User Guide

Sound System Design Reference Manual
Critical distance also is affected by the
absorption coefficients of room boundary surfaces.
Figures 5-17 and 5-18 illustrate the same sound
source in the same size room. The difference is that
in the first illustration the room surfaces are assumed
to be highly reflective, while in the second they are
more absorptive. The density of the black dots
representing the direct field is the same in both
illustrations. In the live room, because energy
dissipates quite slowly, the reverberant field is
relatively strong. As a result, the circle-black
boundary is pushed in close to the sound source. In
the second example sound energy is absorbed more
rapidly, and the reverberant field is not so strong.
Therefore, the circle-black boundary is farther from
the source.
Even though the direct field and the reverberant
field are produced by the same sound source, the
sound is so well scrambled by multiple reflections
that the two components are non-coherent. This
being so, total rms sound pressure measured at the
critical distance should be 3 dB greater than that
produced either by the direct field or reverberant field
alone.
Within the normal variations of statistical
averaging, such is the case in actual rooms. The
behavior of loudspeakers in rooms was described in
great detail in 1948 by Hopkins and Stryker (6). Their
calculations of average sound pressure level versus
distance are illustrated in Figure 5-19. A great deal of
useful information has been condensed into this
single chart. Sound pressure is given in terms of the
level produced by a point source radiating one
acoustic watt. The straight diagonal line shows the
decrease in sound pressure with distance that would
be measured in open air.
The Room Constant (R)
The various shelving curves are labeled with
numbers indicating a new quantity, the room
constant . This will be defined in subsequent
paragraphs. Essentially, R is a modified value of the
total absorption in the room [R = Sα/(1 -α)]. A small
room constant indicates a very live room, and a large
room constant describes a room having a great deal
of absorption.
5-15
Figure 5-19. SPL (point source radiating one acoustic watt)
vs. R and distance from source