"MS" technique?

[PinkStrat]

Hello again Wil & Skippy!

We must be reading tha same recording manuals then? My audio maven pal making me the needed adaptors has this big blue and gold book that outlined the dual figure eight [four leaf clover] EMI technique [among many others]. The mics I will experiment with here are mostly AKGs; C414B-ULS, C 391B pencil, C 4000B. I also own an Equitek E-350 that I will use as the second figure eight microphone. This whole MS concept is really getting exciting now...BTW, I plan to start my testing within a solo acoustic guitar context. Thanks again to both of you--.this is a great forum where many can learn a lot.

 

[dusty reske]

m/s still confusing

Thanks, that's an informative thread, but I'm still confused about what exactly the matrix is doing. I have three textbooks on recording and all but one have inadequate descriptions of the technique behind the m/s configuration and matrix. The one that has more detail is still confusing. The book, Sound Advice, reads: "First, the matrix adds the two signals, reinforcing whatever is common to both signals, namely audio information from the left of center[how can it determine what is left of center since neither of the mics are stereo?]. At the same time, the matrix switches the original phase of the bidirectional mic signal and adds this to the cardiod mic signal again. Because sound from right of center push the bidirectional mic diaphragm to the left, the second addition reinforces signals arriving at both mics from the right. The two resulting signals...represent... an arc of about 270 degrees..." I've used Waves m/s matrix(it's quite good) successfully, but still don't understand how this stereo image is being created. Please, any more info would be appreciated. Thanks, Dusty

 

[skippy]

Their description is accurate, but just wordy enough to be confusing. Let's see if I can make matters worse.

The mid mic is just a cardioid, so its signal contains no information that can be used to differentiate right from left. The side mic is set up perpendicular to the sound source (the source would be in the "null" of the figure-8). Without the mid mic, you'd get no signal to speak of: the side mic signal contains _only_ information that can be used to differentiate left from right.

So let's look at that. The magic of the figure-8 is that the two lobes are 180deg out of phase with each other. Let's take a baloon and pop it- for the purposes of argument, let's pretend that doing that produces a perfect positive-going acoustic pulse. If we carry it over to the left of center and pop it, the side mic will produce a positive-going voltage pulse. if we carry it to the symmetrical position to the _right_ of center, the side mic will produce a _negative-going_ voltage pulse (since we're in the "back", or out-of-phase lobe). The same acoustic signal produces a positive-going response when done left of center, and negative-going when done on the right. Make sense?

So if we add mid + side, we get all the signals that are center and left. If we add mid + (-side), we get all the signals that are center and right. The crux of the matter is this: we have to invert the phase of the side mic signal to derive the right side information, because *by the nature of the mic* everything to the right is inverted. Two inversions make a 0deg phase shift: one in the mic, one in the matrix.

Did that help, or did that screw things up even worse?

 

[dusty reske]

m/s

Actually, that clears things up quite a bit. I never realized the two sides of a bidirectional mic were out of phase...is this electronically created or just the nature of the sound as it passes by the mic? Thanks again, Dusty

 

[skippy]

It is actually in both the acoustic and electrical nature of the mic. A bidirectional mic is basically a velocity transducer, as opposed to a pressure transducer like an omni. It is sensitive to the velocity of the air passing by it as the pressure waves pass. Because velocity is a vector quantity, it has sign: stuff coming at you from in front is positive, stuff coming at you from behind is negative. So the sound itself is primarily responsible for the inversion of phase, if you think of it that way.

Ribbon mics are intrinsically bidirectional: the velocity of the air perpendicular to the ribbon displaces it, while pressure waves from the side (parallel to the ribbon) are ignored. For waves moving parallel to the ribbon, the air molecules are moving exactly the same on both sides of the ribbon, so they don't displace it.

The sign inversion is easy to see, with a ribbon: Pop a baloon in front of it, and the ribbon moves one way in the magnetic field, inducing a positive voltage in the ribbon. Pop one behind it, and the ribbon moves the *opposite* way, inducing a negative voltage. To do a figure-8 (or switchable patterns) with a condenser mic requires a little bit of electronic trickery, and dual diaphragms: but the basic cause of the inversion in the rear lobe is the same, whether it's a ribbon or a diaphragm. The bottom line is that sound back there moves the important stuff in the opposite direction of sound out front!

 

[dusty reske]

m/s, bidirectional mic

Skippy,
Thanks for the info! Dusty

 

[Markd102]

Thanks heaps for all the replies folks. There's heaps of great info here. I'm glad this thread could help others out too.
Mark