Two waves with slightly different frequencies combine to produce beats. When the crests or troughs of the waves coincide, the amplitude and sound level is at its maximum. When the waves are out of phase, the amplitude decreases and the sound level is at its minimum. Beats are the periodic variation in sound level that occurs due to interference between two waves of slightly different frequencies. The beat frequency is defined as the difference between the two frequencies, and the human ear can differentiate beat frequencies up to 10 beats per second. For two tuning forks with frequencies of 200Hz and 204Hz respectively, the beat frequency is 4Hz. A cellist tuning his cello string to a 262Hz tuning fork hears 5 beats per second,