Topic: can a dual diaphragm mic be easily converted to output each diaphragm separately

[dnsacks]

An off the wall question -- have a studio projects lsd 2 and am wondering whether it would be straightforward to convert it to output each of its 4 diaphragms to a separate channel.  Why?  Since it's my understanding that a multipattern dual diaphragm mic (e.g. Neumann u87) creates the subcard or hypercard patterns by summing a bit of the rear diaphragm into/against the front signal, I'd love to experiment with this in post and create the "optimal" pattern after the fact -- kinda a poor man's soundfield/etc. 

[John Willett]

No, you can't do what you want as you need to double-up on the internal electronics.

However, there are a couple of mics that already do this:- the Sennheiser MKH 800 TWIN and the Gefell UM 930 twin.

The Gefell UM 930 twin is switchable as well as being able to access both capsules individually (so you have the best of both worlds) - the Sennheiser TWIN has no control on the mic. at all..

[SmokinJoe]

This is a feature that interests me as well.  The Sennheiser and Gefell are high end mics, too rich for my blood.  I think if someone did this on a set of of mics costing less than $1000 a pair they might have a nice little niche market that appeals to people besides us tapers.  Anyone who is a mid/side fan sees the benefit of being able to dial things in at post, and this should be similar.  Maybe it's not really as useful in practice, and that's why there isn't a big calling.

[DSatz]

Just wanted to note that the very nearly coincident placement of the front and back diaphragms in a dual-diaphragm capsule is very nice if you want good mono compatibility in a stereo recording--but the two membranes aren't acoustically independent. Each one blocks some sound from reaching the other one, and the amount varies greatly at different frequencies. That's a large part of the reason no dual-diaphragm cardioid, or variable-pattern microphone set to a cardioid setting, has a true cardioid pattern at low frequencies; the pattern always spreads out. It is also why a stereo recording made with coincident dual-diaphragm cardioids sounds "more mono" than a stereo recording made with single-diaphragm cardioids, even at the same included angle.

Also, in general it's really difficult to modify the "front end" of a condenser microphone and have it retain its sonic characteristics and operational reliability, because of the extremely high impedance of the circuit environment and the risks of stray capacitance. Not many technicians can do this kind of work well; good intentions don't suffice.

I think it would be easier, less expensive in the end, more reliable, and your recordings would sound better if you took four good, small, single-diaphragm cardioids and arranged their capsules in back-to-back pairs around a common vertical axis. Or you might want to take a look at Ambisonics ...

--best regards

[dnsacks]

thanks all for your clarification -- glad I'm happy with the sound I currently am getting out of my lsd2 setup.

[John Willett]

Just wanted to note that the very nearly coincident placement of the front and back diaphragms in a dual-diaphragm capsule is very nice if you want good mono compatibility in a stereo recording, but the two membranes aren't acoustically independent of one another. Each has a certain degree of what you might call acoustical opaquness or transparency with respect to the other, and this varies greatly at different frequencies.

Except the Gefell UM 930 and UM 930 twin - both these use two separate cardioid capsules rather than a single backplate with two diaphragms.

[Gutbucket]

This is a feature that interests me as well.  The Sennheiser and Gefell are high end mics, too rich for my blood.  I think if someone did this on a set of of mics costing less than $1000 a pair they might have a nice little niche market that appeals to people besides us tapers.  Anyone who is a mid/side fan sees the benefit of being able to dial things in at post, and this should be similar.  Maybe it's not really as useful in practice, and that's why there isn't a big calling.

Well there is this one particular microphone I happen to know of . . .

Jon refers here to his U8 mic system, which he only hints at due to his admirable modesty -
http://www.naiant.com/naiant/microphones.html

[quoted from his website]
U-8S specification

The U-8S capsule contains two 16mm cardioid capsules that are separately routed to tip and ring connectors, and thus may be used independently via the U-A or U-L amplifiers, or directly to a plug-in powered microphone input.  The two resulting signals may be used as a stereo recording, or mixed to create a mono multipattern microphone.

When connected via a U amplifier to a single channel microphone preamplifier, the U-8S will yield a bidirectional (figure-8) pattern.  When connected via the U-8A amplifier, the U-8S can generate a continuously variable polar pattern.  The capsule marked with the blind hole is positive polarity output (front) when used with a U amplifier.

[fguidry]

Line Audio Designs http://www.lineaudio.se/linemic.htm make some elaborate multi-diaphragm mics.

The CM3 is the new darling of the classical/live forum on Gearslutz: http://www.gearslutz.com/board/remote-possibilities-acoustic-music-location-recording/644759-cm3-really-good.html Apparently it's a wide-ish cardioid, it's being compared to a Schoeps MK21 in various posts. And there's a new omni that's not on the Line Audio Designs website: http://www.gearslutz.com/board/remote-possibilities-acoustic-music-location-recording/745131-om1-whats-about.html

Fran

[SmokinJoe]

It would be easier, less expensive, and more reliable and (for the reasons given above) would give better sounding recordings if you would take four well-matched small, single-diaphragm cardioids and arrange their capsules around a common vertical axis, in pairs where the capsules of each pair faces in opposite directions.

--best regards

I have 4  AT853's for just such a purpose... never actually got around to it yet.  Not sure about the well matched part either, but I'll try it someday.

And I remember seeing that on Jon's site a while ago, but I thought it was still in development... not yet released to the wild.

~~~~ later ~~~~~
Now I'm looking at Naiant website and trying to understand the setup.  I think I understand it, but I'm not positive.  I guess for a stereo pair of mics (4 channel output), I would need (2) U-8S capsules, and (2) U-8A amplifiers, and that comes with U-R cables which go between the capsules and the amplifiers.  I'm guessing the U-R is basically a TRS mini-jack to dual RCA cable?

Then I'd probably want a pair of shockmounts. An amplifier body is put into the shockmount, I'm guessing there is an XLR-M going out the "front and back" of the shock mount, and the active cable connections go out through the gap in the "inner holder"?  Then it's not clear to me how to mount the capsules in any kind of organized manner.  I'll guess the U-R cables have clips at the ends where the mics plug in, and I clip it to "something", perhaps the shockmount body, or maybe a homebrew bar.

A complete package would be 2 x $99 for the amplifiers, 2 x $69 for the capsules, and 2 x $19 for the shocks.  Which is about $380, and everything is matched better than my 4 random AT853's. Am I understanding this correctly?

If I do understand it correctly the next question would be "can I get modified U-8A amplifiers so there is 15' of thin balanced cable coming down to XLR-M connectors?"  That way I don't have all the bulky XLR connectors up on top of the stand.  Or probably easier to use minijack or RCA extension cables and put the bodies in my bag.

[DSatz]

John Willett--there can be one backplate between the two membranes (as in the Neumann U 47) or each membrane can have its own backplate (as in the original Neumann U 87) which is electrically isolated from the other backplate. The Gefell models that you mentioned are an instance of the latter. Acoustically it works out to nearly the same thing, and neither type maintains a cardioid pattern at low frequencies very well. If you want that characteristic (very desirable for coincident or closely-spaced stereo recording techniques), choose a good single-diaphragm microphone.

--best regards

[John Willett]

John Willett--there can be one backplate between the two membranes (as in the Neumann U 47) or each membrane can have its own backplate (as in the original Neumann U 87) which is electrically isolated from the other backplate. The Gefell models that you mentioned are an instance of the latter. Acoustically it works out to nearly the same thing, and neither type maintains a cardioid pattern at low frequencies very well. If you want that characteristic (very desirable for coincident or closely-spaced stereo recording techniques), choose a good single-diaphragm microphone.

I agree that a dedicated single-diaphragm mic. with a set polar-pattern is better than a switchable.

But the two capsules in the UM 930 are wider spaced than you would expect (I was surprised) and the cardioid patten holds up very well at low frequencies.

[DSatz]

> the cardioid [pattern] holds up very well at low frequencies.

Well, if so, there's no evidence of that in the graphs you attached. That's because Microtech Gefell doesn't publish polar response graphs for their microphones (at least on the Web) below 250 Hz, which is about middle C on a piano--I'd call that a midrange frequency. Other manufacturers show the polar response of their microphones at 125 Hz, and it's important all the way to the bottom of the microphone's frequency range even though it can be difficult to measure at, say, 50 Hz.

But what you can see is the pattern broadening out around 500 Hz and below. That's potentially advantageous for some applications (close vocal miking in a studio, for example), but not very good, I would say, for coincident or closely-spaced stereo recording. Regardless of the angle you might set between the microphones, your recordings would come out more and more like mono at lower frequencies--the exact opposite of what's desirable for the sake of a sense of spaciousness in the recording.

For comparison, let's take one of the same manufacturer's single-diaphragm cardioids at the same frequencies (see image below). It's far better than the dual-diaphragm (or "dual-capsule") UM 930 in this respect. And this difference becomes more and more pronounced as you go down in frequency--the dual-diaphragm microphone's off-axis response becomes worse and worse relative to the single-diaphragm microphone.

--best regards

[John Willett]

Ahh - but you are comparing an LDC with an SDC.

This is the response of a single capsule LDC cardioid only mic in the same sized housing (M1030):-



It's pretty much the same as the switchable-pattern version.

Here is the capsule in a smaller housing (M930):-



The broadening of the lowest frequencies at the rear seems to be more of a body size thing, rather than an effect of twin diaphragms - as far as I am aware the M930, M1030 and UM 930 all have the same capsule - the M930 is a small body, the M1030 is a large body and the UM930 is a large body with two capsules.

[SmokinJoe]

If you want to record four capsules separately you don't want the U-8As, they will mix the two capsules to a mono output.  A pair of U-As or U-Xs with U-SO splitter cables is what you need.

OK.  I'm beginning to understand.  Just another question of curiosity... looking at this picture,what's the thing on the right?  At first I thought it was 2 XLR's on the ends with RCA's on the top.  From your response, now I'm guessing that's a single XLR, on the far end is a connector for the capsule (I realize now it's smaller), and those things on top are gain knobs for each diaphragm?

[John Willett]

If you want to record four capsules separately you don't want the U-8As, they will mix the two capsules to a mono output.  A pair of U-As or U-Xs with U-SO splitter cables is what you need.

OK.  I'm beginning to understand.  Just another question of curiosity... looking at this picture,what's the thing on the right?  At first I thought it was 2 XLR's on the ends with RCA's on the top.  From your response, now I'm guessing that's a single XLR, on the far end is a connector for the capsule (I realize now it's smaller), and those things on top are gain knobs for each diaphragm?

Where dis the picture come from?

They look like balance/unbalance converters - the thing on the right looks like an in-line XLR-3F to XLR-3M  with what looks like co-axial connectors.  It could be a dual balancing converter - from XLR to phono (cinch) connectors: balanced XLR to unbalanced phono and unbalanced phono to balanced XLR