Page 262 - The Master Handbook Of Acoustics

237
REFLECTION OF SOUND

Doubling of Pressure at
Reflection

The sound pressure on a surface normal to a i
the incident waves is equal to the energy-
density of the radiation in front of the sur-
face. If the surface is a perfect absorber, the a r
pressure equals the energy-density of the
incident radiation. If the surface is a per-
fect reflector, the pressure equals the
energy-density of both the incident and
a r
the reflected radiation. Thus the pressure
at the face of a perfectly reflecting surface
is twice that of a perfectly absorbing sur-
a
face. At this point, this is only an interest- i
ing sidelight. In the study of standing
waves in Chap. 15, however, this pressure
doubling takes on greater significance. FIGURE 10-2
The angle of incidence, a , is equal to the angle of
i
Reflections from Convex reflection, a .
r
Surfaces

Spherical wavefronts from a point source tend to become plane waves
at greater distance from the source. For this reason impinging sound
on the various surfaces to be considered will be thought of as plane
wavefronts. Reflection of plane wavefronts of sound from a solid con-
vex surface tends to scatter the sound energy in many directions as
shown in Fig. 10-3. This amounts to a diffusion of the impinging
sound.
The polycylindrical sound-absorbing system described in the pre-
vious chapter both absorbs sound and contributes to much-needed dif-
fusion in the room by reflection from the cylindrically shaped surface.

Reflections from Concave Surfaces

Plane wavefronts of sound striking a concave surface tend to be
focussed to a point as illustrated on Fig. 10-4. The precision with
which sound is focussed to a point is determined by the shape of the