On of the most interesting psychoacoustic phenomena is the perception of combination tones. Essentially, what this means is that humans can perceive harmonics of sound waves that do not actually exist. When two pure tones are played, two main combination tones are heard: the sum tone and the difference tone. Like the names imply, the sum tone is a frequency perceived as the sum of the frequencies of the original tones, and the difference tone is a frequency perceived as the difference between the original tones. Because of this, two pure tones played simultaneously will cause one to perceive four different frequencies!
Beyond being an interesting characteristic of human hearing, combination tones have played a practical role in music technology. When speaker technology was less advanced, many speakers struggled to properly produce low frequency sounds, but combination tones allowed them to be perceived by the listener.
Works Cited:
1. Gunther, Leon. The Physics of Music and Color. New York, New York: Springer, 2012.
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