using only one diaphragm?

Norm Strong


You make it with a porous backplate, so that there is a limited amount
of sound transmission through it. That will give it a partial response
to pressure, and a partial response to velocity. You can't get the two
to match though, so you need to spend your development money bolting
on tubes and cavities to try and get an equal presure/velocity
response over as wide a range of frequencies as possible.

d

Sorry - that's for a cardioid (didn't read your post properly). For a
bidirectional you make the backplate from mesh that is as transparent
to sound as you can make it. You'd be surprised just how open you can
make it and still retain full electrical capacitance.

d

--
Pearce Consulting
http://www.pearce.uk.com

"Don Pearce" wrote in message
...
On Tue, 06 Feb 2007 17:38:14 GMT, (Don Pearce)
wrote:

On Tue, 6 Feb 2007 09:28:52 -0800, wrote:

How would one go about making a condenser mike with a bidirectional
pattern
using only one diaphragm?

Norm Strong


You make it with a porous backplate, so that there is a limited amount
of sound transmission through it. That will give it a partial response
to pressure, and a partial response to velocity. You can't get the two
to match though, so you need to spend your development money bolting
on tubes and cavities to try and get an equal presure/velocity
response over as wide a range of frequencies as possible.

d

Sorry - that's for a cardioid (didn't read your post properly). For a
bidirectional you make the backplate from mesh that is as transparent
to sound as you can make it. You'd be surprised just how open you can
make it and still retain full electrical capacitance.

Would you use the same mesh on both sides in order to get perfect symmetry?

Norm

[Don Pearce]

On Tue, 6 Feb 2007 09:50:12 -0800, wrote:


"Don Pearce" wrote in message
...
On Tue, 06 Feb 2007 17:38:14 GMT, (Don Pearce)
wrote:

On Tue, 6 Feb 2007 09:28:52 -0800, wrote:

How would one go about making a condenser mike with a bidirectional
pattern
using only one diaphragm?

Norm Strong


You make it with a porous backplate, so that there is a limited amount
of sound transmission through it. That will give it a partial response
to pressure, and a partial response to velocity. You can't get the two
to match though, so you need to spend your development money bolting
on tubes and cavities to try and get an equal presure/velocity
response over as wide a range of frequencies as possible.

d

Sorry - that's for a cardioid (didn't read your post properly). For a
bidirectional you make the backplate from mesh that is as transparent
to sound as you can make it. You'd be surprised just how open you can
make it and still retain full electrical capacitance.

Would you use the same mesh on both sides in order to get perfect symmetry?

Norm


Ideally yes, but in practice that would make the capsule very hard to
assemble.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Scott Dorsey]

wrote:
How would one go about making a condenser mike with a bidirectional pattern
using only one diaphragm?

Diaphragm is suspended between two perforated plates. The rear one is the
stator for the polarization. The front one is used to make the pattern
symmetric.

BUT, because there is static attraction between the stator and the diaphragm,
in order for the pattern to be symmetric, the hole patterns on the stator and
the front baffle plate need to be different from one another.

Making this actually work is left as an excercise to the student, but at
least two manufacturers have successfully done so.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce]

On 6 Feb 2007 13:14:41 -0500, (Scott Dorsey) wrote:

Diaphragm is suspended between two perforated plates. The rear one is the
stator for the polarization. The front one is used to make the pattern
symmetric.

BUT, because there is static attraction between the stator and the diaphragm,
in order for the pattern to be symmetric, the hole patterns on the stator and
the front baffle plate need to be different from one another.

Don't follow that last bit. Could you expand a bit?

d

--
Pearce Consulting
http://www.pearce.uk.com

[Scott Dorsey]

Don Pearce wrote:
On 6 Feb 2007 13:14:41 -0500, (Scott Dorsey) wrote:

Diaphragm is suspended between two perforated plates. The rear one is the
stator for the polarization. The front one is used to make the pattern
symmetric.

BUT, because there is static attraction between the stator and the diaphragm,
in order for the pattern to be symmetric, the hole patterns on the stator and
the front baffle plate need to be different from one another.

Don't follow that last bit. Could you expand a bit?

The diaphragm is pulled toward the stator by the polarization voltage. To
make the system symmetric, therefore, there has to be some corresponding
asymmetry in the two plates.

Sennheiser cheats by not using any polarization voltage.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Don Pearce]

On 6 Feb 2007 13:29:19 -0500, (Scott Dorsey) wrote:

Don Pearce wrote:
On 6 Feb 2007 13:14:41 -0500, (Scott Dorsey) wrote:

Diaphragm is suspended between two perforated plates. The rear one is the
stator for the polarization. The front one is used to make the pattern
symmetric.

BUT, because there is static attraction between the stator and the diaphragm,
in order for the pattern to be symmetric, the hole patterns on the stator and
the front baffle plate need to be different from one another.

Don't follow that last bit. Could you expand a bit?

The diaphragm is pulled toward the stator by the polarization voltage. To
make the system symmetric, therefore, there has to be some corresponding
asymmetry in the two plates.

No, still don't get that. The diaphragm is a perfect electric shield,
and field fringing should guarantee that the electric field covers the
diaphragm evenly whatever the hole pattern. If the second stator is
uncharged, there will be no pull, so no electrical effect.

Sennheiser cheats by not using any polarization voltage.
--scott

Indeed - but they still have the same front/back equalization issues
which are of an acoustic nature, not electrical.

d

--
Pearce Consulting
http://www.pearce.uk.com

[Scott Dorsey]

Don Pearce wrote:

No, still don't get that. The diaphragm is a perfect electric shield,
and field fringing should guarantee that the electric field covers the
diaphragm evenly whatever the hole pattern. If the second stator is
uncharged, there will be no pull, so no electrical effect.

Right. there is an electrical charge that is forcing the diaphragm in
one direction. This alters the microphone pattern, not for electrical
reasons but acoustical. The forces on the diaphragm are no longer symmetric
and the diaphragm is slightly bowed outward and no longer parallel to both
plates and precisely centered between them.

Sennheiser cheats by not using any polarization voltage.

Indeed - but they still have the same front/back equalization issues
which are of an acoustic nature, not electrical.

Yes, but it makes them a little easier to solve.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Scott Dorsey" wrote in message
...
wrote:
How would one go about making a condenser mike with a bidirectional
pattern
using only one diaphragm?

Diaphragm is suspended between two perforated plates. The rear one is the
stator for the polarization. The front one is used to make the pattern
symmetric.

BUT, because there is static attraction between the stator and the
diaphragm,
in order for the pattern to be symmetric, the hole patterns on the stator
and
the front baffle plate need to be different from one another.

Making this actually work is left as an excercise to the student, but at
least two manufacturers have successfully done so.

Scott, could you give me the names of the manufacturers and the model
numbers of the mikes?

Thanks,

Norm

[David Satz]

Norman, I know of only four single-membrane figure-8 condenser capsule
designs in current production. Three are small-diaphragm types from
well-known, first-quality manufacturers:

- Schoeps MK 8 capsule for the CMC ("Colette") modular series, or the
CCM 8 compact microphone (an integral unit with the identical capsule
and flat electronics);
- Neumann KM 120 (= AK 20 active capsule + KM 100 amplifier, which has
flat electronics);
- Sennheiser MKH 30 (an integral unit with active equalization built
into the amplifier).

In addition, "Violet Design" offers a single-diaphragm 1-inch figure-8
capsule which is supposed to emulate the RCA 44 sound to some extent.

--best regards

"David Satz" wrote in message
oups.com...
Norman, I know of only four single-membrane figure-8 condenser capsule
designs in current production. Three are small-diaphragm types from
well-known, first-quality manufacturers:

- Schoeps MK 8 capsule for the CMC ("Colette") modular series, or the
CCM 8 compact microphone (an integral unit with the identical capsule
and flat electronics);
- Neumann KM 120 (= AK 20 active capsule + KM 100 amplifier, which has
flat electronics);
- Sennheiser MKH 30 (an integral unit with active equalization built
into the amplifier).

In addition, "Violet Design" offers a single-diaphragm 1-inch figure-8
capsule which is supposed to emulate the RCA 44 sound to some extent.

Thanks for the info. Violet Design doesn't seem like it would be suitable
for me. :-)

Norm