PURPOSE: To demonstrate interference of sound in a dramatic way.
DESCRIPTION: Two loudspeakers are connected in the monaural mode to the power amplifier. A switch box in the leads of one of the speakers allows reversal of the phase of that speaker. When music with lots of bass is played, flipping the phase reversal switch causes huge reduction in the amplitude of the bass frequencies. This is a very dramatic effect.
A nice experiment shows the relation of phase to physical position. Play an 80 Hz tone into the two speakers, then reverse the phase to reduce the sound to virtually nothing. Uncoil the wire from the back of one speaker and move the speaker 12 or 15 feet across the front of the room; the loud bass tone returns! The waves from the two speakers are no longer out of phase.
REFERENCES: (PIRA unavailable.)
EQUIPMENT: Tape deck with tape of Springsteen "Fire," audio amplifier with speakers, wave generator, phase reversing switch.
SETUP TIME: None.
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