Man's physical universe






The sense of hearing is next in importance to the sense of sight in

enabhng us to receive impressions from a distance. In this Section

we shall consider the nature and characteristics of sound waves, which

are produced by vibrations in matter.

Sound Waves Are Longitudinal or Compressional Waves.

Light waves are transverse to the direction in which the wave travels,

whereas sound waves are longitudinal, i.e., they lie along the direction

in which the wave travels.

Sound waves are compressional waves.

The elasticity of the original

layer of air compressed by the sound source makes it expand, thus

passing on the state of compression to the next layer of air beyond.

Sounds Are Transmitted by Waves in Matter.

Any stimulus which will produce the sensation of hearing is called a

sound. This sensation is produced by vibration of the eardrum, which,

in turn, is caused to vibrate by a wave motion of some medium,

usually the air. The compressional waves in this medium itself are

produced by the vibration of some body. The air is not essential for

the transmission of vibrations to the ear. Any medium which can

transmit compressional waves will serve. A swimmer can hear better

under water than above water, because water carries vibrations faster

and with greater intensity than air. If the ear is placed on a railroad

track, the sound of an approaching train can be heard, because solids

like wood or steel transmit vibrations.

Vibratory motion is the commonest of all motions. Automobiles,

bridges, and buildings may be caused to vibrate all too easily. Nearly

any material is more or less elastic and will resist any attempt to deform

it, thus producing vibrations when the deforming force is released.

Sounds may be produced by blows, as with a hammer, by explosions,

by compression, or by any other type of deformation, such as a pull or a



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