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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

Phy 202: General Physics II

Ch 16: Waves & Sound

Lecture Notes

what are waves
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
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
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
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
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
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
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
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