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College Physics

Introduction to the Physics of Hearing

College PhysicsIntroduction to the Physics of Hearing

Photograph of an old tree in a forest that had fallen some time ago.
Figure 17.1 This tree fell some time ago. When it fell, atoms in the air were disturbed. Physicists would call this disturbance sound whether someone was around to hear it or not. (credit: B.A. Bowen Photography)

Chapter Outline

17.1 Sound
  • Define sound and hearing.
  • Describe sound as a longitudinal wave.
17.2 Speed of Sound, Frequency, and Wavelength
  • Define pitch.
  • Describe the relationship between the speed of sound, its frequency, and its wavelength.
  • Describe the effects on the speed of sound as it travels through various media.
  • Describe the effects of temperature on the speed of sound.
17.3 Sound Intensity and Sound Level
  • Define intensity, sound intensity, and sound pressure level.
  • Calculate sound intensity levels in decibels (dB).
17.4 Doppler Effect and Sonic Booms
  • Define Doppler effect, Doppler shift, and sonic boom.
  • Calculate the frequency of a sound heard by someone observing Doppler shift.
  • Describe the sounds produced by objects moving faster than the speed of sound.
17.5 Sound Interference and Resonance: Standing Waves in Air Columns
  • Define antinode, node, fundamental, overtones, and harmonics.
  • Identify instances of sound interference in everyday situations.
  • Describe how sound interference occurring inside open and closed tubes changes the characteristics of the sound, and how this applies to sounds produced by musical instruments.
  • Calculate the length of a tube using sound wave measurements.
17.6 Hearing
  • Define hearing, pitch, loudness, timbre, note, tone, phon, ultrasound, and infrasound.
  • Compare loudness to frequency and intensity of a sound.
  • Identify structures of the inner ear and explain how they relate to sound perception.
17.7 Ultrasound
  • Define acoustic impedance and intensity reflection coefficient.
  • Describe medical and other uses of ultrasound technology.
  • Calculate acoustic impedance using density values and the speed of ultrasound.
  • Calculate the velocity of a moving object using Doppler-shifted ultrasound.

If a tree falls in the forest and no one is there to hear it, does it make a sound? The answer to this old philosophical question depends on how you define sound. If sound only exists when someone is around to perceive it, then there was no sound. However, if we define sound in terms of physics; that is, a disturbance of the atoms in matter transmitted from its origin outward (in other words, a wave), then there was a sound, even if nobody was around to hear it.

Such a wave is the physical phenomenon we call sound. Its perception is hearing. Both the physical phenomenon and its perception are interesting and will be considered in this text. We shall explore both sound and hearing; they are related, but are not the same thing. We will also explore the many practical uses of sound waves, such as in medical imaging.

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