User Guide

Sound System Design Reference Manual
1-5
Subjectively, the effect of such a comb filter is
not particularly noticeable on normal program
material as long as several peaks and dips occur
within each one-third octave band. See Figure 1-6.
Actually, the controlling factor is the “critical
bandwidth.” In general, amplitude variations that
occur within a critical band will not be noticed as
such. Rather, the ear will respond to the signal power
contained within that band. For practical work in
sound system design and architectural acoustics, we
can assume that the critical bandwidth of the human
ear is very nearly one-third octave wide.
In houses of worship, the system should be
suspended high overhead and centered. In spaces
which do not have considerable height, there is a
strong temptation to use two loudspeakers, one on
either side of the platform, feeding both the same
program. We do not recommend this.
Diffraction of Sound
Diffraction refers to the bending of sound waves
as they move around obstacles. When sound strikes
a hard, non-porous obstacle, it may be reflected or
diffracted, depending on the size of the obstacle
relative to the wavelength. If the obstacle is large
compared to the wavelength, it acts as an effective
barrier, reflecting most of the sound and casting a
substantial “shadow” behind the object. On the other
hand, if it is small compared with the wavelength,
sound simply bends around it as if it were not there.
This is shown in Figure 1-7.
An interesting example of sound diffraction
occurs when hard, perforated material is placed in
the path of sound waves. So far as sound is
concerned, such material does not consist of a solid
barrier interrupted by perforations, but rather as an
open area obstructed by a number of small individual
objects. At frequencies whose wavelengths are small
compared with the spacing between perforations,
most of the sound is reflected. At these frequencies,
the percentage of sound traveling through the
openings is essentially proportional to the ratio
between open and closed areas.
At lower frequencies (those whose wavelengths
are large compared with the spacing between
perforations), most of the sound passes through the
openings, even though they may account only for 20
or 30 percent of the total area.
Figure 1-7. Diffraction of sound around obstacles