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OAO De for an omnidirectional microphone C>e = enhancement critical distance

= distance where the source level and the enhancement system level are the same systems the loop gain should be another 8 dB lower or -20 dB to avoid coloration (Krokstad, 1991). This places rather severe constraints on the loudness of the system.

To overcome the feedback limitations the single microphone systems locate the pickup point close to the source. To meet the -20 dB limit, the ratio of the mic distance to the critical distance must be less then 1/10. In many halls the critical distance is on the order of 7 m (23 ft) (Griesinger, 1991) so an omnidirectional mic must be placed no more than 70 cm (2.3 ft) from the source. Although better for feedback, this close miking technique does not yield a representative sample of the sound in the room.

Directional mics can give some additional gain. A cardioid mic has about a factor of V3 (4.8 dB) increase in distance for the same feedback. For hypercardioids the factor is 2 (6 dB). But at 1.4 m (4.6 ft) you cannot single-mic an orchestra. Using multiple mics close to the sources overcomes some of this problem at a cost of reduced gain and considerable inconvenience. If the mics are mixed before processing as in Fig. 20.25, the nom correction reduces the minimum distance by ^nom.

Figure 20.25 Microphone Mixing before Processing Increases Feedback Stability (Griesinger, 1991)

Figure 20.25 Microphone Mixing before Processing Increases Feedback Stability (Griesinger, 1991)

In an effort to achieve greater gain stability, systems with separate channels of reverberation, illustrated in Fig. 20.26, have been developed. If individual microphones and loudspeakers are separated so they do not influence each other, and if each channel meets the -20 dB gain limit, a channel will not be driven into an unstable feedback condition by the addition of another. Even if a single microphone is used to feed multiple reverberators before mixing, the same effect can be achieved. The overall reverberant level in the room and the reverberation time can be increased by 10 log (number of reverberators) and the source to microphone distance can be increased by a factor equal to the square root of that number. In practice each channel can increase the reverberation time by about 1% (Krokstad, 1991), so the total number of reverberators and loudspeakers needed may range from 50 to 100 in a typical system without additional processing.

Further improvements are possible through processing. The RODS (Reverberation on Demand System) system developed by Acoustic Management Systems uses a sophisticated gating arrangement pictured in Fig. 20.27. The logic module senses whether the sound in the room is rising or falling with time. The input to the delay is gated on when the room level is rising or constant and the output from the delay is connected to the loudspeakers only when

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