Page 171 - Master Handbook of Acoustics

CHAPTER 9

Diffusion

T o make their calculations easier, theorists often assume a completely diffuse sound field, one

that is isotropic and homogeneous. That is, a sound field where at any point, sound can arrive

from any direction, and a field that is the same throughout the room. In practice, that rarely occurs,
especially in small rooms. Instead, as we plainly hear every day, the characteristics of sound are
markedly different throughout most rooms. In some cases, directionality is welcome, because it may,
for example, help a listener localize the source of the sound. In most room designs, diffusion is used
to more effectively distribute sound and to provide a more equal response throughout a room that
immerses the listener in the sound, while preserving a level of directionality that is appropriate for
the application. It is often difficult to provide sufficient diffusion, particularly at low frequencies and

in small rooms, because of the room’s modal response. The goal of most room designs is to obtain
sound energy across the audible frequency range that is uniformly distributed throughout the room.
This is unattainable, but diffusion greatly assists in the effort.

The Perfectly Diffuse Sound Field

Even though unattainable, it is instructive to consider the characteristics of a diffuse sound field.
Randall and Ward have suggested the following ideals:

• The frequency and spatial irregularities obtained from steady-state measurements must be
negligible.

• Beats in the decay characteristic must be negligible.
• Decays must be perfectly exponential (they will appear as straight lines on a logarithmic
scale).

• Reverberation time will be the same at all positions in the room.
• The character of the decay will be essentially the same for all frequencies.

• The character of the decay will be independent of the directional characteristics of the
measuring microphone.

These six factors are observation oriented. More theoretically, a diffuse sound field would be defined
in terms such as energy density, energy flow, and superposition of an infinite number of plane

progressive waves. However, these six characteristics point us to practical ways of judging the
diffuseness of the sound field in a given room.