PZM info

Archived from groups: rec.audio.tech (More info?)

ilClod <me@privacy.net> writes:

> Hello everybody.
> I'm looking for some info about Pressure Zone Microphones: how they
> work, how to design them, how to make some "diy" trial PZM.
> I'd like to discover the boundaty effect in a very practical way and
> would try to build it by myself. Could anyone give me some hints?
>
> Thanks in advance for any help.

Take a look at those links:
http://www.epanorama.net/multi.php?search=search&keywor...
http://www.epanorama.net/multi.php?search=search&keywor...

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

Archived from groups: rec.audio.tech (More info?)

A PZM is nothing but a small electret mic capsule positioned with its
diaphragm as close as possible to a boundary. Any decent electret
circuits (even including some on the www) will work fine. I even get
good room sound from a Panasonic capsule plastered right in the top
corner of the hobby studio, up behind the drum kit - being in the
corner of two walls and the ceiling effectively removed them from the
equation, and made the room sound bigger. In fact any solid wall you
put the electret against will seem to disappear, with the caveat that
it needs to be really close - 1mm is good, maybe a little further is
still OK, but don't go too far or you'll get comb filter artifacts.

On 07 Feb 2005 11:53:35 +0200, Tomi Holger Engdahl
<then@solarflare.cs.hut.fi> wrote:

>ilClod <me@privacy.net> writes:
>
>> Hello everybody.
>> I'm looking for some info about Pressure Zone Microphones: how they
>> work, how to design them, how to make some "diy" trial PZM.
>> I'd like to discover the boundaty effect in a very practical way and
>> would try to build it by myself. Could anyone give me some hints?
>>
>> Thanks in advance for any help.
>
>Take a look at those links:
>http://www.epanorama.net/multi.php?search=search&keywor...
>http://www.epanorama.net/multi.php?search=search&keywor...

Tony (remove the "_" to reply by email)

Archived from groups: rec.audio.tech (More info?)

Il Mon, 07 Feb 2005 22:37:39 +1000, Tony ha scritto:

> don't go too far or you'll get comb filter artifacts.

Tomi: thank you for the links  

Tony: actually, during the weekend I did some more research through the
net, and i found out that there are 2 kinds of boundary microphone (or,
better, this is what I understood  

PZM (Pressure Zone Microphone)
transducer against the boundary, separated by some mm's of air.
Emispherical pattern

PCC (Phase Coherent Cardioid microphone)
transducer inside the boundary, raised some mm's over the boundary.
Slightly directional pattern (IIRC it should be a half-dipole pattern)

Now, from all the pages i've read (i can't remember the sources) i
discovered that, in both kinds of microphone, distance between
diaphragm and boundary is a function of reinforcement frequency and
diaphragm size. I couldn't be able to find more info. How could I
determinate the reinforcement frequency as a function of the diaphragm
diameter? Is that possible?

Am I definitely bound to small size transducers? or there is a way to
work even with 1-inch-wide electrodynamic transducers?

Thanks a lot to everyone who had the patience to answer me :-)

Best Regards

--
il Clod!/
ICQ UIN 97056271

Archived from groups: rec.audio.tech (More info?)

On Mon, 7 Feb 2005 15:57:36 +0100, ilClod <me@privacy.net> wrote:

>Il Mon, 07 Feb 2005 22:37:39 +1000, Tony ha scritto:
>
>> don't go too far or you'll get comb filter artifacts.
>
>Tomi: thank you for the links  
>
>Tony: actually, during the weekend I did some more research through the
>net, and i found out that there are 2 kinds of boundary microphone (or,
>better, this is what I understood  
>
>PZM (Pressure Zone Microphone)
>transducer against the boundary, separated by some mm's of air.
>Emispherical pattern
>
>PCC (Phase Coherent Cardioid microphone)
>transducer inside the boundary, raised some mm's over the boundary.
>Slightly directional pattern (IIRC it should be a half-dipole pattern)
>
>Now, from all the pages i've read (i can't remember the sources) i
>discovered that, in both kinds of microphone, distance between
>diaphragm and boundary is a function of reinforcement frequency and
>diaphragm size. I couldn't be able to find more info. How could I
>determinate the reinforcement frequency as a function of the diaphragm
>diameter? Is that possible?
>
>Am I definitely bound to small size transducers? or there is a way to
>work even with 1-inch-wide electrodynamic transducers?
>
>Thanks a lot to everyone who had the patience to answer me :-)
>
>Best Regards

I've made a few PZMs. I use the best reasonably small electret capsule
I can find, the drill a hole exactly the right size through some
decorative plywood, and mount the transducer absolutely flush to the
wood's surface. A slot along the other side to carry the cable away to
the edge tidily, and it is done. I make them about eight inches
square, and they look just great on a table or the floor. A small hole
near one corner lets me hang them on the wall as an alternative.

They sound - and look - very good indeed. I don't see any reason in
principle why you shouldn't use quite a large transducer - I just
haven't done it yet.

d

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

Archived from groups: rec.audio.tech (More info?)

"ilClod" wrote ...
> Il Mon, 07 Feb 2005 22:37:39 +1000, Tony ha scritto:
>
>> don't go too far or you'll get comb filter artifacts.
>
> Tomi: thank you for the links  
>
> Tony: actually, during the weekend I did some more research through
> the
> net, and i found out that there are 2 kinds of boundary microphone
> (or,
> better, this is what I understood  
>
> PZM (Pressure Zone Microphone)
> transducer against the boundary, separated by some mm's of air.
> Emispherical pattern

Transducer flush with the boundary is reputed to be an
equivalent alternative. But puts the transducer at risk of
damage, so likely the reason Crown, et.al. chose the
alternative configuration.

>
> PCC (Phase Coherent Cardioid microphone)
> transducer inside the boundary, raised some mm's over the boundary.
> Slightly directional pattern (IIRC it should be a half-dipole pattern)

Don't see how you can get a directional pattern without
exposing the backside of the diaphragm?

> Now, from all the pages i've read (i can't remember the sources) i
> discovered that, in both kinds of microphone, distance between
> diaphragm and boundary is a function of reinforcement frequency and
> diaphragm size. I couldn't be able to find more info. How could I
> determinate the reinforcement frequency as a function of the diaphragm
> diameter? Is that possible?

I have seen theoretical discussions of the phenomenon.
You could also grab the Crown, et.al. patents and read
them.

> Am I definitely bound to small size transducers? or there is a way to
> work even with 1-inch-wide electrodynamic transducers?

In the version where the transducer is FACING the boundary,
I believe the larger the transducer, the more space you have
in the "cavity" to produce resonance at an undesirable freq.
At least that is what I recall reading in one of the discussions
from many years ago.

Archived from groups: rec.audio.tech (More info?)

On Mon, 07 Feb 2005 14:55:15 GMT, donald@pearce.uk.com (Don Pearce)
wrote:

>On Mon, 7 Feb 2005 15:57:36 +0100, ilClod <me@privacy.net> wrote:
>
>>Il Mon, 07 Feb 2005 22:37:39 +1000, Tony ha scritto:
>>
>>> don't go too far or you'll get comb filter artifacts.
>>
>>Tomi: thank you for the links  
>>
>>Tony: actually, during the weekend I did some more research through the
>>net, and i found out that there are 2 kinds of boundary microphone (or,
>>better, this is what I understood  
>>
>>PZM (Pressure Zone Microphone)
>>transducer against the boundary, separated by some mm's of air.
>>Emispherical pattern
>>
>>PCC (Phase Coherent Cardioid microphone)
>>transducer inside the boundary, raised some mm's over the boundary.
>>Slightly directional pattern (IIRC it should be a half-dipole pattern)
>>
>>Now, from all the pages i've read (i can't remember the sources) i
>>discovered that, in both kinds of microphone, distance between
>>diaphragm and boundary is a function of reinforcement frequency and
>>diaphragm size. I couldn't be able to find more info. How could I
>>determinate the reinforcement frequency as a function of the diaphragm
>>diameter? Is that possible?
>>
>>Am I definitely bound to small size transducers? or there is a way to
>>work even with 1-inch-wide electrodynamic transducers?
>>
>>Thanks a lot to everyone who had the patience to answer me :-)
>>
>>Best Regards
>
>I've made a few PZMs. I use the best reasonably small electret capsule
>I can find, the drill a hole exactly the right size through some
>decorative plywood, and mount the transducer absolutely flush to the
>wood's surface. A slot along the other side to carry the cable away to
>the edge tidily, and it is done. I make them about eight inches
>square, and they look just great on a table or the floor. A small hole
>near one corner lets me hang them on the wall as an alternative.
>
>They sound - and look - very good indeed. I don't see any reason in
>principle why you shouldn't use quite a large transducer - I just
>haven't done it yet.
>
>Pearce Consulting
>http://www.pearce.uk.com

I've used a similar method as well (actually 300x300x8mm acrylic
sheet, with the edges heavily chamfered and the mic offset from centre
differently in the two directions, just in case that helped minimize
diffraction problems), and it certainly works well. Il Clod also
mentioned having the capsule project a little. I guess that would be
to get the actual diaphragm up to level with the surface, or maybe
slightly further, depending on what's needed to best offset the
effects of the capsule case. The principle is still the same - sensing
right at the boundary.
Tony
Tony (remove the "_" to reply by email)

Archived from groups: rec.audio.tech (More info?)

On Tue, 8 Feb 2005 07:21:05 -0800, "Richard Crowley"
<rcrowley7@xprt.net> wrote:

>"ilClod" wrote ...
>> Il Mon, 07 Feb 2005 22:37:39 +1000, Tony ha scritto:
>>
>>> don't go too far or you'll get comb filter artifacts.
>>
>> Tomi: thank you for the links  
>>
>> Tony: actually, during the weekend I did some more research through
>> the
>> net, and i found out that there are 2 kinds of boundary microphone
>> (or,
>> better, this is what I understood  
>>
>> PZM (Pressure Zone Microphone)
>> transducer against the boundary, separated by some mm's of air.
>> Emispherical pattern
>
>Transducer flush with the boundary is reputed to be an
>equivalent alternative. But puts the transducer at risk of
>damage, so likely the reason Crown, et.al. chose the
>alternative configuration.
>
>>
>> PCC (Phase Coherent Cardioid microphone)
>> transducer inside the boundary, raised some mm's over the boundary.
>> Slightly directional pattern (IIRC it should be a half-dipole pattern)
>
>Don't see how you can get a directional pattern without
>exposing the backside of the diaphragm?
>
Cardioids don't work at the boundary anyway, by definition. They need
a combination of pressure and velocity - at the boundary there is only
pressure.

>> Now, from all the pages i've read (i can't remember the sources) i
>> discovered that, in both kinds of microphone, distance between
>> diaphragm and boundary is a function of reinforcement frequency and
>> diaphragm size. I couldn't be able to find more info. How could I
>> determinate the reinforcement frequency as a function of the diaphragm
>> diameter? Is that possible?
>
>I have seen theoretical discussions of the phenomenon.
>You could also grab the Crown, et.al. patents and read
>them.
>
>> Am I definitely bound to small size transducers? or there is a way to
>> work even with 1-inch-wide electrodynamic transducers?
>
>In the version where the transducer is FACING the boundary,
>I believe the larger the transducer, the more space you have
>in the "cavity" to produce resonance at an undesirable freq.
>At least that is what I recall reading in one of the discussions
>from many years ago.

The transducer facing the boundary thing was the result of a
misunderstanding of how exactly these things worked. It can only
result in unwanted resonances, and will certainly never help the
performance.

d

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

Archived from groups: rec.audio.tech (More info?)

Il Tue, 08 Feb 2005 15:38:56 GMT, Don Pearce ha scritto:

> Cardioids don't work at the boundary anyway, by definition. They need
> a combination of pressure and velocity - at the boundary there is only
> pressure.

Oops... seems i didn't get the right information.
Very interesting anyway. Where could I find some infos about these
working principles? Could you suggest me some reference literature?

> The transducer facing the boundary thing was the result of a
> misunderstanding of how exactly these things worked. It can only
> result in unwanted resonances, and will certainly never help the
> performance.

So the REAL boundary layer microphone is the one whose diaphragm is
flush with the boundary, or at least, a wave fraction (quarter?) raised
up from the boundary itself?
If so, what appreciable effect may a reflected wave have on the
diaphragm? (Maybe i've just asked before, but i'm afraid i don't get
it.)

Or have I misunderstood everything?

If you all have some specific reading to suggest me, i'd be glad to
know them.

Thank you all for your patience.
Best Regards

--
il Clod!/
ICQ UIN 97056271

Archived from groups: rec.audio.tech (More info?)

On Wed, 9 Feb 2005 19:39:48 +0100, ilClod <me@privacy.net> wrote:

>Il Tue, 08 Feb 2005 15:38:56 GMT, Don Pearce ha scritto:
>
>> Cardioids don't work at the boundary anyway, by definition. They need
>> a combination of pressure and velocity - at the boundary there is only
>> pressure.
>
>Oops... seems i didn't get the right information.
>Very interesting anyway. Where could I find some infos about these
>working principles? Could you suggest me some reference literature?
>
Not really, I'm afraid - I tend to work things out from first
principles of acoustics.

>> The transducer facing the boundary thing was the result of a
>> misunderstanding of how exactly these things worked. It can only
>> result in unwanted resonances, and will certainly never help the
>> performance.
>
>So the REAL boundary layer microphone is the one whose diaphragm is
>flush with the boundary, or at least, a wave fraction (quarter?) raised
>up from the boundary itself?
>If so, what appreciable effect may a reflected wave have on the
>diaphragm? (Maybe i've just asked before, but i'm afraid i don't get
>it.)
>
Yes - any distance above the boundary results in a compromise in the
purity of the pressure/velocity ratio. And certainly anything
approaching a quarter of a wavelength will have an effect on response
by comb filtering.

>Or have I misunderstood everything?
>
>If you all have some specific reading to suggest me, i'd be glad to
>know them.
>
Draw pictures of waves, would be my advice ;-)

d

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