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beats arising from the mistuned unison are called first-order beats, and are both an acoustic and psychoacoustic phenomenon. beats also may be heard between pure tones that are nearly an octave, fifth or fourth apart. these are called secondary or second-order beats, where the beat frequency is equal to the frequency difference e between the upper tone and the exact interval for the octave, 2e for the fifth and 3e for the fourth. however, no amplitude modulation is present, and the beating results from effects of neural processing.

sound example: beats produced by 100 and 110 hz, with the higher tone gradually reduced to 100 hz causing the beats to slow down and disappear.

see: tuning. compare: grain, tremolo,[link widoczny dla zalogowanych], vibrato.
beats recur at a rate equal to the difference between the two frequencies,[link widoczny dla zalogowanych], called the beat frequency. thus the beat frequency produced by 500 hz and 496 hz is 4 hz. piano tuners use the phenomenon of beats when bringing strings into unison.

although two tones with a constant phase difference sound the same, independent of the degree of phase difference,[link widoczny dla zalogowanych], in the case of secondary beats, the phase difference is constantly changing, producing a changing waveform, as can be seen in the diagram below. the phenomenon occurs only where the lower tone is below about 1500 hz. however, it also occurs with dichotic listening where it is called binaural beats and where the phase difference is interpreted as in binaural hearing, i.e. as a spatial difference. see also: critical bandwidth.