Physics 312 Lecture: "Sound Waves II." Apr 9, 1998

• The decibel system measures sound intensity on a logarithmic scale
• The intensity of an isotropic sound wave decreases as the square of the distance from its source
• Since intensity is proportional to the square of the amplitude, the amplitude of a sound wave decreases as the inverse of the distance from its source
• Close to their source, sound waves are best described as spherical waves with a decreasing amplitude
• Far from their source, sound waves may be approximated as plane waves with constant amplitude
• The Doppler effect is the change in frequency of a sound if the source, or the observer (or both) is moving
• Relative motion towards causes frequency to increase; relative motion apart causes frequency to decrease

This lecture discusses material in Chapter 17 of Serway.

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