Chapter 13- Sound

Section 13-1: Sound Waves

  • Sound waves are longitudinal
  • Consist of compressions (areas of high pressure) & refractions (areas of low pressure)
  • Sound waves radiate out in spherical wavefronts from a vibrating object.
  • Pitch is known as the frequency that a sound wave is perceived to have.
  • Sound waves travel outward in 3 dimensions.
  • Frequencies between 20 Hz and 20000 Hz can be picked up by humans.
  • Frequencies lower than 20 Hz are identified as infrasonic.
  • Frequencies higher than 20000 Hz are identified as ultrasonic.
  • The speed of the sound wave depends upon the medium.
  • The speed of sound at sea level & room temperature is about 343 m/s.
  • The freeuncy shift between the relative motion between an observer and the source of waves is known as hte Doppler Effect.
  • The Doppler Effect can be seen if a sound source and the detector are moving relative to one another.
  • Formula for pitch:

Section 13-2: Sound Intensity

  • As a sound wave travels thorough a medium,energy is transfered to the next molecule.
  • Intensity is known as the rate which the energy is transferred.
  • Intensity is measured in watts per square meter.
  • Formula for intensity:

  • threshold of hearing: the quietest sound audible to humans
    • Approx. 1.0 X 10^-12 W/m2
  • threshold of pain: the loudest sound a typical person can tolerate
    • Approx. 1.0 W/m2
Perceived Intensity
  • The perceived loudness of a sound is dependent of the logarithm of hte intensity.
  • It is measured in decibels (dB).
  • The sound level measured in decibels is:
    • dB = 10log)_10 intensity
  • "Intensity" is the intensity of sound being measured.
  • "Intensityr k"is the relative intensity
  • Decibel level is a measure of relative intensity in terms of logarithm.
  • A forced vibration at the natural frequency produces resonance.

Section 13-3: Harmonics

  • Fundamental Frequency- the lowest frequency of vibration of a standing wave.
  • Fundalemental frequency is the lowest possible frequency of a standing wave.
  • Harmonics of a vibrating string or pipe open at both ends is found by the equation:

n = 1, 2, 3...
  • Harmonics of a pipe closed at one end is found by the equation

n = 1, 3, 5...
  • Harmonic Series- a series of frequencies that includes the fundamental frequency and integral multiples of the fundamental frequency.
  • Timbre- the quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities.

Practice Problems:

  • The fundamental frequencies produces by a typical piano range from 26.0 Hz to 4435 Hz. What are the wavelengths of these frequencies? (Answer: wavelength= 13.3 m, wavelength= 0.0778 m)
  • Two cars are moving towards each other, the one on the left at 20.0 m/s and the one on the right at 30.0 m/s. The car on the left sounds its horn with a frequency of 440.0 Hz. What pitch will the driver in the car on the right hear? (pitch= 508 Hz)

    Works Cited:
    Faughn, Jerry S. and Raymond A. Serway. Physics. New York: Holt, 2004.
    Strong, Tom. Course notes. Honors Physics, Dept. of Science, Mount Lebanon High School. May and June 2009.