Audio Spectrum and Beats

Have a play with one, two or three tones at once:

images/audio-spectrum-n.js

(Note: below 20 Hz is Infrasound, and above 20,000 Hz is Ultrasound.)

Start with one tone.

Now add a second tone and bring it close to the first tone.

You will notice "beats" as the two sounds interfere with each. Try for example 400 and 402.

Also try things like 800 and 402 and so on.

You can add a third tone for fun, and make weird sci-fi sounds.

Because the waves interact to create a new more complex wave.

Here we see how two waves (in this case of very different frequencies) make a new wave:

Two sine waves of different frequencies adding together to form a complex wave pattern

Now let's see what happens when the tones are really close in frequency.

They will sometimes be "in sync" and make a bigger wave when added together:

Two identical waves in phase adding together to create a wave with double the amplitude

Or they can be "out of sync" and cancel each other out:

Two waves out of phase canceling each other out to create a flat line

And we hear that change as beats.

Have a play with This Beats Graph (drag it left and right) to undertsand it better

The sounds played here aren't an accurate hearing test!

Also we can't hear all tones equally. We are most sensitive to sounds between 1,000 and 4,000 Hz.

Graph of sound intensity versus frequency showing the ear's sensitivity at different pitches
A Typical "Equal Loudness" Curve