Page 265 - The Master Handbook Of Acoustics

240 CHAPTER TEN

Reflections Inside a Cylinder

St. Paul’s Cathedral in London boasts a whispering gallery. The way this
whispering gallery works is explained in the diagram of Fig. 10-8. Reflec-
tions from the exterior surfaces of cylindrical shapes have been men-
tioned in the treatment of “polys.” In this case the source and receiver are
both inside a mammoth, hard-surfaced cylindrical room.
At the source, a whisper directed tangentially to the surface is
clearly heard on the receiver side. The phenomenon is assisted by the
fact that the walls are dome-shaped. This means that upward-directed
components of the whispered sounds tend to be reflected downward
and conserved rather than lost above.

Standing Waves

The concept of standing waves is directly dependent on the reflection
of sound as emphasized in Chap. 15. Assume two flat, solid parallel
walls separated a given distance. A sound source between them radi-
ates sound of a specific frequency. The wavefront striking the right
wall is reflected back toward the source, striking the left wall where it
is again reflected back toward the right wall, and so on. One wave trav-
els to the right, the other toward the left. The two traveling waves
interact to form a standing wave. Only the standing wave, the interac-
tion of the two, is stationary. The frequency of the radiated sound is
such as to establish this resonant condition between the wavelength of
the sound and the distance between the two surfaces. The pertinent
point at the moment is that this phenomenon is entirely dependent on
the reflection of sound at the two parallel surfaces.

Reflection of Sound
from Impedance Irregularities

The television repairman is concerned about matching the electrical
impedance of the television receiver to that of the transmission line,
and matching the transmission line to the impedance of the antenna
(or cable). Mismatches of impedance give rise to reflections, which
cause numerous undesirable effects.