john klett wrote on Fri, 23 April 2004 21:55 |
If you have a pair of omni's closer together - like about 5 inches apart or so - and place a baffle between them - a foot square hunk of 1/4" luan plywood with some 1/2" open cell foam on both sides is a decent place to start experimenting - you are sort of making a simple form of dummy head.
The wavelengths at low frequencies are so long they will ignore the baffle and reach both mics. For all intents that information is in phase. As wavelength gets shorter the baffle becomes more significant and the mics essentially go cardioid. Our ears are omni but all that stuff around them (head, pina etc.) make 'em more directional simply by blocking and diffracting (refracting?).
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This is roughly the description of Jurg Jecklin's "OSS" technique and the infamous "Jecklin Disk", although the "official" technique is a bit more refined.
The idea behind Jecklin's technique is to record directional sounds in a way that your ear/brain will be able to coherently determine the direction. For very low frequencies (<80Hz), consider them omnidirectional. For frequencies above that point, but whose wavelengths are still larger than the width of one's head, your brain uses mostly on phase difference, this means frequencies up to about 400-600 Hz or so, or into the midrange. For higher frequencies, your brain uses mostly amplitude difference to determine direction.
The Jecklin Disk is intended to address these issues by making a baffle that's essentially transparent acoustically at frequencies below about 400-600 Hz, and absorbtive at frequencies above that. This way your brain can use phase differences in the range of frequencies where they are important (both mics hear the same sound, only difference is phase), and amplitude differences where they are important (high end... sound coming from the right is attenuated in the left mic).
What you said about the mics behaving as if "cardiod" above the baffle's cutoff, that's true in the sense of amplitude axial response, but in terms of phase response, they are still much more linear than a cardiod, and their off-axis response is also very linear.
With a spaced omni setup, you essentially get only phase/time differences so directionality at high frequencies is compromised. If you use cardiod mics coincident pair then you only have amplitude difference and you compromise directionality in the lower midrange. Depending on the source, either of these conditions may be acceptable. If you are recording an instrument with nothing below 400 Hz then spaced omni will work fine. If you're recording a tambourine or shaker, then coincident pair will work fine. However, for acoustic guitar or drum kit, you can't beat the OSS technique.
This all works fine for listening with headphones but things become very problematic when summing to mono. You are intentionally introducing a phase difference with the OSS-type technique simply by virtue of the spacing of the capsules. This phase difference is going to cause comb filtering and cancellation, other issues when summed to mono. The two solutions to this problem are either to use spaced omni with enough space between the mics such that the secondary sound in any mic sounds like ambient sound when summed to mono (Haas effect), or coincident pair (or M-S, same thing) which sums to mono perfectly.
The other major problem with any stereo recording technique is the problem of playback on stereo loudspeakers, where the variability of listening environments, reflected sound, and crosstalk (the left ear can easily hear the right speaker) means that the only things that are going to translate accurately are mono sounds panned hard left or right so that they really only come from one speaker at a time. All of these detailed phase techniques, all that, goes out the window. Wide spaced omni technique translates best to loudspeaker playback imho but also is maybe the worst for mono summing and definitely the worst for headphones. I am still grappling with this issue myself. Everything is a compromise. I guess this is why you must know your target audience, how & where they listen to music, and go for it.
See ya-