The three noises
White noise has every frequency at the same level, so it sounds bright, like a radio between stations. Pink noise drops the highs by half with every octave and sounds like steady rain. Brown noise drops them twice as fast and sounds like wind or a waterfall, deep and soft. The page makes each one fresh as it plays, so nothing repeats, and the sliders mix them: a little white over brown fills in the top, a lot of brown under pink turns rain into a storm.
A tone at any frequency
Type a frequency or drag the slider, pick a sine, square or triangle wave, and the tone plays until you stop it. 440 Hz is the A a tuner uses, 1 kHz is the test tone studios reference, and the sweep button runs from 20 Hz to 20 kHz so you can hear where your speakers or your ears give up.
Two tones, a few hertz apart
Binaural beats are two tones, one in each ear, set a small difference apart: 200 Hz on the left and 206 Hz on the right, say. The ear hears a slow pulse at the difference, 6 Hz in that case. Set the base tone and the difference, put headphones on (it needs one tone per ear), and that’s all it is: two frequencies and the pulse between them.
Timer and mixer
The timer fades everything out over the last minute of the time you set, from ten minutes to eight hours. The mixer holds the levels of each noise and tone together, and the page remembers your mix in this browser so it’s set the same way tomorrow. On a phone, keep the browser on this page while it plays.