Chapter 20 Sound

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