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Chapter 20 Sound

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. Main topics. Origin of sound Nature of sound in air Speed of sound in air Natural frequency

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Chapter 20 Sound

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  1. 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. PHY 1071

  2. Main topics • Origin of sound • Nature of sound in air • Speed of sound in air • Natural frequency • Resonance PHY 1071

  3. 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. PHY 1071

  4. 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 PHY 1071

  5. 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. PHY 1071

  6. 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. PHY 1071

  7. 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. PHY 1071

  8. 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. PHY 1071

  9. 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. PHY 1071

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