I found some boundary microphones

[Brian Gaff]

What exactly are they, and what is the point of them? The instructions say
put them on the walls and they record a much less reverberent sound. Well
having tried this. I have to say, oh no they don't. Am I missing something?
These were branded Realistic and were Electrets, looked like a thick ally
plate with a mound in the middle where at one end the sound went in.
Brian

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[Roderick Stewart]

What you describe sounds like what I know as a PZM, or Pressure Zone
Microphone. They seem to be used a lot in police interview rooms
because they pick up everything, every cough sniff or shuffle that
takes place in the room, though they are not much use for music or
drama for exactly the same reason.

Rod.

[charles]

In article <tjin89$3dgst$1...@dont-email.me>, Brian Gaff

<brian...@gmail.com> wrote:
> What exactly are they, and what is the point of them? The instructions
> say put them on the walls and they record a much less reverberent sound.
> Well having tried this. I have to say, oh no they don't. Am I missing
> something? These were branded Realistic and were Electrets, looked like a
> thick ally plate with a mound in the middle where at one end the sound
> went in. Brian

Reaistic was a brand name used by Tandy.

I've never heard of boudary mics being used forthat. As I understood them,
they were omnidirectional - idealfor zoom meetings since the mic captured
everybody round a table (if the mic was in the middle).

--
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[SimonM]

On 29/10/2022 10:18, charles wrote:
> In article <tjin89$3dgst$1...@dont-email.me>, Brian Gaff
> <brian...@gmail.com> wrote:
>> What exactly are they, and what is the point of them? The instructions
>> say put them on the walls and they record a much less reverberent sound.
>> Well having tried this. I have to say, oh no they don't. Am I missing
>> something? These were branded Realistic and were Electrets, looked like a
>> thick ally plate with a mound in the middle where at one end the sound
>> went in. Brian
>
> Reaistic was a brand name used by Tandy.
>
> I've never heard of boudary mics being used forthat. As I understood them,
> they were omnidirectional - idealfor zoom meetings since the mic captured
> everybody round a table (if the mic was in the middle).
>

The Realistic PZMs were made for Radio Shack /
Tandy by Crown, and are capable of excellent results.

As sold they were unbalanced, but there's a mod
that basically converts them to the electrical
equivalent of Crown's professional version (and
IIRC will run them on phantom power, too).

I used them in one odd task: building a TV studio
that doubled up as a conference hall, the mix
point could either be at the back of the hall, or
in a separate control room. In the latter case I
had a well-spaced pair of PZMs mounted up on the
back wall of the auditorium, feeding into one of
the monitor inputs of the mixer (Soundcraft Delta,
which was quite flexible). It gave an acceptable
idea of what was happening in the room, for
whoever was mixing in the control room to assess
the PA (as long as nobody messed with amplifier
gain etc.). To my surprise it worked rather well.

They are also handy for under pianos, etc. They
have a hemispherical polar pattern, and a good
even frequency response, but because they're
essentially half of an omni mic, they are hard to
apply in real life. They are far less useful as a
spot mic than you might think.

I've got the conversion information somewhere, but
no idea where presently! It must be on the interweb...

There was a DIY equivalent: ElectroVoice sold a
"Mic Mouse" accessory in the 1970s and 1980s. Just
a rounded block of acoustically transparent foam,
that would accept a 635A (half-omni) or a C451 EB
(CK1, half-cardioid). You could theoretically put
them on the front of a stage to pick up theatrical
dialogue or singing, but they also picked up
footfalls rather too well, sadly. Both that and
the PZMs used the boundary effect, however.

[Liz Tuddenham]

SimonM <some...@large.in.the.world> wrote:

[...]
>... They are far less useful as a

> spot mic than you might think.

I made one many years ago and tried it out in various situations such as
sound reinforcement for widely-spaced fast-moving performers* and
recording a small group of performers gathered around in a circle. I
was distinctly unimpressed and came to the conclusion it was either a
solution looking for a problem or simply a sales gimmick.


*A morris side in a vilage square who thought it was funny to smash it
with their sticks at the end of the performance. It was no great loss.


--
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www.poppyrecords.co.uk

[Brian Gaff]

Yes I know they were Tandy. I had one of their three head cassette decks,
one of the best decks I ever had in fact, a name much maligned by many, but
they made good well built nice sounding Hi Fis and many things.
As for the microphones, yes they seemed to pick up even the rustling of
clothing, which was not of much help when trying to make recordings.
I think they were a solution looking for a problem!

Brian

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"charles" <cha...@candehope.me.uk> wrote in message
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[Brian Gaff]

Interesting, but would not some omni mikes have worked much the same? These
I am reliably informed were originally bought for the local talking
newspaper but picked up far too much echo outside noise and clothing rustle.
Does the boundary effect really work?

Brian

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"SimonM" <some...@large.in.the.world> wrote in message
news:tjiven$3efg8$1...@dont-email.me...

[SimonM]

On 30/10/2022 09:12, Brian Gaff wrote:
> Interesting, but would not some omni mikes have worked much the same? These
> I am reliably informed were originally bought for the local talking
> newspaper but picked up far too much echo outside noise and clothing rustle.
> Does the boundary effect really work?

Yes the boundary effect is real, and you can
duplicate it, simply by putting a small personal
mic such as an ECM77 on a flat surface (with
masking tape or similar).

I am fairly certain the hype surrounding the Crown
stuff, was just that - based on copy written by
the sales department.

"Mic mice" are more useful, since you can get half
a cardioid out of them (Crown's design was indeed
a hemisphere) I have never tried a hypercardioid,
but I don't see why it shouldn't work.

Obviously you need some sort of steep HP filter to
eliminate footfalls, but obviously they are handy
for picking up tap dancing (now an almost lost
art, obviously).


The issue is that there's nowhere to put one in
most situations. Never on a lectern! The only
application I can think of is for a stage play or
similar, or a real police interview room. Several
manufacturers made models specifically for that
application, because you can build them right into
a wall, with nothing sticking out at all, making
them less vulnerable. You also sometimes see the
equivalent in the meter bridge of a console.

I have a vague memory of spotting some on "The
Good Old Days", but I think it was a test, as that
show usually had short guns or cardioids hidden
behind set decoration at the front of the stage.
And I think they also had boom coverage too.

[Roderick Stewart]

On Sun, 30 Oct 2022 09:12:07 -0000, "Brian Gaff"
<brian...@gmail.com> wrote:

>Does the boundary effect really work?

Yes, and if you fix one to a wall, it picks up structural noise too.

Rod.

[Mary Wolstenholme]

Like next doors bed creaking. It's too near the wall.

[Liz Tuddenham]

Brian Gaff <brian...@gmail.com> wrote:

> Yes I know they were Tandy. I had one of their three head cassette decks,
> one of the best decks I ever had in fact, a name much maligned by many, but
> they made good well built nice sounding Hi Fis and many things.
> As for the microphones, yes they seemed to pick up even the rustling of
> clothing, which was not of much help when trying to make recordings.
> I think they were a solution looking for a problem!

The pressure-doubling efect should mean they generate 6dB more signal
than the equivalent omnidirectional mic in free space - but unless mic
noise levels are important, this is little advantage.

In theory there should also be a 6dB gain for signals originating in the
neighbourhood of the surface they are attached to, so a group of
speakers around a table should sound 6dB louder than they would with an
omnidirectional mic located further away from the table top. The room
noise and echoes should remain at the original level, so the result
should be a 6dB improvement in the "S/N ratio." of the room.

I have my doubts about whether that effect really takes place to any
noticeable extent - and it would only apply above frequencies where the
wavelength was significantly smaller than the smallest dimension of the
table top.