Page 268 - The Master Handbook Of Acoustics

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REFLECTION OF SOUND

source. This is an example of a benevolent
mismatch as the air-conditioner noise is
reduced in the room.

The Corner Reflector Receiver

In an art museum with large Dutch paint-
ings on display, the eyes of certain subjects
seem to follow as one walks by. Corner
reflectors are like that. There seems to be
no way of escaping their pernicious effect.
Source
The corner reflector of Fig. 10-9, receiving
sound from the source S, sends a reflection
directly back toward the source. If the
angles of incidence and reflection are care- FIGURE 10-8
fully noted, a source at B will also send a
Graphic explanation of the “Whispering Gallery” of
direct, double-surface reflection returning St. Paul’s Cathedral, London. A whisper directed tan-
to the source. A source at C, on the oppo- gentially to the cylinderical surface is readily heard
site of B, is subject to the same effect. by the receiver on the far side of the room.
You might be instinctively aware of per-
pendicular (normal) reflections from surrounding walls, but now consider
reflections from the four corners of the room that follow the source around
the room. Corner reflections suffer losses at two surfaces, tending to make
them somewhat less intense than normal reflections at the same distance.
The corner reflector of Fig. 10-9 involves only two surfaces. How
about the four upper tri-corners of the room formed by ceiling and
walls and another four formed by floor and wall surfaces? The same
follow-the-source principle applies. In fact, sonar and radar people
have long employed targets made of three circular plates of reflecting
material assembled so that each is perpendicular to the others.

Echo-Sounding

Objects can be located by sending out a pulse of sound and noting the
time it takes for the reflected echo to return. Directional sources of
sound make possible the determination of both the azimuth angle and
the distance to the reflecting object. This principle has been widely
applied in water depth sounders, sonar on submarines, etc. All