PURPOSE: To quantitatively demonstrate sum and difference tones.

DESCRIPTION: Set the two oscillators to equal amplitudes with frequencies of 500Hz and 700 Hz, so that the combination tones are not masked by being related harmonically. When the volume of the system is turned sufficiently high, several differ ence tones will immediately be heard: 200 Hz, 300 Hz, and 400 Hz. Move the frequency of one of the oscillators back and forth by a small amount to call attention to the difference tones. The extra oscillator and speaker can be set to the difference frequ ency so that it can be identified by the observer. Sum tones also occur, the best example being at 1200 Hz, but it cannot be heard due to masking by the two louder original tones. However, tuning the extra oscillator to a frequency of 1200 Hz at a low a mplitude will allow it to beat with the sum tone, thus indicating that the sum tone is actually there!

SUGGESTIONS: Seat the amplifier to monaural.

REFERENCES: (PIRA 3C55.30) See Demonstration Reference File for further information. This experiment is discussed in Berg and Stork, The Physics of Sound, 2nd edition.

EQUIPMENT: Two oscillators with audio amplifier and loudspeakers on Audio Cart, extra oscillator with small speaker.

SETUP TIME: 5 min.

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