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Multichannel sound systems
The quality of live music is strongly dependent on the acoustics of the venue. Different music types require different acoustic conditions to sound optimal. Passive methods have been employed for many years to the alteration of room acoustics to suit each performance type. However, passive methods such as movable curtains and reflectors, or reverberation chambers with adjustable openings, do not provide a sufficiently large range of acoustic conditions for all performances.
Multichannel sound systems are being increasingly applied to the control of room acoustics in multi-purpose venues. Electroacoustic systems enable a greater variation of room acoustics than traditional passive techniques, and changes can be made instantly under computer control.
There are two general approaches to room enhancement systems. In the first, microphones are placed near the stage area to detect predominantly direct sound from the performers. The microphone signals are used to generate reflections or reverberation which are then broadcast from a large number of loudspeakers positioned throughout the room. Such systems are termed in-line because they aim to avoid feedback from the loudspeakers to the microphones. This feedback can produce instability of the sound system. However in-line systems produce a high direct sound amplitude meaning that the loop gain can be kept low enough to maintain sound quality.
In the second approach, the feedback between loudspeakers and microphones is usefully harnessed to provide reverberation gain by placing microphones throughout the room volume. Such systems are termed non-in-line, or regenerative systems. Regenerative systems have the advantage that the reverberation enhancement occurs for both the performers and the audience (reverberation is a global property of a room which does not depend on the sound source position). Feedback with multiple loudspeaker-microphone channels behaves in a more natural way than single channel feedback, and the sound system stability is easily controllable with fewer colouration artefacts.