Daft idea?: A ribbon boundary mic.

[k brown]

If there's any truth to Crown's marketing claims, a pressure zone microphone (PZM) is distinct from a boundary layer micropone (BLM), which is what this would be. They claim special properties (which I've always taken with a grain of salt) for a diaphragm that's VERY close to a boundary and parallel to it; facing it. BLMs have their diaphragms flush with the boundary, facing out, or at right angles to it (as in this case). Crown claimed the 'pressure zone' was created in the tiny gap between the boundary and the diaphragm facing it; by this definition, a BLM doesn't have a 'pressure zone'.
I think there has been a lot of smoke and mirrors over the years about boundary mics - all they do is remove one reflecting surface from the space it's operating in. If on a floor, floor reflections no longer affect the 'roominess' of the sound, compared to a conventional stand-mounted mic.

 

[Marik]

Pressure meeting velocity messes with the pattern; an angle I hadn't considered. In the cardioid and supercardioid boundary mics, pressure meets gradient and the patterns don't change (other than being sliced in half at the boundary, obviously). Is velocity a special case in this regard?

It is not about the pattern. The principle of mass operated 'true' fig8 system is pressure gradient between front and back of the ribbon, where air particles travel from its front to back (velocity). The forces on the diaphragm double with each octave going up. That's why by tuning the ribbon to the very low frequencies results in a flat response. Also, the shorter the path, the higher cut off frequency. Obviously, for a normal operation the air particles should 'wrap around' the ribbon frame symmetrically from both sides. The arrangement you propose by itself introduces an obstacle on one side, which effectively cuts entire half of the 'wrapping' signal, reducing the forces. As I mentioned above, you will get severe output reduction and poor top frequency response.

The only way to make it work in such arrangement is to load the back of the ribbon with infinite acoustic resistance (for omni, or cardioid operation). The complication will be considerable and perhaps is not worth it.

Best, M

 

[k brown]

Would seem that a figure 8 mic set against a boundary works just fine; Schoeps does it with their MS-BLM. If it doesn't result in a fairly 'clean' 1/2-fig 8 pattern, it wouldn't do MS very well.

It's also done with the KFM 360, which is baffled double-MS.

Some reason it would work with a dual-diaphragm condenser fig 8, but not a ribbon?

The advantages of a figure 8 ('half fig 8') boundary mic certainly don't require that it be done with a ribbon.

EDIT - Shure has already done a purpose-built condenser boundary fig 8 - the Microflex MX395 is available with figure 8 pattern (would love to see what that capsule looks like - doesn't look large enough to contain back-to-back cards). Not Schoeps quality, of course, but proves the concept works. DIY!

 

[mappee]

I don't think Dave Royer is open minded to new ideas. He great at marketing. He took the Speiden ribbon mic, (which was a clone of B&O BM5), and sold more than his predecessors did. Dave knows production and marketing make a winner.
Now as far as PZM ribbon... seems that if you put your RCA 77DX with null to the wall wouldn't that be it?

 

[k brown]

Having to carry a wall around with you wouldn't be very convenient, though. ; - )

In case that was a serious suggestion, it would sound pretty bad, as the edge of the ribbon would be at least 2-3" from the wall, which create a lot of HF combing.

 

[mappee]

Pink Floyd used to carry a wall with them so what's to stop the determined. And a well combed sound may be perfect.
Coif? Quaff? idk

 

[k brown]

To derail my own thread a bit, I'd like to amend a statement I made a couple of posts ago about "all a boundary mic does..." - that's true for when a boundary mic is used on a wall or the floor, but -

Free standing boundary mics (with omni capsules) have the unique property of being omni at low frequencies, and directional at high frequencies - like APEs, but the transition happens at lower frequencies (determined by the size of the boundary plate). Coincidentally (or maybe not?), for Neumann GFM 132-sized boundaries this transition takes place at just about the same frequencies as a human head - from ~350Hz to 1400Hz. You get the smooth richness and extended low end of omnis, plus the articulation and detail of directionals, from a single pair of mics. Graph below shows patterns of an approx. 4" square free standing boundary (for GFM-size boundaries, the freqs will be almost an octave lower).
For this reason, they are very effective in ORTF or NOS-like arrays. The Crown SASS had the foam barrier to make it more mono-compatible, and to allow the boundaries to be aimed at much more forward angles than ORTF or NOS, for added rear-rejection. I assume they considered that would make it applicable to a wider range of uses. The pre-SASS stereo boundary array was known as the ‘wedge’: two boundaries aimed at 110 to120 degrees, with no barrier. Produces a 360 degree sound field very close to a dummy head, especially if done with GFM-sized boundaries, due to their transition to omni occurring at about the same frequencies as a dummy head. A little shuffling improves the LF spread on loudspeakers. Mono compatibility same as a dummy head. My favorite mic for outdoor recording is a ‘wedge’ of GFM-style boundaries fitted with PZM mics; PZMs are more inherently wind-resistant than BLMs, so don’t need such heavy wind-proofing. A bit of foam on the cantilevers, plus lycra-like material stretched over the whole unit is usually all that’s needed. Dead cat only in extreme conditions.

Free field boundary mics are another very useful (and under-appreciated) tool in the shed. Pierre Sprey of Mapleshade Records uses a large PZM wedge as his main array, and modified 'Sound Grabbers' as spot mics. The acoustics he records in are not to everyone's liking, but don't blame the boundary mics for that!

 

[mappee]

Pierre Sprey of Mapleshade Record ***Thanks for hipping me*** That pzm boundary plexi setup looks interesting.
Kinda like Sweiden did with the Billie Jean drum sound, while purpose of his technique was max seperation of HH
snare. Not sure where you're heading with this PZM Boundary thinking but new thoughts are worthwhile and considerable.

 

[k brown]

What do you feel needs further explanation?

 

[mappee]

I'll read thru thread again and see if I get point. It started with approaching Royer about an idea of a ribbon pzm and
took off from there. So is there a conclusion beyond Royer being dis-interested and .......or are we just being daft?

 

[zebra50]

I built a prototype ribbon-floor mic for a customer about about a decade ago, based on a Beyer M260 element.
It didn't sound great and one big problem is that it hoovered up al the magnetic crap from the floor!

 

[k brown]

The main point of my OP was the idea of a figure 8 boundary mic; I focused too much it being done with a ribbon.

For those unaware, the M260 is a hypercardioid.