Chapter 31, Problem 25A

Suppose a pair of loudspeakers a meter or so apart emit pure tones of the same frequency and loudness. When a listener walks past in a path parallel to the line that joins the loudspeakers, the sound is heard to alternate from loud to soft. What is going on?

To determine

To Explain: The sound is loud to soft alternately when listener walks past in a path parallel to the line that joins loudspeaker.

Explanation of Solution

Introduction:

The superposition of two or more sound waves occupying the equal space can be defined as interference of sound waves. Hence, a pattern of interference is formed within which the amplitudes of waves may be increased, decreased or completely cancel.

Interference is said to be constructive when the crests of the two waves overlap each other resulting in the addition of individual effects. Similarly, interference is said to be destructive when the crest of one wave is overlapping with the trough of the other resulting in reduction of individual effects.

In case of interference pattern resulting from the double slit experiment, a series of interference fringes occur due to the various path lengths from the slits to the screen. Hence, in the given situation the listener walks past in a path of interference fringes due to sound waves. Hence, the sound is heard to alternate from loud to soft.

Conclusion:

Hence, due to interference fringes of sound waves the sound is heard to alternate from loud to soft.