Boundary microphone advice

[Colin McDonald]

Following on from the discussion in this thread, I now find that I have need of a boundary mic (but not for school nativity plays :-)

The location is the raised and carpeted chancel of a church with a very high roof (mic on stands and overheads ruled out) and a small amateur singing group needs to be part of the induction loop feed, and occasionally the recording feed. It would also be helpful if it could be included in the main PA sound support feed.

The local dealers tend not to keep much stock of better quality boundary mics, so it's special order only (and no free trials). I was considering some of these mics:
Crown PZM-185
Crown PZM-30D/60D
BARTLETT TM-125 - "not recommended for recording" and further down the same website BARLETT recording microphone - "not recommended for PA". (Not sure how to reconcile that one). The latter has less self noise, has a cardioid rather than supercardioid polar pattern and has a better frequency response.

Should I also be considering the offerings from the big names such as AKG, Shure, and Sennheiser?

[Paul R Johnson]

Bruce Bartlett was behind the most successful boundary mic the Crown PCC-160. Used in countless theatres for recording and PA. It was for many years the industry standard. The TL-125 was his own companies new version, and they're really solid, reliable and good sounding mics. In comparison with others I have tried, the Bartlett is the best made. He has a habit of rebranding things when he comes up with new stuff, and has produced a different version specially for recording, and the other renamed as a stage mic. In practice, you could use either for recording, but the stage version is designed to have a little more reach, which helps with amplification, and the frequency response is designed to gently roll off the LF, which is necessary to prevent low frequency takeoff when the gain is pushed. It's also got a bit of a presence peak to help clarity. The recording version, if used for PA would just need the LF rolling gently off - but having both versions is quite a sensible idea. John Willett sells metal 'frames' that can get a small condenser element into almost the right place, close to the floor so your existing mics could be used to take advantage of the boundary effect - they are, however, expensive.

Bruce used to have this video on his website - which I applied a bit of UK humour to for fun.

TM 125 truck rollover - YouTube

Then watch the UK version

blue room microphone - YouTube

[Colin McDonald]

Ho ho!
I've certainly come across a few mics that richly deserve having a van driven over them.

Thanks for your comments, Paul. I feel minded to go for the Barlett recording mic in the first instance (and if the singers can't be heard clearly enough on the PA, then they can jolly well learn to sing properly!)

After reading specs and reviews for a number of other mics, I remain unconvinced that they are as effective for the cost involved. It is rather a specialist area I suppose, and perhaps that is why some well respected manufacturers choose to stay out of it.

[Richard Crowley]

IMHO, boundary mics are a one-trick pony, and the places where you REALLY need that one trick are not nearly as numerous as they would want us to believe. Using boundary mics for reinforcement seems particularly problematic.

It is hard to imagine a situation where you aren't permitted visible mics, yet they use PA, induction loop hearing systems, and recording. As I age, I have decreasing tolerance for that kind of foolishness. I tend to just let them stay back in the 19th century and move on.

[Les Wilson]

Boundary mics are useful when you have actors and vocalists spread in a large area, moving around and individual radio mics are not an option. If the "small singing group" is clustered together and stationary, I use a couple of Shure KSM-137 mics on straight stands. There are other choir mics as options.

[Colin McDonald]

Like I said. No stands and no overhead mics. None of the places I would normally put microphones are able to be used.
Do pay attention at the back! :-)

[Greg Miller]

Originally Posted by Colin McDonald

I now find that I have need of a boundary mic ((snip))

The location is the raised and carpeted chancel of a church with a very high roof

How well do boundary mics work on a carpeted floor? I thought the principal relied on reflections from a very close hard surface. Wouldn't the carpeting significantly change the physics of the "boundary" layer? I would guess that, since the carpeting will absorb, rather than reflect, the higher frequencies, the resulting sound would be LF-heavy. (i.e., the "boundary" effect will continue to boost the lower frequencies which are not absorbed, but will not boost the higher frequencies which are absorbed.)

Of course one could mount the mics on a large sheet of plex, say 24" x 24" or bigger, depending on the desired LF pickup, but that would rather negate the "invisibility" of the mics in the first place.

[Richard Crowley]

Originally Posted by Greg Miller

Of course one could mount the mics on a large sheet of plex, say 24" x 24" or bigger, depending on the desired LF pickup, but that would rather negate the "invisibility" of the mics in the first place.

Perhaps one would get the effect of the floor "boundary" at low frequencies, but the carpet would presumably soak up high frequencies.

I attended an outdoor concert (in Portland Forest Park) where they were using a pair of PZM mics mounted on 3ft/1m square clear plastic boundaries at the top of tall stands. A slight breeze came up and caught one of the boundaries and flew the whole stand into the trees, and caught the other one and sent it spinning/sailing down very close to the audience. It could have decapitated someone if it fell in the audience. Alas, just another ridiculous application of boundary mics where they don't belong.

Its hard to do anything effective in a "microphone-free zone".

[Greg Miller]

Originally Posted by Richard Crowley

Perhaps one would get the effect of the floor "boundary" at low frequencies, but the carpet would presumably soak up high frequencies.

Yes, that's exactly what I fear. So the result would be very boomy/muddy.

I have sometimes been on the receiving end of "those are ridiculous requirements" comments here. (e.g. when a client says, in essence, "it should sound like a pair of U87s but should fit in my pocket.") But in this case, I can't resist the temptation to say, "If they want the advantages of microphones, they need to have microphones." The people wearing the induction loop receivers will know they are wearing them, how can they reasonably object to the other half of the chain?

Admittedly, you might be able to work around the carpeting issue by using the right EQ. At any rate, unfortunately for Mr. McDonald, this is certainly an "interesting" situation.

[John Willett]

There are very few really good boundary mics - eg: Neumann GFM132 and the Schoeps - and they are rather costly.

There is a more versatile option - the Maier Sound "Turtle" - this will convert almost any small diaphragm condenser mic. into a boundary mic.

The Sound On Sound review is HERE.

It protects the mic. in a hard steel "shell" and also shockmounts the mic. with the patented Rycote "Lyre" suspensions.

And when you no longer need a boundary mic., you can unclip the mic. and use it as normal.

[Paul R Johnson]

Carpet actually makes very little difference, because the idea is to place the mic element on the boundary so that any contribution to the audio that the boundary of the area creates is missing - so unless the carpet has long fibres that attenuate sound coming from the direction of the plane, nothing much happens.

Without doubt a decent mic on a stand as close to each performer as convenient is the best way, but very impractical. Reducing mic count means sharing, which means balance problems. Reducing mic count further requires proper internal balance, but it's difficult to get people out of the nulls between mics. Shotguns reach a bit further but comb filtering starts to be objectionable.

Boundary mics are a good compromise - they are low visibility and compared to a cardioid on a short stand, they sound much better. In reverberant spaces spaces they sound better than non boundary types in so much as intelligibility is better.

The turtles John distributes are great products as I mentioned above, but they are not cheap!

Boundaries are great for certain things, better than trying to get decent sound out of other concealed or semi-descrete mics, and much better than hanging lavs from overhead - which work great for tap, but are horrible for voices.

All this said, for video purposes, two or three SM58s on those little 3 leg mini stands work quite well.

If you want to try boundaries, then buy a couple of these.
BM-38 - PULSE - UNI-DIRECTIONAL BOUNDARY CONDENSER | CPC
They are very cheap, very solid and heavy and sound surprisingly good. I've had a couple of these in a theatre for over ten years feeding the loop system and back stage relay, and the quality is not at all bad, and they cost less than thirty quid! While the Bartletts and the PCC-160s before them were dancer proof, the turtles do expose the capsule a small amount, and I'd be worried putting in a decent condenser that could still be stood on.


Richard is quite right in that they are a one trick pony. They work well, on the floor, or stuck to the wall. That is pretty well all they do!

[Greg Miller]

Originally Posted by Paul R Johnson

Carpet actually makes very little difference, because the idea is to place the mic element on the boundary so that any contribution to the audio that the boundary of the area creates is missing - so unless the carpet has long fibres that attenuate sound coming from the direction of the plane, nothing much happens.

Mr. Johnson, I certainly don't mean to disagree, but with your indulgence I would like to ponder this a bit further.

I recall reading that when the mic is at the boundary, there is a 6dB increase in output level, supposedly because the direct sound and the reflected sound now arrive at essentially the same time, in phase, and the sum of these two waves results in a two-fold or 6dB level increase. (Does everyone agree with that?)

Given the above, I'd think that placing a boundry mic in the middle of a carpeted area (such as Mr. McDonald's carpeted chancel) would not result in any such increase, within the range of frequencies where the carpet is absorptive... i.e. frequencies where there is no significant reflection. If you tend to think I'm wrong about this, would you please have another go at explaining why, as I'd like to understand this better.

[Rick Reineke]

I used to have a pair of Crown PCC-160 and used them on grand pianos when the lid had to be closed.. it always sounded very good IMO.. and Isolation was a bonus.
See photo: UCC Synod 24, Minneaplis, MN
I would suspect Bruce's 're-issues' sound good as well.

[Colin McDonald]

After a bit of digging about, I found this PDF on Boundary mics from ElectroVoice
and it has this to say at the bottom of page 2 on the subject of carpets and boundary mic placement:

WHAT EFFECT DOES CARPETING HAVE ON THE PERFORM-
ANCE OF A BARRIER MOUNTED MICROPHONE?
Since the strength of the reflected wave determines the additional
sensitivity, anything that reduces the reflected wave energy will
reduce the sensitivity of the microphone. Carpeting or acoustic tiles
tend to reduce the intensity of the higher frequencies; therefore,
the output from a barrier microphone on a carpeted floor will have
a marked reduction of high-frequency sensitivity.

Fair enough, but what I'm wondering is that a mic such as the Barlett recording mic comes mounted on a steel plate (13 cm long x 7.6 cm wide) so will that provide some offset for this HF loss?
For example, by my calculations, a 5KHz sound has a wavelength of .06864m so the steel plate might still be able to reinforce sounds of that kind of pitch or above.

[Warren Kawamoto]

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