idea
Beats
When two notes are very close in pitch, for example two violin strings that are almost in tune, their vibrations go in and out of step. When they are in step they add up and the sound gets louder, and when they are out of step they cancel and it gets softer. You hear one note pulsing: wah, wah, wah. The number of pulses per second is the difference between the two frequencies, so the closer the notes, the slower the pulsing. Musicians use this to tune: they adjust until the beats slow down and disappear.