How it works
Plays a steady tone at any frequency from 20 Hz to 20,000 Hz until you stop it. Type the frequency, drag the slider or pick a note. The slider moves by musical steps rather than equal hertz, so 20 to 200 Hz gets as much of it as 2,000 to 20,000. In the box, the arrow keys move by 1 Hz, or 10 with Shift, and Space plays and stops. Under the number is the nearest note and how many cents off it is, with A4 at 440 Hz.
Waves and volume
A sine is a pure tone with nothing but its own frequency. Triangle adds a few soft overtones, and square and saw add many, which makes them buzz and sound much louder at the same setting, so they are turned down to match. Start quietly, especially in headphones: a high tone can be unpleasant well before it seems loud.
What it is handy for
Tuning by ear against a held note, checking both sides of a pair of headphones, finding what rattles in a room by sweeping the slider until something buzzes, and testing how low a speaker really goes. Most laptop and phone speakers give little below about 150 Hz, so a 50 Hz tone may be silent on them and plain in headphones.
Questions
What frequency is middle C?
261.63 Hz, when A4 is 440 Hz. Each octave doubles the frequency, so C5 is 523.25 Hz and C3 is 130.81 Hz. Pick C4 from the note list to hear it.
Can I use it to test my hearing?
Roughly. Most adults stop hearing somewhere between 14,000 and 17,000 Hz, and you can sweep up to find where you stop. Speakers, headphones and volume all change the answer, though, so it is no medical test.
Why do some frequencies sound louder?
Ears are most sensitive between about 2,000 and 5,000 Hz, so a tone there sounds louder than one at 100 Hz on the same setting. Speakers also boost some frequencies and cut others.