SAT Subject Physics Practice Question 113: Answer and Explanation

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Question: 113

8. In the figure below, two speakers, S1 and S2, emit sound waves of wavelength 2 m, in phase with each other.

Let AP be the amplitude of the resulting wave at point P, and AQ the amplitude of the resultant wave at point Q. How does AP compare to AQ ?

A. Ap < AQ
B. Ap = AQ
C. Ap > AQ
D. Ap < 0, AQ > 0
E. Ap and AQ vary with time, so no comparison can be made.

Correct Answer: A


The distance from S2 to P is 5 m (it’s the hypotenuse of a 3-4-5 triangle), and the distance from S1 to P is 4 m. The difference between the path lengths to point P is 1 m, which is half the wavelength. Therefore, the sound waves are always exactly out of phase when they reach point P from the two speakers, causing destructive interference there. By contrast, since point Q is equidistant from the two speakers, the sound waves will always arrive in phase at Q, interfering constructively. Since there’s destructive interference at P and constructive interference at Q, the amplitude at P will be less than at Q.

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