DJL said:
I would think that if the capsule is getting hit from the front and the reflected back signal (without something to control it) the mic could sound like mud... by-the-way, how much of a delay are we talking about... and is the delay controlled only the acoustic delay chamber or is there an electronic delay circuit also?
There's no electronic delay, it's all acoustic. And the circuit is made up of the side openings (size, distance, and spacing), and the rear chamber. Chamber size (determined by the specific mic design) is a factor, but the chamber is fairly small. so any peaking (usually easily heard) or dips (less obvious) will be up pretty high.
The "muddiness" (usually due to "proximity effect" which is caused by the velocity component of a pressure gradient mic design) will actually be less of a problem. You can put a thimble or small shot glass near the front of an omni mic to hear the effect of a small chamber resonance bouncing into the diaphragm.
DJL said:
ADDED: here's another thought... what would be the results of having a mic with an adjustable acoustic or electronic delay chamber?
Good thinking, DJL !!! That's EXACTLY how the RCA 77DX works, by having a series of internal vanes and shutters inside (to change the chamber and openings) to make a simple figure 8 ribbon mic into a multi-pattern mic. Open the shutters and vanes all the way and it's a figure 8 pattern. Close down some of the shutters a bit and it's cardioid. Close off the back side entirely, and it's an omni.
On the RCA 77DX, these patterns are marked "B" for "Bi-directional" (figure 8), "U" for "Uni-directional" (cardioid) and "N" for "Non-directional" (omni). The switch that changes the patterns is a mechanical switch - no electronic connections.
On condenser mics, the patterns are changed by changing the level and polarity of the signal coming from the rear diaphragm. The front diaphragm signal remains constant. The various polar patterns are derived as follows:
Positive signal from both diaphragms = Omnidirectional
Positive front, slightly lower back signal = Wide Cardioid
Positive front, no back signal = Cardiod
Positive front, slightly negative back = Hyper Cardioid
Positive front, fully negative back = Figure 8
Continuously variable pattern mics use a similar arrangement, but they all work by varying the back diaphragm voltage relative to the fixed front diaphragm voltage.
On a condenser mic, you'll find holes that go all the way thru the back plate (that are used as part of the the delay network to achieve a cardioid pattern), and holes that don't go all the way thru the back plate (that are used to tune the resonant front chamber, created by the spacing between the diaphragm and the back plate).
. They go omni and get the monitors squealing. I didn't know why till I read Harvey's mic thread. The theory is correct, but Harvey seemed to be saying that mic designs were a compromise in the first place to even out frequency response, and that turning a cardioid, for instance, into an omni by covering the holes would result in very strange peaks and dips. Thanks for the Tape-Op link. I don't have a subscription but would love to see that article. Did they address anything like Harvey wrote?
