Man's physical universe

xanabras

ACOUSTICS 461

tuning fork and then holding it against the table top. The table top

is caused to vibrate and thus becomes a vibrating medium of greater

intensity because it has a greater surface to agitate the air against it.

Inasmuch as the energy of vibration is greater, it must last for a

shorter time, the vibrations dying out much faster than when the

tuning fork is held in the hand. The original vibrating body is called

the generator, while the sounding board is called the resonator. Thus

the strings in a piano make up the generator, while the sounding board

is

the resonator.

There are two distinct kinds of resonators.

One kind has no vibration

frequency of its own and responds to all of the vibrations of the generator.

Such vibrations are called forced vibrations. A second type of

resonator possesses a natural frequency

and strengthens sounds

of its own pitch only. Such vibrations

are called sympathetic

vibrations or resonance. A tuning

fork is usually mounted in a

hollow box whose size is such

that the air column will vibrate

in resonance with the fork.

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Sympathetic vibrations.

Just as a child in a swing can be made to swing farther by applying

a gentle impulse at the proper time, so one tuning fork can be caused to

vibrate by the vibrations of another fork in tune with it.

If the loud

pedal of a piano is held down so that all of the strings are free to vibrate,

that string will vibrate whose natural frequency corresponds to the

frequency of a briefly sung note.

Sound Waves May Be Canceled by Interference.

Mufflers used to absorb or minimize sound energy from internalcombustion

engine exhausts depend on the cancellation of sound waves

by interference.^ In one type of muffler the length of the closed tube is

one fourth of the wave length to be canceled. Waves entering the tube

are reflected and return to the main tube 90 degrees out of phase, thus

canceling the incoming sound wave. In a second type of muffler one

path is longer than the other by one half of the wave length to be canceled,

and the waves uniting at the outlet cancel each other. Such muf-

' Interference may be explained by referring to a boy in a swing. If a person gives the

swing a push each time it reaches the end of a swing, the swing will swing farther and farther

with each push; but if a person gives the swing a push when it is not at the end of the swing,

the amplitude of the swing will be decreased. A push equal to the force of the swing, but in

the opposite direction, when applied as the swing comes to the position closest to the

ground, will stop the motion entirely. Sound waves are canceled in the same way by other

sound waves which are out of phase with the incoming sound waves.

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