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Bluelighter
Place a tape recorder there. Run the output into an oscilloscope. You'll see the waveform, therefore see the sound, so you know it's there even though no-one has heard it.
Tape recorders record pressure waves, not "sound" and when played, they recreate pressure waves, which are then turned into sound in our heads.
EDIT it's kind of obvious when you consider how a microphone and loudspeaker work, i.e. one is moved by moving air and one moves air.

