Handbook for Sound Engineers

Fundamentals of Audio and Acoustics 29

If the obstruction is smaller than the wavelength of
the wave striking it, the wave diffracts around the
obstruction and continues to propagate. Both effects are
complex and frequency (wavelength) dependent,
making them difficult to calculate, Fig. 2-11.
The reflected wave will be strong if the surface is
large and has low absorption. As absorption is
increased, the level of the reflection is reduced. If the
surface is random, the wave can be scattered depending
on the size relationship between the wave and the
surface relief. Commercially available diffusors can be
used to achieve uniform scattering in critical listening
spaces, Fig. 2-12.

2.6 Surface Shapes

The geometry of a boundary can have a profound affect

on the behavior of the sound that strikes it. From a

sound reinforcement perspective, it is usually better to

scatter sound than to focus it. A concave room boundary

should be avoided for this reason, Fig. 2-13. Many audi-

toriums have concave rear walls and balcony faces that

require extensive acoustical treatment for reflection

control. A convex surface is more desirable, since it

scatters sound waves whose wavelengths are small rela-

tive to the radius of curvature. Room corners can

provide useful directivity control at low frequencies, but

at high frequencies can produce problematic reflections.

Electrical reflections can occur when an electromag-

netic wave encounters a change in impedance. For such

waves traveling down a wire, the reflection is back

towards the source of the wave. Such reflections are not

usually a problem for analog waves unless there is a

phase offset between the outgoing and reflected waves.

Note that an audio cable would need to be very long for

its length to cause a significant time offset between the

Figure 2-9. The wavelength of an event determines how it interacts with the medium that it is passing through. Courtesy

Syn-Aud-Con.

1 wavelength Increasing

time

Phase angle

(deg)

T = 1/f

f = 1/T

= Tc

where,

T is the time in seconds,

f is frequency in hertz,

c is propagation speed

in feet or meters.