Page 141 - Master Handbook of Acoustics

Whispering Galleries

St. Paul’s Cathedral in London, St. Peter’s Basilica in Vatican City, the Temple of Heaven in Beijing,
Statuary Hall in the U.S. Capitol Building, Grand Central Station in New York City (in front of the
Oyster Bar & Restaurant), and other structures contain a whispering gallery; the acoustical
mechanism is diagrammed in Fig. 6-7. The source whisperer and the receiver may be separated from
each other by a great distance; both are located at the focal points of hard-surfaced, paraboloid-
shaped walls, and face the walls. 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
paraboloid-shaped. This means that upward-directed components of the whispered sounds tend to be
reflected downward and are conserved and transmitted rather than lost above. Although acoustically
interesting, architectural configurations such as this are usually undesirable. Except for unique
applications, concave surfaces such as sections of cylinders, spheres, paraboloids, and ellipsoids
should not be used in acoustically important architecture. The sound focusing effects are exactly

counter to the usual goal of providing uniform, diffuse sound throughout a space.

FIGURE 6-7 Graphic example of a whispering gallery showing symmetrical sound focusing points.
A whisper directed tangentially to the paraboloid-shaped surface is readily heard by the receiver on

the far side of the room. More generally, concave surfaces pose acoustical problems.

Standing Waves

The concept of standing waves directly depends on the reflection of sound. Assume two flat, solid
parallel walls separated a given distance (as in Fig. 6-3). A sound source between them radiates

sound of a specific frequency. As we observed, 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 travels to the right, the other toward the left. The two traveling waves interact to form a
standing wave. Only the standing wave, the interaction of the two, is stationary. The frequency of the
radiated sound establishes this resonant condition between the wavelength of the sound and the
distance between the two surfaces. This phenomenon is entirely dependent on the reflection of sound

at the two parallel surfaces. As discussed in other chapters, standing waves require careful design
scrutiny, particularly in terms of a room’s low-frequency response.