Surround Microphone Technique for the Direct/Ambient Approach

Using the direct/ambient approach, pan pot stereo, spaced omnis, and coincident techniques can all be used in the frontal stage LCR stereo. However, coincident microphone techniques should be done with knowledge of the relative pickup angles of the microphones in use; achieving good enough isolation across LCR is problematic with normal microphones. In playback, the addition of the center channel solidifies the center of the stereo image, providing greater freedom

in listening position than stereo, and a frequency response that does not suffer from crosstalk-induced dips in the 2kHz region and above described in Chapter 6. The surround loudspeaker channels, on the other hand, generally require more microphones. Several approaches to surround channel microphones, often just one pair of spaced microphones, for the direct/ambient approach are:

• In a natural acoustic space like a concert hall, omnis can be located far enough from the source that they pick up mostly the reverberation of the hall. According to Jonathan Stokes³ it is difficult to give a rule of thumb for the placement of such microphones, because the acoustics of real halls varies considerably, and many chosen locations may show up acoustic defects in the hall. That having been said, it is useful to give as a starting point something on the order of 30-50 ft from the main microphones. Locating mikes so far from the source could lead to hearing echoes, as the direct sound leakage into the hall mikes is clearly delayed compared to the front microphones. In such a case it is common to use audio delay of the main microphones to place them closer in time to the hall microphones, or to adjust the timing in postproduction on a digital audio workstation.This alone makes a case for having a digital console with time delay on each channel, so long as enough is available, to prevent echoes from distantly spaced microphones.

Of course, in live situations if the time delay is large enough to accommodate distantly spaced hall microphones, the delay could be so large that "lip sync" would suffer in audio for film or video applications. Also, performers handle time delay to monitor feeds very poorly, so stage monitors must not be delayed.

• An alternate to distantly spaced omnis is to use cardioids, pointed away from the source, with their null facing the source, to deliver a higher ratio of reverberation to direct sound. Since this is so they can be used closer to the source, perhaps at 1/2 the distance, of an equivalent omnidirectional microphone. Such cardioids will probably receive a lower signal level than any other microphone discussed, so the microphone's self noise, and preamplifier noise, become important issues for natural sound in real spaces using this approach. Nevertheless, it is a valid approach that increases the hall sound and decreases the direct sound, something often desirable in the surround channels. One of the lowest noise cardioids is the NeumannTLM 103.

³ A, multiple Grammy award winning classical music engineer with experience in many concert halls.

• The IRT cross, an arrangement of four spaced cardioids facing outwards arranged in a square about 10 in. (25cm) on a side at 45° incidence to the direct sound has been found to be a useful arrangement for picking up reverberation in concert halls; one could see it almost as double ORTF. Also, it may be used for ambi-ences of sound effects and other spatial sound where imaging is not the first consideration, but spaciousness is. The outputs of the four microphones are directed to left, right, left surround, and right surround loudspeakers. A limitation is that some direct sound will reach especially the front facing microphones and pollute its use as a pickup of principally reverberation.

• The Hamasaki square array is another setup useful in particular for hall ambience. In it, four bidirectional mikes are placed in a square of 6-10 ft on a side, with their nulls facing the main sound source so that the direct sound is minimized, and their positive polarity lobes facing outwards. The array is located far away and high up in the hall to minimize direct sound. The front two are routed to L and R and the back two are routed to LS and RS. Side-wall reflections and the side component of reverberation are picked up well, while back wall echoes are minimized. In one informal blind listening test this array proved the most useful for surround.

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