When two loudspeakers emit pure tones of nearly—but not exactly—the same frequency, a listener standing midway between them hears the sound intensity rise and fall periodically. Which of the following best accounts for this observed "fading in and out" of the sound?
(a) Because the two sound waves travel slightly different distances through the air, their phase difference slowly changes even when the listener remains still; the varying path difference causes alternating constructive and destructive interference that makes the sound fade in and out.
(b) The amplitudes of the two sound waves alternately add and cancel owing to constructive and destructive interference, and because sound intensity depends on the square of amplitude, the perceived loudness varies accordingly.
(c) The listener experiences successive Doppler shifts as air currents move toward and away from the speakers, periodically altering the apparent pitch and loudness.
(d) Resonance between the listener’s ear canal and one speaker alternates with resonance with the other speaker, creating the perception of varying loudness.