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Chapter 20 Sound

The physicist usually takes the objective position and defines sound as a form of energy that exists whether or not it is heard and goes on from there to investigate its nature.

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Main topics

• Origin of sound

• Nature of sound in air

• Speed of sound in air

• Natural frequency

• Resonance

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Origin of sound• Most sounds are waves produced by the vibrations of

material objects. For example,– In a piano, violin, and guitar, the sound is produced by the vibrating strings.– Your voice results from the vibration of your vocal cords.

• In each of these cases,– The original vibrations stimulates the vibration of something larger or more

massive, such as the sounding board of a stringed instrument, or the air in the throat and mouth of a singer.

– This vibrating material then sends a disturbance through the surrounding medium, usually air, in the form of longitudinal waves.

• Under ordinary conditions, the frequency of the vibrating source and the frequency of the sound waves produced are the same.

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Pitch and frequency of sound

• Pitch: we describe our subjective impression about frequency of sound by the word pitch.

• Frequency corresponds to pitch:– A high-pitched sound like that form a piccolo has a

high frequency of vibration.– A low-pitched sound like that form a fog horn has

a low frequency of vibration.

• The human ear of a young person can normally hear pitches corresponding to the range of frequencies between 20 and 20,000 hertz.– Infrasonic: frequencies below 20 hertz.– Ultrasonic: frequencies above 20,000 hertz.

A picture of piccolo

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Nature of sound in air• Compression and rarefaction

• When the door is opened, a compression travels across the room.

• When the door is closed, a rarefaction travels across the room.

• It is not the medium itself that travels across the room, but the energy-carrying pulse. The pulse (disturbance) travels from the door to the curtain.

• A continual swing of the door open and closed in a periodic fashion will set up a wave of periodic compressions and rarefactions that will make the curtain swing in and out of the window.

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Nature of sound in air (cont.)

• When the prong of the tuning fork next to the tube moves toward the tube, a compression enters the tube.

• When the prong swings away in the opposite direction, a rarefaction follows the compression.

• As the source (the prongs of the tuning fork) vibrates, compressions and rarefaction travel in the same direction from the tuning fork through the air.

• The frequency of the vibrating source and the frequency of the wave it produces are the same.

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Speed of sound in the air

• Thunder is heard after a flash of lightning is seen.

• Sound requires a recognizable time to travel from one place to another.

• The speed of sound does not depend on the loudness or frequency of the sound.

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Natural frequency and resonance• Natural frequency: any object

composed of an elastic material will vibrate when disturbed at its own special set f frequencies, which together form its special sound.

• Resonance: when the frequency of forced vibrations on an object matches the object’s natural frequency, a dramatic increase in amplitude occurs.

Pumping a swing in rhythm with its natural frequency produces a large amplitude.

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The effect of resonance

• Resonance is not restricted to wave motion. It occurs whenever successive impulses are applied to a vibrating object in rhythm with its natural frequency.

• In 1940, four months after being completed, the Tacoma Narrows Bridge in the state of Washington was destroyed by wind-generated resonance.