Boundary microphone advice
 

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?

Re: Boundary microphone advice
 

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.


Then watch the UK version

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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! :-)

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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".

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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!

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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.

Re: Boundary microphone advice
 

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:
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.

Re: Boundary microphone advice
 

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Re: Boundary microphone advice
 

Aha, that's exactly what I suspected. So, unfortunately, this will require a little more thought if Mr. McDonald's mics are located on a carpeted chancel.

I found an interesting paper here: https://pantherfile.uwm.edu/type/www...yMicsStudy.htm
which cites that "4 foot - 40 Hz rule." The formula given for the -6dB point, for a "too small" boundary area, is this: f(-6dB) = 188/D, where D is the side length in feet of a square boundary. Thus for a boundary plate 4' x 4' , D=4, so the -6dB point is 188/4 = 47Hz. (But they round this off to 40 Hz for mnemonic purposes.)

Can we roughly apply this for the dimensions of the Bartlett plate? 13cm = 5.1in = 0.43 ft. 7.6cm = 3in = .25ft. The area of the plate is 0.11 sq. ft. A square plate with that area would be 0.33ft on a side. So for the equation given, the -6dB point would be 188/.33 = 560 Hz.

The transition frequency, where LF rolloff begins, is given as 750/D. With the above dimensions, the mic would be flat down to 2,250 Hz. I would guess that a lot of carpet begins attenuating at a lower frequency than that. But if you make the plate somewhat bigger, say 1ft square, then the transition frequency moves down to 750 Hz, and the -6dB point moves down to 180 Hz. I would guess that's well below any significant attenuation from most carpet.

OK, there are a lot of assumptions and averaging there, but if all of that is true, could Mr. McDonald live with a boundary plate 1 ft square on each mic? It could, of course, be painted in a flat color that closely approximates the color of the carpeting. "Invisible"? No. "Unobtrusive"? I'd say yes. Out of everyone's sight lines? Most surely.


When you stop to think about it, sound technology is lagging far behind picture technology. The picture guys have perfectly silent cameras... why don't we have invisible microphones?

Re: Boundary microphone advice
 

This is a very unscientific comparison between 6 Shure SM-81s against 1 Bartlett TM-125 on a carpeted stage, in a high church of a children's choir concert I shot last night. Raw footage, no color correction or eq applied yet.

Microphone test - YouTube

Re: Boundary microphone advice
 

But *very* much cheaper than a dedicated top end boundary mic. like the Neumann GFM 132 (which was about £2k +VAT when I last looked) and the Schoeps BLM3.

The reason I like the Turtle so much is that you can use any top-end SDC mic. that you already have when you need a boundary mic., and still use the mic. normally when you don't.

Re: Boundary microphone advice
 

Thank you Warren, that was most interesting.The comparison is very useful - I've watched and listened to it a good few times.

Re: Boundary microphone advice
 

If you are using floor mics on a carpeted stage, the high frequencies tend to be absorbed by the carpet, causing a dull or muffled sound. It helps to put the mic on a hard, foot-square panel such as 1/8” thick masonite, plywood, Plexiglas or Lucite. That keeps the sound clear.

Even though the floor mic has its own metal boundary built in, there is still some loss in high-frequency response when the mic is placed on a carpet.

The attached graph shows how the frequency response of a floor mic CHANGES when you move the mic from a hard panel to directly on top of ½” thick carpet. Specifically, there's a 5.5 dB loss at 5kHz and a 4 dB loss at 10 kHz. (The effect varies with the carpet thickness). The high frequencies are diminished, weakening the sibilance (“s” sounds) in the actor’s voices. So be sure to place a hard, thin panel under each mic when the stage is carpeted.

Re: Boundary microphone advice
 

Has anyone used this DPA boundary layer mount? I is meant to work with the DPA 4060/4061 tie microphone.

Re: Boundary microphone advice
 

I'm intrigued by the carpet 'issue' I have never encountered any drop in performance from boundary mics on carpet. Surely the entire point is that having the microphone on the boundary layer prevents refections from the boundary surface being captured by the mic. With the mic in the surface, these reflections cannot happen. So with a table, or a stage - the big reflective surface is effectively removed. Carpet doesn't change this? - it attenuates HF reflections of course, but these reflections don't hit the mic. Carpet also helps footfall noise.

Bruce is the expert, of course - but although the carpeted surface alters the sound in the room, I've never found much impact (if any) on the boundary mic sound.

Re: Boundary microphone advice
 

I have a pair of the DPA mics with the boundary mounts. One of them sort of lives under the lid of my wife's grand piano. It works OK, but I think being so close to the strings gives a more percussive sound which would be fine for some types of music but isn't my first choice for classical. I prefer the sound with an open lid and the mics at least a few feet away, but when that isn't an option for whatever reason I use the DPA.

Re: Boundary microphone advice
 

Note that many (most?) "boundary mics" come with their own reflective surface of some limited size. One could argue that at high frequencies, the mic's own boundary is in play, while at lower frequencies, the wavelength exceeds the mics hard surface diameter.

Re: Boundary microphone advice
 

Mr. Crowley,

That is exactly what's stated in the link that I posted above (post #16). So if the external surface is somewhat absorptive (carpet) rather than entirely reflective (e.g. hard floor) then it won't act as a proper boundary.

Re: Boundary microphone advice
 

Colin , I just noticed this thread and don't know if you are still looking for the boundary mic , noting you started the thread at the end of November ?

Anyway , I have an Audio Technica boundary mic ( can't remember the model but will check when I get home ) which I have used for recording interviews and it works quite well ; I also have a cheap and cheerful Realistic PZM . At work we have a Shure boundary mic , again I will need to check the model but it is very similar in appearance to the Audio Technica .

If you are still needing one of these you are very welcome to try one out .

PS , what about a miniature rifle mic such as the C747 or suchlike - almost invisible and might just work pointing up at an angle from the floor ?

Edit - just checked and the Audio Technica is an AT-851a , it comes with the AT 8531 power module which has XLR output .

I also have an AT-935 lectern mic on a short gooseneck ; this again is a miniature rifle mic and fairly inconspicuous , being about the size of a pencil and black , with the AT 8533 phantom powering module .

You are welcome to try either of these if you think they might work for you ?

Just send me a PM if you want to try them .