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ACOUSTICS
B&C SPEAKERS, BAGNO A RIPOLI, ITALY
Arrays of speakers for public venues must have wavefronts that combine to work as if they were a single
speaker. Modeling helps in the design of an acoustic lens and dramatically shortens the time needed to
find the best design.
BY MATTIA COBIANCHI AND ROBERTO MAGALOTTI, B&C SPEAKERS S.P.A.
Projecting high-quality sound to large
audiences often involves a linear array
of speakers. However, to make sure
that every audience member hears every
word clearly and every sound with full
fidelity, the wavefronts from the individual
speakers inside the array must be in
phase with each other, and the frequency
response must be as smooth as possible
— ideally flat. The traditional method
has been to combine megaphone-variant
horns in a coherent array, but even the
best such horns produce unintended side
effects such as diffraction, reflection and
distortion. These waveguides leave considerable
room for improvement when
trying to combine multiple loudspeaker
enclosures in arrays without destroying
the coherency of the wavefront.
In an ideal waveguide, the signal
throughout the driver outlet arrives in
phase. Over the years, speaker manufacturers
have employed a variety of techniques
to achieve that effect such as by
shaping the waveguides a certain way
or by using variable-density foam. With
the help of COMSOL Multiphysics and
the Acoustics Module, our research team
at B&C Speakers S.p.A. has taken a new
approach by designing an acoustic lens in
a waveguide (Figure 1, left) that achieves
the required phase coherence in the output
sound field. This allows a line array
of individual speakers (Figure 1, right)
to work together as an extended sound
source, which is particularly important
for the high frequencies from roughly 1
to 20 kHz. The final design of our waveguide
is now patented in Europe and patent
pending in China.
Getting Line Arrays to Function as One
In a line array, a series of speaker modules
are stacked on top of each other, and
the slot in the center projects the higher
frequencies. This high-frequency source is
nearly as tall as the entire module so that
stacked modules in essence form an uninterrupted
line from top to bottom and function
as a single source. They must have
phase coherence so that the sound field outside
the waveguide outlet is isophasic (that
is, the sound at all frequencies presents a
flat waveform upon exiting the horn).
One issue is that our compression drivers
consist of a membrane with a plug-like
transformer, and the exit at the end of the
Figure 1. A B&C waveguide undergoing acoustic testing (left). A line array using this type of waveguide (right) consists of several speakers each with one
or more compression drivers stacked so the thin rectangular exit at the center of each is aligned to form an extended sound source going from top to bottom.
Photo courtesy of LSS Advanced Speaker Systems — Italy. Employing 90 people, B&C Speakers is one of largest and most prestigious manufacturers of
professional loudspeaker transducers in the world, and our core business is in professional audio for large systems intended for nightclubs, concert halls and
stadiums. Besides designing and distributing components under our own brand name, we also supply OEM components to most of the top professional audio
brands in the market today.
// COMSOL NEWS 2010
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