Handbook for Sound Engineers

138 Chapter 6

strictly speaking, reverberation and should be treated as
such. The Sabine equation and its offspring are not
going to help in predicting how much absorption is
going to be needed to modify a room. More discussion
and research are needed to be able to fully quantify the
behavior of absorption in small rooms.

As is mentioned in Chapter 5, the standard way to
measure acoustical treatment is according to ASTM
C 423-00. This is an indirect method that looks at the
impact that the material has on a diffuse sound field. In
rooms that do not exhibit any diffuse sound field in the
frequency range of interest, we need another way to
measure acoustic treatment. An alternative method is
outlined in Chapter 5.

6.7.1 Room Shape

We have referred to the statistical approach (e.g., rever-
beration) and the wave approach (modes) to acoustical
problems, and now we come to the geometrical approach.

The one overriding assumption in the application of

geometrical acoustics is that the dimensions of the room

are large compared to the wavelength of sound being

considered. In other words, we must be in region 3 of Fig.

6-15 where specular reflection prevails and the short

wavelengths of sound allow treating sound like rays.

A sound ray is nothing more than a small section of a

spherical wave originating at a particular point. It has a

definite direction and acts as a ray of light following the

general principles of geometric optics. Geometrical

acoustics is based on the reflection of sound rays. This

is where the shape of the room is the controlling acous-

tical aspect. Like the quest for room ratios the search for

the perfect room shape is also ellusive. Some have

suggested that nonparallel surfaces are a must, however

there are no perfect shapes. There are some shapes that

work well for some applications.

6.7.2 Reflection Manipulation

In open space (air-filled space, of course) sound travels

unimpeded in all directions from a point source. In real

rooms we don’t have point sources, we have loud-

speakers or other sound sources such as musical instru-

ments that do not behave like the theoretical point

source. Real sources have characteristic radiation or

directional patterns. Of course in real rooms the sound

does not travel unimpeded for very long, depending on

the MFP. After the sound leaves its source it will bounce

off of some surface and will interact with the unre-

flected sound. This interaction can have a profound

impact on the perception of the original sound. There is

Figure 6-14. ETC of a small room, approximately 250 ft^2 area.

Figure 6-15. Regions for room treatment.

Region 1

C/

2L f 4f

Region 2 Region 3 Region 4

Modes Dominate Surface Treatment