dr. jie zouphy 10711 chapter 20 sound the physicist usually takes the objective position and defines...
Post on 20-Dec-2015
213 views
TRANSCRIPT
Dr. Jie Zou PHY 1071 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.
Dr. Jie Zou PHY 1071 2
Main topics
• Origin of sound
• Nature of sound in air
• Speed of sound in air
• Natural frequency
• Resonance
Dr. Jie Zou 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.
Dr. Jie Zou 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
Dr. Jie Zou 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.
Dr. Jie Zou 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.
Dr. Jie Zou 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.
Dr. Jie Zou 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.
Dr. Jie Zou 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.