Renowned recording engineers Jack Renner of Telarc Records and Marc Aubort of Nonesuch Records describe spaced omnidirectional microphones as their favorite technique for stereo recording of symphonic ensembles.
Of all the stereo mic techniques available, the “spaced-omni” method is especially good at providing a warm, full sound (deep low-frequency response) and a spacious sense of ambience. It adds up to a pleasant listening experience.
We’ll explore how this technique - and others - can be beneficial.
With the spaced-pair method (also called AB), you place two identical mics a few feet apart and aim them straight ahead (Figure 1). The mics can have any polar pattern, but omni is most popular for this method. The greater the spacing between mics, the greater the stereo spread.
How does this method work? Instruments in the center of the group produce the same signal from each mic.
When you monitor the mics, you hear a phantom image of the center instruments midway between your loudspeakers.
If an instrument is off-center, it is closer to one mic than the other, so its sound reaches the closer microphone before it reaches the other one. Both mics produce the same signal, except that the farther mic’s signal is delayed compared to the closer mic’s signal.
If you send the same signal to two speakers with the signal in one channel delayed, the sound image shifts off center. With a spaced-pair recording, off-center instruments produce a delay in one mic channel, so they are reproduced off center.
The spaced pair codes instrument positions into time differences between channels. During playback, the brain decodes these time differences back into corresponding image locations.
A delay of 1.2 millisecond (msec) is enough to shift an image all the way to one speaker. You can use this fact when you set up the mics. Suppose you want to hear the right side of the orchestra from the right speaker.
The sound from the right-side musicians must reach the right mic about 1.2 msec before it reaches the left mic. To make this happen, space the mics about 2 to 3 feet apart.
This spacing makes the correct delay to place right-side instruments at the right speaker. Instruments partway off center produce interchannel delays less than 1.2 msec, so they are reproduced partway off center.