The authors of the February article “Visualizing Sound Directivity via Smartphone Sensors,”1 which brings smartphones into the physics classroom, are to be congratulated. They could go further and suggest, as Paul Hewitt does in any of his Conceptual Physics editions, and show interference when switching the wires of one speaker so it is out of phase with the other. When monaural sound emanates from both speakers, the loudness of sound varies with the closeness of the speakers. When they are brought face to face, very little sound is heard. Interference is nearly complete as the compressions for one speaker fill in the rarefactions of the other.

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