View Full Version : Omni Ambient Mics
04-29-2002, 11:24 PM
Nikka (and everyone else!)
I was wondering about using a pair of Earthworks omnis, TC30?, as a pair of ambient mics. If I'm not mistaken, using two omnidirectionals creates a binaural, as opposed to stereo, effect. Is there something special that Earthworks does to address this? How does this effect instruments that are also close miked?
Using two omni microphones does not make a binaural recording, unless those omni microphones happen to be placed in the ears of a dummy of a human head. Binaural recording is supposed to replicate as closely as possible the way we hear things, including the timing and directional cues caused by the head itself. Binaural recordings sound great over headphones, but often don't translate well to speakers.
As for using Earthworks omni microphones for ambient microphones, they're great...since they pick up sound from all around, they really do a good job of picking up the sound of the room. Also, since they're true omnidirectional microphones, their bass response is as good twenty feet away as it is twenty inches away. Since they're omnidirectional, though, the directional cues are not as strong as they would be with cardioid microphones, especially when the microphones are relatively close together. That may have been what you were wondering about when you said using two omni's wouldn't create a stereo image. They do, but they rely more on timing differences rather than directional to place different instruments in the stereo soundstage.
You can adjust the level of ambient microphones relative to close mikes and come up with a variety of different sounds, some more useful than others.
04-30-2002, 08:32 PM
For ambient mics I use a pair of Earthworks QTC-1s and a Jaecklin (sp.?) disk. The disk is basically a 12" diameter absorptive/diffusive disk placed between the mics (which should be about 8" apart) to increase stereo seperation. I made my own disk by gluing 2 pieces of acoustical cieling tile to both sides of a piece of 1/4" plywood - I left a bit of the wood sticking out to screw onto a homemade threaded mount.
This set-up gives amazing results.
05-09-2002, 05:52 PM
Originally posted by Ted
Since they're omnidirectional, though, the directional cues are not as strong as they would be with cardioid microphones, especially when the microphones are relatively close together. That may have been what you were wondering about when you said using two omni's wouldn't create a stereo image. They do, but they rely more on timing differences rather than directional to place different instruments in the stereo soundstage.
"rather than directional"? Rather than directional what??? It's *directional* cues that all manner of stereo microphony are trying to capture!
All stereo mic techniques represent directional cues using some combination of Timing Differences, Amplitude Differences, and Timbral Differences -- this last more commonly referred to as Head Related Transfer Function (HRT), the phenomenon of the human head filtering out high frequencies from Left sounds arriving at the Right ear & vice-versa.
Spaced omni mics rely on timing differences to impart directional cues, period.
Omnis used in conjunction with a Jecklin Disc rely on timing differences as well as timbral differences (HRT) to impart directional cues. Same goes for binaural heads, spheres, Schneider Discs, and any other "baffled omni" pair.
Near-coincident cardioids (eg., O.R.T.F. or N.O.S.) use a combination of amplitude differences and timing differences to impart directional cues. (Timbral differences also make a very minor contribution to any stereo mic technique using cardioids, which is why large-diaphragm condensors don't always make the best stereo mic arrays; their uneven off-axis response interferes with the representation of convincing HRT.)
Coincident arrays (X/Y, Blumlein, and M/S) rely on amplitude differences to impart directional cues...though the above sentence about off-axis response interfering with convincing HRT holds truth here too.
The only situation where Ted's sentence
>>Since they're omnidirectional, though, the directional cues are not as strong as they would be with cardioid microphones<<
makes sense would be if describing a pair of *coincident omnis*, a technique that is considered INVALID for precisely that reason: it doesn't work! It doesn't reproduce directional cues at all (Bruce Sweiden's (sp?) experiments with the Andre Crouch Choir notwithstanding).
Regarding Ed's post:
There are a couple of ways of picking up stereo: There's "difference in time" and "difference in amplitude". XY microphones only use the latter. ORTF microphones use more of the difference in time. Spaced omnis utilize both of these. The closer the spaced omnis get the less the difference in amplitude AND the less the difference in time, to the degree that it's quite ineffective.
I think that Ted may have been referring to "difference in amplitude" when he said "directional".
Thanks Nika...I'd typed up a response to Ed's message earlier today and somehow it never was posted. How odd.
Anyhow, yes, I did mean "rather than amplitude" instead of "rather than directional". I've had a bad head cold this week and seem to be hitting a couple below par. Sorry about that.
Also, I did assume that Shane was referring to a coincident or near-coincident stereo pair in his initial post (I guess saying "especially when the microphones are relatively close together" was a little vague). So I am in agreement with Ed's post, and apologize for any confusion I may have caused.
You are forgiven this time.
Have a good weekend, all!
05-11-2002, 10:38 AM
I was talking about spaced omnis, but I didn't want to put in a comment after things got heated.
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