Man's physical universe

xanabras

'

456 ENERGY MAY BE PROPAGATED BY VIBRATIONS

speeds of the two cars.

The same effect may be observed as the whistle

of an approaching locomotive decreases in pitch as it passes and recedes

from an observer. This apparent change in frequency, produced by

the relative motion of a sound source and an observer, is called the

Doppler effect.

The Doppler eflfect results from the fact that when the distance

between the sound source and the observer is decreasing, the number

of waves entering the ear per second is increased, and vice versa.

The same eflfect produced in light rays was discussed in Unit II,

Section 3.

The Ear Includes a Device to Convert Compressional Waves into

Mechanical Vibrations.

The perplexing problem of how we hear has been the object of a great

How does the ear differentiate between the buzz of a

deal of research.

mosquito, the shrill shriek of a siren, the high tones of a violin, and the

Ear Drum J

/ / 7^

Middle Ear^//

Oval Window — / /

Round Window-/ ,

Eustachian Canal—

Inner Ear

Tynfipanic Membrane-

FiG. 219.

Diagram of the ear.

deep rumble of the bass drum? How can all of these things be heard

at once and be recognized? How can the student hear a speaker when

his neighbors are carrying on a competitive conversation? What is

the advantage of two ears?

One hypothesis that helps to answer some of these questions is that

parts of the membrane of the ear are tuned to a certain frequency and

vibrate when compressional waves of that frequency are received.

A nerve fiber attached to that part of the membrane would then

conduct the impulse to the brain.

A study of the ear will show the basis for this hypothesis.

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