Page 137 - Master Handbook of Acoustics

In a rectangular room, there are six surfaces and the source has an image in all six surfaces,
sending energy back to the receiver, resulting in a highly complex sound field. In computing the total
sound intensity at a given receiving point, the contributions of all these images must be taken into
consideration.

Flutter Echoes

Returning again to Fig. 6-3, we note that parallel walls such as these present an acoustical problem. If
the distance between the walls is large enough so the time between reflections is outside the Haas
fusion zone, a flutter echo is created as sound bounces back and forth from one wall to the other.
Because of the regularity of these reflections, the ear is very sensitive to the effect. In fact, even if the
time delays are otherwise in the fusion zone, the effect may still be audible as an echo. This echo can
be very prominent in an otherwise diffuse sound field and is highly undesirable. In theory, with

perfectly reflective walls, there would be an infinite number of images. The acoustical effect is the
same as being between two mirrors and seeing the series of images. In practice, successive images
attenuate because of absorption or diffusion at the walls. Where possible, parallel walls should be
avoided, and when unavoidable, they should be covered by absorbing or diffusing material. Splaying
walls by a small amount of perhaps 5° or 10° can also avoid flutter echoes.

When sound strikes a boundary surface, some sound energy is transmitted or absorbed by the
surface and some is reflected. The reflected energy is always less than the incident energy. Surfaces
that are made of heavy materials (measured by surface weight) are usually more reflective than lighter
materials that tend to absorb or transmit sound. Sound may undergo many reflections as it bounces
around a room. The energy lost at each reflection results in the eventual demise of that sound.
Reflection depends partly on the size of the reflecting object. Sound is reflected from objects that

are large compared to the wavelength of the impinging sound. Generally speaking, sound will be
reflected from a rectangular panel if each of its two dimensions is five times the wavelength of sound.
Thus, objects act as frequency-dependent reflectors. This book would be a good reflector for 10-kHz
sound (wavelength about an inch). While facing a sound source, moving the book in front of and away
from your face would result in significant differences in high-frequency response because of

acoustical shadowing. At the low end of the audible spectrum, 20-Hz sound (wavelength about 56 ft)
would sweep past the book and the person holding it as though they did not exist, and without
appreciable shadows.

Doubling of Pressure at Reflection

The sound pressure on a surface normal to an incident wave is equal to the energy density of the
radiation in front of the surface. If the surface is a perfect absorber, the pressure equals the energy
density of the incident radiation. If the surface is a perfect reflector, the pressure equals the energy
density of both the incident and the reflected radiation. Thus the pressure at the face of a perfectly
reflecting surface is twice that of a perfectly absorbing surface. In the study of standing waves, this

pressure doubling takes on great significance.