Specifications

6
Designing the distributed sound system
There are several main steps in designing a distributed sound system:
• Determining loudspeaker coverage and placement
• Determining power levels for each loudspeaker
• Choosing the right amplifier
Loudspeaker coverage and placement
In placing loudspeakers in a distributed system, the goal is to provide coverage effectively but economically. An
effective coverage would be one where the sound from the loudspeakers is not only audible, but also intelligible,
wherever needed. An economical coverage would tend to be one that achieves the goal using the fewest
loudspeakers necessary.
A loudspeaker in an enclosed area produces two sound fields. The main one is the direct field—sound coming
directly “line-of-sight” from the loudspeaker. Primary and secondary reflections can also be considered part of
the direct sound field, as long as their delays are short enough to psychoacoustically reinforce the original sound.
The other is the diffuse field (sometimes called the reverberant field), which is sound that you might call “post
direct.” This diffuse field of reverberation is sound that has bounced around the room, reflecting off surfaces
such as floors, walls, tables, ceilings, etc., until it is absorbed by the air, other objects, and the room itself. The
diffuse field is comprised of multiple sound wave fronts traveling in different directions, each taking a slightly
different length of time to arrive at the listener (or microphone). As a result, a common characteristic of the diffuse
field is “image smearing,” which reduces the intelligibility of the sound.
Thus, to keep intelligibility high, you should maximize
the ratio of direct field to diffuse field. As the Inverse
Square Law dictates (see the sidebar on page 8), the
direct field sound falls off as the distance from the
loudspeaker increases. The diffuse field is also sub-
ject to the Inverse Square Law, but moving away from
one reflective surface often moves you towards
another; as a result, the intensity of the diffuse field
usually doesn’t vary significantly throughout a room.
The graph at right shows the direct, diffuse and
combined sound fields (direct and diffuse, summed)
of a single loudspeaker in a large, fairly reverberant
room. Closer to the speaker, the direct field is much
stronger than the diffuse field; intelligibility here will
be very good to excellent, but it will drop off as you
move further away. At the critical distance, D
C
, the
direct and diffuse fields are equal in intensity, and
beyond D
C
the diffuse field overpowers the direct. At
this position a person speaking clearly though the
sound system might be heard, but not clearly enough
-35.0
-30.0
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
110
Critical distance, D
C
Diffuse (reverberant) sound fieldDiffuse (reverberant) sound field
Direct + diffuse fieldsDirect + diffuse fields
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D
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Distance from loudspeaker, in meters
Relative SPL (referenced to 1 meter from loudspeaker)
20 30 40
Intensity of direct and diffuse sound fields in an enclosed space