PHY238Y Lecture 8

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PHY238YLecture 8

• Sound waves
• The speed of sound
• Traveling sound waves
• Intensity and sound level
• References Haliday, Resnick, Walker
  Fundamentals of Physics, 6th ed., Wiley 2003, Ch.
  18 (18.1 18.3, 18.5)
• Hallett et al. Physics for the life sciences,
  4th ed., Ch.2 (2.1 2.5)
• Thanks to dr. R. Nave for the permission to use
  some of the pictures from Hyper Physics
  http//hyperphysics.phy-astr.gsu.edu

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PHY238YLecture 8

• Sound waves are longitudinal
• A single-frequency sound wave traveling through
  air will cause a sinusoidal pressure variation in
  the air.
• The air motion which accompanies the passage of
  the sound wave will be back and forth in the
  direction of the propagation of the sound.

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PHY238YLecture 8

• The elastic property of a gas or liquid is called
  bulk modulus
• The speed of sound

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PHY238YLecture 8

• Sound pressure Audible sound consists of
  pressure waves
• Sound pressure pm is defined as the amplitude of
  the pressure wave
• The power carried by the wave is proportional to
  the square of the amplitude

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PHY238YLecture 8

• Traveling sound waves
• Displacement and pressure variations

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PHY238YLecture 8

• Sound intensity is
• Power/unit area
• or time rate of energy transfer/unit area
• Many sound intensity measurements are made
  relative to a standard threshold of hearing
  intensity I0,
• The most common approach to sound intensity
  measurement is to measure the sound level on the
  decibel scale

• The factor of 10 multiplying the logarithm makes
  it decibels instead of Bels, and is included
  because 1 decibel is the just noticeable
  difference (JND) in sound intensity for the
  normal human ear

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PHY238YLecture 8

• The threshold of hearing
• The measured threshold of hearing curve shows
  that the sound intensity required to be heard is
  quite different for different frequencies.
• The standard threshold of hearing at 1000 Hz is
  nominally taken to be 0 dB (it actually is 4 dB)

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PHY238YLecture 8

• Threshold of pain is 120 130 dB
• The dynamic range of human hearing extends from
  zero dB to the threshold of pain.
• Example problem 18-2 p. 404 from
  Halliday/Resnick/Walker

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PHY238YLecture 8

• The audible sound a pressure wave with frequency
  between 20 Hz and 20,000 Hz and with an intensity
  above the standard threshold of hearing
• Frequency 20 Hz - 20,000 Hz (corresponds with
  pitch)
• Wavelength (at 20oC in air) 0.0172m 17.2 m
• Intensity 10-12 - 10 W/m2 (0 to 130 dB)
• Pressure 2 x 10-5 - 60 N/m2 (2 x 10-10 - .0006
  atm)
• Sounds may be generally characterized by
• Pitch (frequency)
• Loudness (is NOT simply the intensity level)
• Quality (timbre determined by the harmonic
  content of the sound)

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PHY238YLecture 8

• Variation of intensity with distance the inverse
  square law

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PHY238YLecture 8

• Inverse square law plot