Why Two Nearby Notes Seem to Breathe
Play two steady tones whose pitches are close but not identical. At first, their sound waves nearly rise and fall together, so the combined sound is strong. As the faster wave gradually slips ahead, its peaks meet the slower wave’s troughs more often. The tones partly cancel, and the sound softens. Then the waves line up again, and the volume swells.
This repeating change is called beating. It is not either note changing its own loudness; it is the way their waves combine in the air and at the listener’s ear.
The Pitch Gap Sets the Pulse
Each tone produces a repeating pressure wave. A higher-pitched tone completes more cycles per second than a lower-pitched one, so even a small pitch difference makes their alignment shift steadily. The size of that difference determines how quickly the loudness rises and falls: tones separated by two hertz produce two beats per second.
With pitches very close together, the effect sounds like a slow, distinct throbbing. As the gap widens, the pulses speed up until they are no longer heard as separate swells; the listener instead hears a rough or wavering sound.
A Useful Way to Tune
Musicians can use beats to tune two notes to the same pitch. If the notes are slightly off, the pulsing reveals the mismatch. Adjusting one note toward the other slows the beats; when the pitches match, the regular pulsing disappears. The quiet moments are not silence, because the two tones do not usually cancel perfectly—but the changing loudness makes even a tiny pitch difference easy to notice.
