Phy 202: General Physics II

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Phy 202: General Physics II. Ch 16: Waves & Sound Lecture Notes. What are waves?. Waves are traveling disturbances or vibrations Waves carry energy from place to place There are 2 types of waves: Transverse (example: light, electromagnetic waves)

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## Phy 202: General Physics II

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### Phy 202: General Physics II

Ch 16: Waves & Sound

Lecture Notes

What are waves?
• Waves are traveling disturbances or vibrations
• Waves carry energy from place to place
• There are 2 types of waves:
• Transverse (example: light, electromagnetic waves)
• Disturbance is perpendicular to direction of wave propagation
• Longitudinal (example: sound waves)
• Disturbance is parallel to direction of wave propagation
Properties of Periodic Waves
• All waves share 3 properties:
• The propagation speed of the wave, vwave
• The spacial length of a wave from crest to crest is called its wavelength, l
• The rate (or frequency) of vibration describes how fast a wave oscillates, f
• The propagation of a wave is related to its wavelength & its frequency:

vwave = l.f

• The speed of wave depends on the properties of the material (medium) where the wave travels.
• For a wave on a string:
• Where:
• F = tension in the string
• m = mass of the string
• L = length of the string
Sound
• Longitudinal wave
• Produced by vibrations in a medium
• The disturbance is the local change in pressure generated by the vibrating object
• It travels because of the molecular interactions.
• The region of increased pressure( compared to the undisturbed pressure) is called condensation
• The region of lower pressure is called rarefaction.
• The maximum increase in pressure is the amplitude of the pressure wave. (measurable)
• Frequency of the sound 20Hz to-20000Hz.
• Pressure waves below 20 Hz are called infrasonic waves
• Pressure waves over 20,000Hz are called ultrasonic waves.
The Speed of Sound
• Speed of sound depends on the compressive properties of the medium.
• Because of the high frequencies, the compression/expansions are fast and no heat is exchanged (adiabatic).
• Sound can travel in gases, liquids and solids.
• The speed of sound in gases:
• The speed of sound in liquids:
• The speed of sound in solids:
Sound Intensity
• Waves transport energy without transporting mass. The amount of energy transported per second is called the power of the wave (W) (power of the sound)
• The power of the wave is determined by the source. The power is distributed (spreads) in all directions. Far away from the source, the power is spread over a greater area.
• Intensity is a measure of power transmitted by a wave per unit area:

I = P/A

• The human does not sense sound intensity linearly but rather logarithmically
• The decibel scale has been developed to “linearize” the human ear response to sound intensity

b = (10 dB) . log (I/Io)

where Io is the minimum intensity of sound the “average” human ear can detect

Doppler Effect
• The frequency of the source producing the wave equals the number of cycles per second.
• The frequency measured by an observer is the number of crests (condensations) encountered per second.
• When both the source and the observer are at rest, the 2 frequencies are equal.
• When one or both are in motion, the 2 frequencies are different.
• The difference between source and observed frequency is called Doppler shift
Doppler Shift(Moving source, observer at rest)
• The wavelength is changed because of the relative motion of source and observer

Source moving toward observer

Source moving away from the observer

Notes:

vs = speed of source

v = speed of sound

fs = frequency of source

Doppler Shift(Moving observer, source at rest)
• The time between encountering 2 crests changes for observer

Observer moving toward source

Observer moving away from source

Notes:

vo = speed of observer

v = speed of sound

fs = frequency of source