Chapter 10
Joseph F. Alward, PhD
Department of Physics
University of the Pacific
| Sound
Waves Chapter 10 Joseph F. Alward, PhD Department of Physics University of the Pacific |
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Water Wave
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A stone dropped in water causes a disturbance to travel outward in an expanding circle. The water doesn't go anywhere; it is only the energy which moves. The speed of this disturbance-- this flow of energy--is about 2 m/s. |
Tsunamis
 Shallow pond: d = 0.5 meter v = (9.8 x 0.5)1/2 = 2.2 m/s In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or about 720 km/hr. More about tsunamis |
Period and Frequency
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If a succession of stones were dropped, one each second, a wave train would be created. The expanding circles in the wave train are called wave fronts. The period of the wave motion would be one second. The frequency of the wave motion would be one per second, or one hertz (Hz). Frequency = 1 / Period |
Period and Frequency
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Frequency = 1 / Period One stone is dropped into the water every 1/5 second. What are the period and frequency of the wave motion? Period = 1/5 second Frequency = 1 / period = 1 / (1/5) = 5 Hz |
Wavelength
 Snapshot in time of water wave (top view). |
The distance between successive peaks --or maxima-- is called the wavelength of the wave. Distance = speed x time Wavelength = wave speed x period Assuming the wave speed of the disturbance on water is 2 m/s, and the period of the wave motion is 1/5 s, what is the wavelength of the wave motion? Wavelength = (2 m/s) x (1/5 s) = 0.4 m |
Transverse and Longitudinal Waves
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Longitudinal:
displacements are parallel to direction of propagation. Transverse: displacements perpendicular to propagation direction. |
Condensation and Rarefaction in Sound Waves
 Speaker membrane expands, creating a region where the air molecules are packed closely together, a "condensation". The air pressure in a condensation is higher than normal. |
 As the membrance moves back, a region is left behind where few molecules are located, a "rarefaction". Meanwhile, the condensation moves forward. |
Sound Waves
Compressions are regions of above-normal air pressure. Rarefactions are regions of below-normal air pressure. |
Calculating Wavelength
from Frequency
| The speed of sound in air is about 340 m/s. What is the wavelengh of sound created at 1000 Hz? |
Measuring Sound
 Oscilloscope displays period, from which frequency may be calculated. Amplitude measures of the energy carried in individual compressions; the greater the sweep of the speaker membrane, the greater will be the amplitude of the sound. |
Sound Reflection
 Reflection of sound is like reflection of light. Angle of reflection = angle of incidence |
 Opera hall in San Francisco. Hanging plastic reflects light and sound. |
| Reflection of Sound Applet |
The Rule of Five for Lightning
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Speed of sound = 343 m/s at 20
C Speed of light = 300,000,000 m/s
Rule : See lightning, start
counting Example: 10 / 5 = 2 miles
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Sonar
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Sound navigation and ranging Clock measures round-trip time. Half the travel time, times the speed of sound gives the depth.
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Ultrasound Measuring Devices
 Ultrasound is sound at a frequency which is outside of the range of human hearing. |
Bats
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Sound Refraction
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Sound travels faster in warm air. Top left: Lower part of wave front gets ahead of upper part, so front turns upward. |
Forced Vibrations and Resonance
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Forced Vibrations and Resonance
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1940 collapse of the bridge across the Tacoma Narrows in Washington. |
Interference
 These are plots of the amplitudes of sound waves, not the actual waves themselves. When compressions from one source overlap rarefactions from another, cancellation occurs. |
| Superposition
Superposition. |
Interference of Two Sound Sources
 Dark lines show sound cancellation. |
Constructive Interference
 Compressions overlap compressions: Waves are in phase. |
Destructive Interference
 Compressions overlaps rarefactions: Waves are out of phase. |
Positive and Negative
Wire
Speaker Connections
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Both speakers should be connected the same way. |
Speaker Connection Wires are not the Same
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Monaural sound is weaker, or completely cancelled. Would cancellation occur with stereo sound? |
Noise Abatement
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Sound emanating from outside headphones is cancelled. |
Beats
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Beat frequency is the difference between the frequencies of the two tuning forks. |
Frequency and Loops
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At twice or three times the frequency, you get twice or three times as many loops. |
Standing Waves
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Positions of zero rope displacement in a standing wave are called nodes. At each end of any loop there is a node, and two loops makes one wavelength.
Distance between adjacent |
Doppler Effect
 A bug stationary on water is jiggling, causing circular wave fronts to spread out. The frequency of the disturbance is the same for observers at A and b. |
 If bug is moving, wavefronts arriving at B are closer together in time; the reverse is true for the observer at A. |
Doppler Effect
 Approaching, the frequency is higher because the wavefronts are closer together in time. Departing, the frequency is lower. |
| Doppler:
Source Doppler: Observer Police car siren (very good, but may be slow loading). Doppler (Good)User controls speed of moving source. |
Noise versus Music
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Noise is random, music is not. |
Modes of Vibration of a Guitar String
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The first harmonic is also called the fundamental frequency. Multiples of the fundamental frequency are called overtones.
The 2nd harmonic, for example, has |
Composite Vibrations
 The 2nd overtone (the 3rd harmonic) superimposed on the fundamental. |
Oscilloscope Display of
Composite
Wave and its Constituents
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Musical Instruments Produce Different Sounds
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Each has the same fundamental, but different overtones mixed in. |
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