Alice places two stereo speakers a distance d apart. She sends a signal which is a sine wave of frequency f, so that the speakers are producing the same pure tone in phase. The sound wave in air has a wavelength λ.
She enlists her friend Bob to do an experiment. Alice sits directly in front of the speakers on the line which bisects the line segment connecting the speakers (see figure) but relatively far away from the speakers. Bob starts at the same place, but he slowly moves to the right. For him the sound gets quieter as he moves right, until he can barely hear it. Then it begins to get louder again. The figure shows Bob's position: where he first can barely hear the sound. This is the set up.
In Experiment 1, Alice and Bob keep their positions, and Alice changes the frequency of the signal sent to both speakers, so that both speakers are still producing a sound in phase at the same frequency (a different frequency from the set up).
In Experiment 2, Alice changes the frequency of the second speaker slightly, but the first speaker remains at the original frequency.
The distance d is large compared to the wavelength, and it is small compared to the distance Alice and Bob sit from the speakers. The distance from Bob to the left speaker is d1, and his distance to the right speaker is d2.
Which is the best explanation that Bob hears little sound where he sits in the set up?