Topic: Boundary Mics - Recommendations?

[John Kelly]

I'm looking to pick up a boundary mic (maybe a pair) for recording some classical music and opera.  I'm not allowed to run a stand up front, not even a small stand, so my options are to pick up boundary mics or stealth it - and I'm not a fan of stealthing. 

Anyone used a boundary mic in that setting?  Any recommendations on a specific model?

[mfrench]

they're not boundary mics, but I use my DPA 4060's in the boundary layer technique, by simply spacing them and taping them to the rear wall of the venue. Depending on the room, I've had some fantastic results.

[DSatz]

I recently started using a pair of boundary microphones for opera recording, which I mix in with the main stereo pair. It makes a huge difference in the clarity and intelligibility of the singing, and I only wish I had started doing it sooner. I may also place the main pair a little farther back from now on, "just because I can."

The performances that I record are given in a church. The first time I used boundary mikes, they were Schoeps BLM 03 Cs, which are virtually invisible and built to resist damage even if stepped on. The sound was lovely, but the orchestra was directly in front of the slightly raised "stage" (altar) area, so I decided that some directivity and ability to aim the mikes away from the orchestra was desirable.

Schoeps makes a small accessory plate called a BLC, with a built-in holder for one of their CCMs or a capsule on a Colette active cable, so I've started using a pair of those with speech cardioid capsules on them. At first I was worried that people could step on the capsules and break them, but then Jerry Bruck gave me the idea of putting a headlight guard on top of the mike (see photo--this is from before I gaffer-taped stuff down, of course). I've also sent this photo to friends at Schoeps to see whether they might not want to come up with their own solution. Anyway, that has worked quite well and it's what I will very likely continue to use.

--best regards

P.S.: Looks as if photos are OK to upload again now, so:

[John Kelly]

they're not boundary mics, but I use my DPA 4060's in the boundary layer technique, by simply spacing them and taping them to the rear wall of the venue. Depending on the room, I've had some fantastic results.

I appreciate it, but I've already recorded from the back with my Senns and the results are less than optimal.  I need to be close, and in order to be close I need to be extremely low profile.  The 4060s aren't a bad choice and I'll definitely consider them, but I think boundary mics may work better in this scenario.

DSatz - I'll check into the Schoeps, thanks!

[mfrench]

While I'd agree with the rear of the room as being less than optimal - I think you'd be surprised with the technique. It produces allot better results than you might think.
I'll very typically fly one pair of mics up front (or closer) and use the boundary pair as a secondary reference. I can't properly express the amount of times that the musical director has suggested loving the rear of hall capture.

boundary mics are omnis used on a boundary - i don't see much of a difference at that level.

[John Kelly]

While I'd agree with the rear of the room as being less than optimal - I think you'd be surprised with the technique. It produces allot better results than you might think.
I'll very typically fly one pair of mics up front (or closer) and use the boundary pair as a secondary reference. I can't properly express the amount of times that the musical director has suggested loving the rear of hall capture.

boundary mics are omnis used on a boundary - i don't see much of a difference at that level.

I just recorded a recital at the rear of the room - the problem I have with it is the vocalist is not mic'ed, so I have to crank the gain from the rear and really ride the levels due to the crowd.  Kind of hard to enjoy the show if I'm constantly worrying about the recording.  From the front I'm pretty sure I can dial in to a comfortable level and leave it throughout the performance.

And the boundary mics I've been looking at (really only a few so far) are cards, not omnis. 

I was thinking about picking up a pair of Sennheiser e912s, but the frequency response graph didn't really impress me much.

[Walstib62]

If I'm not mistaken, boundary mics are neither cards nor omni's. They actually have a hemispherical pickup pattern, which is more similar in practice to an omni depending on placement. The advantage to boudary mics is that the surface they are mounted to becomes the pickup area. The larger the surface, the better the low end response is in particular.

[Walstib62]

mfrench,
Could you decribe how you are mounting those mics in a little more detail? Sounds interesting.
Thanks

[mfrench]

If I'm not mistaken, boundary mics are neither cards nor omni's. They actually have a hemispherical pickup pattern, which is more similar in practice to an omni depending on placement. The advantage to boudary mics is that the surface they are mounted to becomes the pickup area. The larger the surface, the better the low end response is in particular.

I was going to follow up my comment with something quite similar to this. The boundary takes a 360º pickup orb, and reduces it to a hemispherical pattern.

Let me see if I can find pictures; but, I literally take the 4060's, spread them about 4', and take a couple of inches of gaff tape and tape them to the surface, just behind the capsule.
In one venue, I have the ability to utilize the hall design as baffles. The hall design has tall vertical wood slats at the rear of the hall, spaced about 9" apart.  I can keep the mics closer together, and, baffle them from each other by nestling them tightly to the wood baffle slats.
let me see if I have any pictures available.

[John Kelly]

If I'm not mistaken, boundary mics are neither cards nor omni's. They actually have a hemispherical pickup pattern, which is more similar in practice to an omni depending on placement. The advantage to boudary mics is that the surface they are mounted to becomes the pickup area. The larger the surface, the better the low end response is in particular.

Typically, yes.  But the ones I was looking at were actually cardiod.  Odd, I know.

[DSatz]

Walstib, no, actually that's not what boundary layer microphones do. First of all, any pattern of microphone can be used, and its directional pattern will then be cut in half since the other half of it would theoretically lie behind/beneath/beyond the room boundary that it's mounted in/on. So for an omni mike the pattern becomes hemispherical, but that's only to be expected since sound can't reach the microphone from beneath the floor / behind the wall that it's flush with.

Second, if the microphone is mounted on (or within) a large enough, rigid enough surface, its frequency response will be the same as it would be with normal free-field mounting. But it won't have the usual narrowing of its pattern at high-frequencies any more, and it will now be 6 dB more sensitive to direct sound energy and 3 dB more sensitive to diffuse sound energy across the frequency range. The fact that the sensitivity to direct sound rises 3 dB more than the sensitivity to diffuse sound means that the pickup will be somewhat clearer, all other things being equal (which they won't be, since you don't normally record from positions near a floor or wall, do you? I didn't think so).

What you may be thinking of when you mention the (very real) relationship between the size of the surface that the microphone is mounted in/on and the low frequency response is that for flat frequency response down to a given low frequency, the size of the room boundary must be at least half a wavelength at that frequency in both dimensions. Since (say) 55 Hz has a wavelength in air of about 20 feet, you can well imagine that mounting a boundary layer microphone (say) on the lid of a piano is going to cause severe low-frequency losses (although advocates of "PZMs" do that all the time). And it's not just a simple rolloff--there can be peaks and valleys of 12 dB or more at particular frequencies.

So if you're trying to make a full-range recording, you need to be particular about where the microphones are located. For providing reinforcement for the vocalists in an opera recording, though, you don't need (or want) anything below about 80-100 Hz anyway.

--best regards

[John Kelly]

Oh noes!  Post with the Neumann boundary mic is gone. 

[Gutbucket]

John,
If you go the DPA miniature omni (4060/1/3) route, they have a boundary mounting acessory which is basically a tapered rubber disk into which the mic is inserted, which protects the capsule and makes for a smooth transition to the bondary surface.  The disks are lightweight and need to be gaffer taped down.  They are available seperately or as part of the stereo mic'ing kit.  I posted photos if them here a few years ago and will look for the thread.

DPA now makes a dedicated boundary model, which uses the same capsules in a weighted housing of similar dimentions which doesn't need to be taped down unless you choose to do so.

[Gutbucket]

Found those old 4060 boundary mount accessory photos-

top-


bottom-


mic inserted-


top with mic inserted (ignore the cork base and foam ball baffle)-


More photos and usage details are in this post from that original thread.

[John Kelly]

Very nice.  I think that may be the best way to go, that way I'll also have a set of tiny ass stealth mics as well when I don't want to run the Senns.

[John Kelly]

Any idea what the DPA part numbers are for those adapters?

[Gutbucket]

I couldn't find the individual parts listing on the DPA site, but found it referenced in this snipet (As I recall, cost was around $25 each a few years ago)-

The SMK4060 combines two DPA 4060s omnidirectional miniature microphones, Hi-Sens, with a variety of mounting accessories. These include the BLM6000 Boundary Layer Mount, which can be used in conjunction with a 4060 (or any of DPA’s miniature mics) to place the mic on a reflective boundary such as a floor, wall, ceiling, piano lid etc, enabling them to capture the ambient sound of the area. The sound captured from this “pressure zone” has higher sensitivity, clarity and intelligibility compared to the sound captured from the "free air" and serves as a kind of acoustical zoom.

The dedicated boundary mic is the BLM4060.  Same rubber disk as above plus mic, permanently mounted in a larger metal casing with a removable cable.

[Walstib62]

I did a little reading and realized that I was confusing boundary mics and PZM's. Since the OP is wanting to mic an unamplified performance, wouldn't PZM's be better since there is a boost in output signal over that of a boundary mic?

[Gutbucket]

Just another term for the same thing. PZM = Pressure Zone Microphone which is Crown's term for their specific design of omni mics capitalizing on the boundary layer effect. Boundary layer = pressure zone.  Other manufacturer's boundary mounted omnis may look different and be constructed somewhat differently but utilize the same acoustic principle.

[Walstib62]

Apparently, the pressure zone is the area between the mic capsule and the boundary surface when the capsule is parallel to the boundary, pointing towards it. Boundary mics sometimes orient the capsule perpendicular to the boundary, wherein there is no pressure zone. At least that is what is described by Ron Wickersham, who co-patented the PZM design.

[Gutbucket]

The is still a pressure zone there, it is just not effective to as high a frequency. 

The depth of the pressure zone is determined by the frequency being measured.  At low frequencies the wavelenghts are large so the boundarly layer or pressure zone is deep.  The same acoustic principle relates to 'Room Modes' which are active when wavelengths correspond to room dimentions and why speakers sound bassier when placed against the wall or in the corner. 

At very high frequencies the wavelengths are short and the boundary layer or pressure zone is thin.  The speaker acoustic analogy in this case is the 'baffle-step' in the speaker's frequency respose determined by the cabinet dimentions, and explains why soffit mounted monitors must be flush mounted without any gap between the cabinet and the wall surface.  The same principle explains why omni mics become directional at high frequencies and at what frequency they will do so in relation to their diameter.

To effectively harness the boundary / pressure zone effect in a microphone at the highest audible frequencies the capsule must be very close to the surface.  PZMs do that by mounting the capsule a tiny fraction of an inch above the surface facing toward it.  The other  omnidirectional boundary mics mentioned in this thread accomplish that by mounting the capsule flush in the surface itself, pointing out.  Both methods place the capsule fully in the boundary / pressure zone for the full audible range.

Boundary mounted directional mics may use different physical configurations (probably with the capsule perpendicular to the surface like you mention) which would produce response variations up top if their capsule diameter is large in relation to the wavelength of the highest frequency of interest. They are still using the same effect, just less effectively,  up to a lower effective frequency.
 
Still that's not the case with any of the boundary mics mentioned or pictured here so far.

[edit- oops, just went back to check the earlier posts in the thread. What I said above holds true for the Neumann and the DPAs, but it appears from the photo that the Schoeps BLC mount does position the capsule perpendicular to the surface instead of flush in it. Not sure about the dedicated Schoeps boundary mic mentioned but not pictured.  The design tradeoff Schoeps made with that mount was the ability to use existing capsules. I trust their judgement. 

In the case of the DPA mini mic mounts which also use existing capsules, the mics are tiny enough to mount inside the bulge in the rubber boundary surface so the opening on that surface blends seemlessly into the larger surface on which it is placed.]

[Walstib62]

There ya have it.
Thanks

[illconditioned]

The is still a pressure zone there, it is just not effective to as high a frequency. 

The depth of the pressure zone is determined by the frequency being measured.  At low frequencies the wavelenghts are large so the boundarly layer or pressure zone is deep.  The same acoustic principle relates to 'Room Modes' which are active when wavelengths correspond to room dimentions and why speakers sound bassier when placed against the wall or in the corner. 

At very high frequencies the wavelengths are short and the boundary layer or pressure zone is thin.  The speaker acoustic analogy in this case is the 'baffle-step' in the speaker's frequency respose determined by the cabinet dimentions, and explains why soffit mounted monitors must be flush mounted without any gap between the cabinet and the wall surface.  The same principle explains why omni mics become directional at high frequencies and at what frequency they will do so in relation to their diameter.

To effectively harness the boundary / pressure zone effect in a microphone at the highest audible frequencies the capsule must be very close to the surface.  PZMs do that by mounting the capsule a tiny fraction of an inch above the surface facing toward it.  The other  omnidirectional boundary mics mentioned in this thread accomplish that by mounting the capsule flush in the surface itself, pointing out.  Both methods place the capsule fully in the boundary / pressure zone for the full audible range.

Boundary mounted directional mics may use different physical configurations (probably with the capsule perpendicular to the surface like you mention) which would produce response variations up top if their capsule diameter is large in relation to the wavelength of the highest frequency of interest. They are still using the same effect, just less effectively,  up to a lower effective frequency.

OK, speed of sound is approx 1ft/ms, so wavelength at 1kHz is 1ft.  Wavelength at 12kHz is 1in.  If you are 1/2 or 1/4" above a surface, it should be no better than being 1 or 2mm above the surface like the Crown PZM mics.  It should not matter whether the mic points down, forward, or up, as long as the capsule is close to the surface.  Of course a cardioid mic should point forward if we want directionality.

By the way, most mics mounted flush on the surface are called "phase coherent", because I think the PZM name is a trademark.  Paging DSatz or someone else to clear this up.

  Richard

[Gutbucket]

I think the PZM trademark the biggest part of the nomenclature confusion. 

I specifically tried not to use actual measurements in my post becasue I didn't want to go look up and check the actual details, but following Richard's line of thought above, I believe the 'safe' in-the-boundary-layer factor is to locate the capsule less than a 1/4 wavelength from the boundary for the highest frequency of interest.  I'll let you do the math for what that spacing would be for a 20khz wavelength.. or maybe a more reasonable real-world frequency of, say 15khz.

[DSatz]

Hi, gutbucket. You're right that for a pressure transducer, which has only a front sound inlet, the membrane should be part of (i.e. flush with) the room boundary. That's how Schoeps' three types of dedicated BLM capsules or microphones are arranged--see my little photo montage below. But that approach can't be used with pressure-gradient capsules, since they require a rear/side sound inlet along with the front inlet.

So for pressure-gradient capsules the BLC arrangement is about as good as you can get, while for pressure transducers I prefer the dedicated capsules or microphones (personally I own a pair of BLM 03 C, which is basically a capsule with a permanently attached active extension cable).

The "Pressure Zone Microphone" was patented by Ken Wahrenbrock and the patent was licensed to Crown. The patent has long since expired but the name and its acronym "PZM" are still registered trademarks of Crown International. Thus other manufacturers use either "boundary layer" or some other similar term. I'm fairly certain that that term originated with Schoeps. Neumann's "Grenzflächenmikrofon" (as in their GFM 132 microphone) translates as "boundary surface microphone," which may be an even better term.

The Wahrenbrock design, as you said, uses a very small pressure transducer mounted within an "arm" spaced a few mm away from a small, flat mounting plate. The microphone element faces the plate rather than outward. Those design features form the essential basis of the patent, since the technique in general was known for decades beforehand and could not be patented. But those features demonstrably harm the performance of the microphones, especially at high frequencies. To my mind this was the use of someone's design talent to make an already existing technique patentable for purely commercial reasons. Of course it was presented as a great advance, but it was a step backward in terms of objective performance.

--best regards

[Gutbucket]

Thanks for the expert word and clarification!

For anyone interested, I dug around and found a the link to Crown's Boundary Microphone Application Guide: Boundary microphone theory and applications for Crown boundary microphones-  http://www.crownaudio.com/pdf/mics/127089.pdf

It's an interesting read on the topic.

[DSatz]

> It's an interesting read on the topic.

Unfortunately it is also misleading in terms of basic technical facts. Take for example the first drawing in the book--Figure 1 on page 1, which I find to be typical of Crown's entire P.R. campaign. It shows how the path for direct sound is shorter than the path for reflected sound (duh), and how mixing a signal with its delayed counterpart causes comb filtering.

Part "A" of the diagram shows only two paths for the sound, but in reality there would be infinitely many. Whether you place your microphone on a room boundary or not, sound will still bounce off of every other room boundary, creating a more or less continuous range of path lengths. The only reflections that a boundary layer microphone can suppress are the ones that bounce directly off the surface that the microphone is mounted on. And when those reflections reach other room surfaces, they will be reflected right back into the microphone anyway.

Most room reflections will still be picked up, in other words; a boundary microphone reduces reflected sound pickup by only a rather modest amount relative to direct sound--generally about 3 dB. Yet these diagrams and verbal explanations make it seem as if PZMs can fundamentally eliminate path length differences (i.e. suppressing reflected sound by ~20 dB or more, like the null at the back of a good cardioid), and this has misled many naive people into expecting PZMs to compensate for miking distances that in fact are simply too great.

I could go on, but my normal vision is finally coming back after an arduous eye exam this morning, and I want to go watch some "Ad Men" episodes (why didn't I discover that show sooner?). Meanwhile I recommend a 1981 paper, "The Acoustical Behavior of Pressure-Responding Microphones Positioned on Rigid Boundaries - A Review and Critique," which is available from the AES as preprint #1796.

--best regards

[Gutbucket]

Good points. I'll put AES#1796 on my list.