L-Acoustics ARCS, Руководство оператора

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L-ACOUSTICS ARCS Manual V2.0 2/13/2003
4
0. INTRODUCTION
0.1 WAVEFRONT SCULPTURE TECHNOLOGY ®
a) The Sound Reinforcement Problem
Effectively covering an audience is the goal of any sound reinforcement system design. This is
straightforward in small spaces where the left/right stereo configuration is suitable provided that the
available power is sufficient, i.e., a stereo pair of loudspeakers is a relatively easy system to install and
the results are fairly predictable. Things become more complex when larger audience area coverage is
required and there are two possible approaches:
1) Multiplying the number of sound sources by dividing the audience into areas which are covered by
individual sources. In this case, the Haas effect is exploited and delay lines can also be introduced to
provide proper localization. This is the distributed sound reinforcement, or multiple sound source
approach and the L-ACOUSTICS MTD or XT lines of coaxial loudspeakers is highly suited to this type
of sound design.
2) Coupling a number of individual sound sources to form a loudspeaker array with the objective that
each array becomes the equivalent of a single sound source.
Typically, the multiple sound source concept is best suited to medium-sized venues, whereas in very
large venues it can be impractical and expensive to install and tune a large number of loudspeakers
with many delay lines. In addition, the distributed sound reinforcement approach can be unsuitable for
open air applications or in a daily touring situation.
Although coupling a number of loudspeakers to provide the required sound pressure and angular
coverage has long been regarded as the most practical approach for high power sound reinforcement,
the condition that "each array becomes the equivalent of a single sound source" has often been comprised
or overlooked.
When arrayed, most conventional speakers do not couple acoustically at frequencies having a
wavelength smaller than the physical size of the enclosure. Typically, problems occur at mid and high
frequencies, accounting for roughly two thirds of the audio spectrum. The net result is a chaotic sound
field that exhibits severe comb filtering, uneven frequency response that varies with listener position
and a significant loss of energy. In practical terms, this results in efficiency and intelligibility losses plus
uneven coverage.
b) Wavefront Sculpture Principles
Conditions for achieving proper coupling of individual arrayed sound sources have been defined by Dr.
Christian Heil and Professor Marcel Urban, in "Sound Fields Radiated By Multiple Sound Source
Arrays", AES paper preprint 3269, presented at the 92nd AES convention in Vienna, 1992.
The theory that was developed defines the acoustic coupling conditions required for effectively
arraying individual sound sources. Relevant parameters include: wavelength, the shape of each source,
the surface area of each source and the source separation.
WST coupling conditions can be summarized as follows:
An assembly of individual sound sources arrayed with regular separation between the sources on a plane or
curved continuous surface is equivalent to a single sound source having the same dimensions as the total
assembly if, and only if, one of the two following conditions is fulfilled:
1) Shape: The combined surface area of the wavefronts radiated by the individual sources of the array fills at
least 80% of the target radiating surface area (see also Condition 3).