Man's physical universe

xanabras


450 ENERGY MAY BE PROPAGATED BY VIBRATIONS

There Are Many Interesting Consequences of the Relatively Small

Velocity of Sound in Air.

Many curious effects can be explained by the relatively small velocity

of sound in air. An observer can see that the marchers in a parade

some distance behind the band lag in their step because they are

marching with the music. It seems to a listener at some distance and

to one side of a large chorus or orchestra that the most distant musicians

lag behind those who are closer, although they are all

kept together

by the conductor.

All Sound Waves in a Given Medium Travel at the Same Speed.

It will be recalled that a prism refracts light into a colored spectrum

band because the longer waves travel with a greater speed than the

short waves through any material.

In the case of sound, the velocity

in a given medium is not affected by the wave length, except for very

intense sounds. If this were otherwise, the music from an orchestra,

for example, would be considerably distorted.

Compressional Waves May Be Produced by Vibrating Bodies.

Pitch {i.e., the sensation of highness or lowness) depends upon the

number of times a body vibrates per second {i.e., the frequency), and

ft

W^

T^^^

the frequency of the vibrations

.-''' ''--.

determines the number of compressional

waves set up in the

atmosphere.

Pitch is subjective,

while frequency is the objective

physical magnitude. Inasmuch

as sound always travels at the

same rate through the atmosphere

under the same condir^

r^^. r^y. •. i •

i . •

J u .u tions. It follows that the more

'

Fig. 216. The pitch is determined by the

number of vibrations per second.

waves there are per second the

shorter will be the wave length

{i.e., the distance from rarefaction to rarefaction, or from compression

to compression).

The amplitude of the vibrations of a body determines the energy

carried by compressional waves. The total amount of energy carried

by compressional waves per second will be a product of the frequency

by the energy carried by each wave. The total amount of energy per

second therefore depends upon both the square of the frequency and

amplitude of the generator.

The intensity of compressional waves, like the intensity of any kind

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