Introduction - L-Acoustics MTD Series Operator's Manual

0. INTRODUCTION

Effectively covering an audience is the goal of any sound reinforcement system design. This is
straightforward in small spaces where a 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 the goal is to reduce audible
interference effects by dissociating or decoupling the individual sound sources (delay lines can also be
introduced to provide proper localization). This is the distributed sound reinforcement, or multiple
sound source approach, and the MTD line of enclosures is highly suited for 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.
For the second approach, 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). Additional conditions were published in the AES preprint ''Wavefront Sculpture
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Technology'' (WST) that was presented at the 111 Convention, NYC, 2001 (preprint 5488). The
theory that was developed defines the acoustic coupling conditions required for effectively arraying
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individual sound sources. These conditions are satisfied by the ARCS , dV-DOSC™ and V-DOSC
products which are intended for medium- to large-scale sound reinforcement applications. However,
in most cases, it is not feasible to meet WST criteria while at the same time having a sufficient level of
versatility for small- to medium-scale applications. In other words, if a product is to be arrayable, it
typically leads to an enclosure design that cannot be used in single or very small configurations. A
different set of loudspeaker enclosure design specifications apply that are more suited to the multiple
source sound design approach.
The L-ACOUSTICS approach to distributed sound reinforcement using multiple sound sources starts
with the specification that each individual loudspeaker enclosure should behave as a totally coherent
source. This criterion can be achieved using coaxial components which are well-suited to the design of
highly versatile, small format systems. The use of coaxial components has been popularized over the
years by a famous British manufacturer for studio monitoring applications – to the best of our
knowledge, L-ACOUSTICS was the first manufacturer to use coaxial technology in professional sound
reinforcement applications and the current MTD line is a continuation of the heritage that was
introduced in 1989.
Coaxial, dual concentric components provide a smooth transition between the LF and HF sections
since, by definition, the directivity of the two transducers is matched at the crossover frequency. In
addition, the directivity is horizontally, vertically and diagonally symmetric (axi-symmetric). This results
in true, single source behavior and the performance obtained is superior in terms of coherence when
compared with any combination of two independent sound sources (separate woofer plus horn-
loaded compression driver, for example). This is the case even if the independent sources are
designed to provide the same directivity behavior (which is rarely the case) since the acoustic centers
of the two sources are not located at the same physical location.
Other benefits obtained using the coaxial, axi-symmetric configuration are smooth acoustical
impedance loading for the compression driver and a short time window of reflections which is far
more acceptable than the longer reflection sequences that are produced by traditional horn designs. In
addition, the wavefront radiated by an axi-symmetric sound source has directivity that is smoothly
increasing with frequency which helps to match the acoustical environment of a typical auditorium.
Normally, the reverberation time in auditoria decreases smoothly above 1 kHz and at greater
distances in the venue, the low frequency energy is fairly constant due to the reverberant field.
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L-ACOUSTICS MTD Manual V1.2 11/30/2004