Man's physical universe

xanabras

452 ENERGY MAY BE PROPAGATED BY VIBRATIONS

The walls of a megaphone prevent the sound waves from spreading

out as soon as they would spread from the mouth.

Sound Waves May Be Refracted.

Lake

Inasmuch as sound travels at different speeds in layers of air of different

temperature and relative humidity, sound waves may be bent

as they pass from one medium to another, as shown in Fig. 218, to the

extent that the convex waves

Warm Air /rfFX/n^

bccome concavc in nature

Fig. 218.

Sound waves are refracted as they

pass from cold air through warm air.

and thus focus the sound at

a given point. The sound

thus appears to travel farther

under these conditions.

The sound of a guitar played

on a boat on a lake on a

hot summer night will travel

long distances with remarkable clearness because of the refraction of

the sound waves by the warm air over the cooler water.

Sound Waves May Be Analyzed by Methods Which Use Compressional

Waves to Produce Corresponding Wavelike Patterns in Beams of

Light.

Curves showing the nature of sound waves can be obtained by an

instrument called the "phonodeik." The principle here employed is

that the vibrations in the air move a sensitive diaphragm, w^hich is

attached to a mirror reflecting a beam of light to a motor-driven

revolving mirror, which, in turn, throws the beam of light on a screen

in the form of a long wave. It is important to keep in mind that the

waves in a beam of light do not resemble compressional waves in the

atmosphere but rather serve as a diagrammatic representation of

sound waves.

Sound waves may also be directly recorded by the phonautograph,

in which a stylus attached to the diaphragm traces a path on a smoked

paper carried on a rotating cylinder.

The telephone diaphragm and the microphone generate oscillating

currents from sound waves, which may be received by a cathode-ray

type of oscillograph. This oscillograph is of great value in telephone

and radio research and testing.

The phonograph, invented by Thomas A. Edison in 1877, recorded

the movements of a stylus controlled by a diaphragm, by indentation

in a sheet of tinfoil supported over a spiral groove on a metal cylinder.

Such

Later machines made the records on wax cylinders or disks.

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