Grades 3–5 · Arts & Music
Why do two steady musical tones seem to swell and fade?
Two musical tones can each stay steady while their combined sound swells and fades. This pulsing is called beating. It can happen when the tones have slightly different frequencies, meaning slightly different numbers of vibrations each second. One vibration per second is one hertz (Hz).
In the diagramSteady tones can make a changing combination440 Hz and 442 Hz; equal individual amplitudes440 Hz442 HzCombined amplitude envelope200 s0.5 s1 sEach tone keeps a steady individual amplitude.2 swells per second; 0.5 second per swell.
At a listening point, the two sound-pressure changes add together. Sometimes their peaks arrive together and reinforce each other. Later their timing drifts, so one wave partly opposes the other. Because one tone cycles a little faster, the relationship keeps changing and then repeats.
For ideal tones at 440 and 442 vibrations per second, the difference is two. Their combined amplitude swells twice each second. The pulsing rate is not the pitch of either tone. The diagram shows the changing amplitude envelope, the outline of the vibration’s size, rather than every rapid vibration. Real instruments have additional overtones and unequal strengths, so the quieter moments need not become complete silence.
About this explanation
Starting knowledge: Knowing that vibration rate relates to pitch and that vibration size differs from rate.
Sources
- UNSW Sydney, Physclips — Interference beats and Tartini tones
- UNSW Sydney, Music Acoustics — Acoustics FAQ