Part 3 (or 4 or whatever)
Well, I haven't heard from David Satz as far as permission to reprint his post here, but I don't think he'll mind (since he's a nice guy), so here it is:
RockyRoad wrote:
Could some kind person explain to me how the physics of these things work, and how sound from behind an omni mic such as the KM183 can get around the metal side casing and into the mic.
Sorry for the dumb questions, but I'd like to know why things arent as they seem on the surface.
"Why are things not the way they seem?" is a question that I _so_ wish people would ask more often than they do. Most folks seem to stop noticing that things aren't the way they seem, and start behaving as if that appearances are all that matter. To me that's the essence of that form of spiritual death which we in this society call "adulthood." It's why I believe that only children should be allowed to vote or own property--but failing that, there should be a law (or better yet, a general agreement) that grown-ups ought to answer all honest questions honestly. Then maybe we would not be such a culture of deception and self-deception, and people would retain their ability to notice things that don't make sense.
The replies from Sean and Scott are spot on, but I'd like to try to help you visualize what these two types of microphone are doing. Again, the relevant 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.)
That should be enough to establish a basic viewpoint, I hope. (End of David Satz' post)
And we'll now head into "when and why to use what, and how" when we do the next posting on this.