Man's physical universe

xanabras

462 ENERGY MAY BE PROPAGATED BY VIBRATIONS

flers do not build up back pressures and should be of value in silencing

airplane exhausts.

Reverberations Are Produced by the Reflection of Sound Waves.

In the open country one can often hear several echoes produced by a

loud sound, as it is reflected back and forth between mountains or

hills; a dynamite blast in the mountains often produces such reverberations.

Reverberations are not desired in broadcasting studios and

auditoriums.

Depth-sounding devices used by many ships depend upon the

measurement of the time required for sound waves to travel to the

ocean bottom and back to a receiving microphone. Airplanes use

similar devices to determine their distance from the ground.

Sound reflection is also used in geophysical prospecting. Vibrations

produced by explosions in the ground are recorded by seismometers

placed at various positions. Sound waves travel faster through salt

domes and other possible oil-bearing formations than they travel

through solid rock.

The presence of approaching airplanes can be detected by the use of

large concave mirrors, five feet or more in diameter, which converge

the sound waves on a sensitive sound-detector.

The Reflection of Sound Waves Causes Acoustical Problems.

Megaphones, trumpets, and horns of various kinds act as reflectors,

increasing the intensity of the sound in one direction, just as the reflector

in a flashlight concentrates the light in one place.

Sounds are more intense in a room or hall, because they are reflected

back by the walls. Sometimes, however, the sound has to travel so far

that the reflected waves are not received at the same time that the

original waves arrive. These echoes are one of the sources of poor

acoustics in rooms. This difficulty can be remedied in part by hanging

draperies against the walls or by lining the walls with materials which

absorb rather than reflect sound. People's clothes absorb sound; the

echoes characteristic of a large empty hall are not so noticeable when

it is filled with people.

The scientific basis for acoustics in rooms and halls has been worked

out. In order to obtain proper acoustical properties, the size, shape,

and the materials of construction must be considered. After a hall is

once built, consideration of the furnishings and the location of the

audience and the sound source are important. It is needless to say

that only highly trained specialists can be depended upon to solve

these problems satisfactorily. Many large auditoriums that have been

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