C4's for spaced omni AB

[Image_Engine]

Hi All from Australia!
Was on the vovox site the other day listening to a demo that was done using AB spaced omni's (Brauner VM1's). It was a vocal/guitar duo; Vocalist on the front side and guitar on the rear. How to do that with the c4's??? And I dont really understand how the omni caps work when the rear is sealed and the cardioid is not? Anyone know that one?
Enough questions.
Thanks
Mark

[hargerst]

A-B spaced omnis is simply a recording technique using two mics widely spaced to capture the music, and give a listener a sense of the room the recording was made in. Since omnis are "omnidirectional", they capture sound from all directions; pointing them at the source is less critical, and you adjust the loudness of the players by moving the players closer or further from the mics - a very simple, yet elegant solution to getting the right mix. You would simply set up two C4's (with the omni capsules) anywhere from 6 feet apart to as wide apart as practical, till you get the sound you want.

Your second question is a little more difficult to answer easily, but David Satz (of Schoeps) did a remarkably good job of it. Here's David's explanation:

The two relevant mic design categories are "pressure transducer" (basically omnidirectional) and "pressure gradient transducer" (basically figure-8, but by using dual diaphragms and other tricks, any other first-order directional pattern can be synthesized including cardioid and super- or hypercardioid).

The model of a pressure transducer is a barometer. It measures air pressure in the space around it. The simplest, grade-school science barometer is a sealed tin can with air in it. The lid of the can will flex in proportion to air pressure changes in the room around it; you can attach a stick to the lid, and calibrate the stick's motions in terms of whatever units of air pressure you want to use (inches of mercury or the standard metric unit, which is "bars").

The thing is, the can will get squeezed by increasing air pressure or it will expand in times of low air pressure, regardless of which way you "aim" it. In fact the concept of "aiming" a barometer doesn't really exist because it's integrating and responding to a phenomenon that is all around it. You just set it up in whatever physical orientation is convenient for you, and it works.

You could think of the barometric pressure in a daily weather report as being the response of the barometer at 0.000011574 Hz if you want (one cycle per day). Essentially a barometer is a microphone with response down to DC. And that is a real-world characteristic of pressure transducers: their low-frequency response can be extended as far down as you like. Most pressure microphones have some small vent built in to prevent them from bursting when transported by air, but they can very well be dead flat to below 1 Hz or 5 Hz, certainly to any audible frequency.

OK. So the pressure transducer works precisely _because_ only one side of the diaphragm (the lid of the can) is exposed to the air pressure that is to be recorded; the air on the other side of the diaphragm is a constant mass, and the diaphragm flexes in order to equalize the pressure on both its sides.

The other major category of transducer is pressure-gradient, which is a fancy way of saying that its diaphragm is exposed to the sound field both on the front and the back, so it responds to the difference between the pressure that exists on the front and the pressure on the back. If the pressure presented on both sides at a given moment is identical, there is no net motion and no output. If the pressure on the front is greater than the pressure on the back, the diaphragm will move toward its backplate (assuming a condenser microphone). If the opposite is true, the diaphragm will move outwards, away from the backplate.

The thing is, if you just hang a microphone diaphragm out in space, it will be pushed around by wind or by air currents of any kind (including if you just blow on it) but it won't pick up much in the audio frequency band because it's a thin element and the pressure from sound waves will tend to be identical on both sides of the diaphragm, at least until you get up to the high frequencies (which we'll talk about some other day), and when the pressure is the same on both sides of the membrane there is no net movement and no output. But before I explain why this type of arrangement picks up sound at all, let's observe that we've actually encountered something that is true of pressure gradient microphones generally, which is that they are much more sensitive to wind, breath noise and "popping" of consonants in vocal pickup than their omnidirectional counterparts are (when the omnis are pressure transducers).

The trick which makes a pressure-gradient arrangement work for recording sound is that the sound reaching the back of the membrane is delayed momentarily, by setting up a delay chamber in between the back vents of the microphone and the back of the diaphragm. If you can make the pathway for sound even just a tiny fraction of an inch longer before the sound reaches the rear of the diaphragm, then you will cause a phase shift between the sound reaching the front and the sound reaching the back. That phase shift will be different at different frequencies, of course, so there will really be only one frequency (plus its exact integer multiples) at which a maximum of difference in pressure will result between the front and back of the diaphragm. At that frequency the resulting microphone will have its highest sensitivity to sound. But if you arrange things so that this frequency occurs somewhere other than at the very top or the very bottom of the audio range, you can do other tricks with damping and filtering so as to flatten the overall response.

The thing is, this more complicated type of microphone is also sensitive to the direction from which sound is arriving, because if sound is arriving from in front, it will strike the front of the diaphragm immediately, then when it reaches the rear input ports it will pass through the acoustic delay chamber and eventually reach the back of the diaphragm--so there will be a continually varying difference in the air pressure on the two sides of the diaphragm, and that's what moves it and produces a signal. But if the sound is coming from behind the microphone, it will reach the back inlets first, and pass through the delay chamber at the same rate of speed as the original wave is traveling outside the microphone; by the time both waves reach the two sides of the diaphragm, they will be in phase with one another and the result is no net motion of the diaphragm. (That's if the microphone is a single-diaphragm cardioid.)

[Image_Engine]

A-B spaced omnis is simply a recording technique using two mics widely spaced to capture the music..
Wow, that was quite an answer. Have to say the most comprehensive Ive ever received!
I was aware of the normal AB ie overheads etc but with the Vovox/Brauner demo it sounded like they were a lot closer. I experimented and it seems they must have had them much closer; more like 300mm at a guess (well that gives a similiar effect). The players were definitely much closer than a normal spacing, almost close mic'd and too wide makes even the slightest movement of the vox VERY noticeable; the 300-450mm seems to stop that and still be spacious (one channel phase inverted though)
I suppose too I was a bit perplexed (was using 2 old NT2's for the experiment) when facing the diaphragms from either side it seems quite natural. However with the C4s, would it be best then to point them straight up or....?
Thanks so much for prompt and thorough reply
Regs
Engine

PS Im doing a cable upgrade in my project studio; have some SP litz cables on the way and Im putting them up against the Analysis Plus Pro Oval Stage and a Vovox direct P. Ill let you know how it goes. There is just so little info on cable shootouts. I was so impacted by the difference between good (Im currently using planet waves) and GOOD cable ie the above.