The design of a concert sound system, especially for stereo sound, requires close attention to issues such as room acoustics, headroom, loudspeaker levels, timing, and coverage.
For a surround system, those basic issues are even more important and complex.
This is because the timing and level distribution of the sound are dependent on the direction of wave propagation. Mono and stereo sound can propagate generally from the stage toward the more distant part of the audience, where the level may decrease and the delay will increase.
But in surround sound, wave propagation occurs in various opposite directions, therefore, the level balancing and synchronization of loudspeakers across the audience is more difficult.
There are distinct types of surround systems suitable for different applications. For example, an ambience surround system may provide additional room sound (reverberation and early reflections) for improving the room response or to change its “liveness” for a particular performance.
Cinema surround systems typically provide left-center-right for front localization, and additional channels to cover the sides and rear. Since the sides and rear surround channels are primarily used for sound envelopment and effects, the problems with varying delay from multiple sources are not very noticeable. A music surround system, in contrast, needs better time alignment between all of the audible sources. In addition, the surround loudspeakers will need to match the main loudspeakers’ frequency response and headroom.
Let’s look at conceptual approaches to creating this type of system in venues that are too large for conventional loudspeaker layouts.
Any Audible Experience
One of the most convincing sound reproduction methods is binaural sound. When implemented to the fullest, this method is capable of accurately creating or recreating just about any audible experience. But among other things, it requires the signals heard by the left and right ears to be kept separate. Headphones can provide total separation but their use by a large audience is usually not very practical or desirable.
The unwanted interaural crosstalk that occurs when loudspeakers are used can be canceled with digital processing, but this is only practical for a specific listener at a specific location. Even then, it can only be an approximation that is not likely to perform well for side or rear directions, and cannot tolerate much listener movement. For a large audience, this method is not yet practical, although holding great potential, it may become viable in the future.
The precise reconstruction of any sound wave from a group of loudspeakers is possible, but only at a single point in space. An “Ambisonic” system achieves this using a mathematically correct operation to create each loudspeaker’s drive signal based on its location with respect to the listening point.
If the angular spacing between adjacent loudspeakers is not too large, this approach can work quite well. But for a listener who is some distance from the ideal point, neither the mathematical process nor the listening experience is optimum.
For significantly off-center listeners, there are other methods that can provide a better surround experience, primarily by virtue of higher channel separation allowing clearer perception of opposite-side sound-source locations.
A practical approach to sound staging used in cinema and live theater is placing loudspeakers in specific desired locations and simply assigning the signal to them as needed. This is particularly effective in theatrical productions, where certain sounds require precise point-source localization for a very wide audience area, and loudspeakers can be arbitrarily placed.