Tone Generator

A pure tone is one frequency with nothing else in it: a sine wave that repeats the same number of times every second. The number of repetitions per second is the frequency, measured in hertz (Hz). The ear turns frequency into pitch and level into loudness. Most people can hear from about 20 Hz to 20 kHz, and the top of that range fades with age and with exposure to loud sound.

#Waveforms and harmonics

Every periodic sound can be built from a fundamental, the lowest frequency, and harmonics, which are whole-number multiples of it. The waveform decides which harmonics are present and how strong they are, and so it decides the tone colour. The pitch stays with the fundamental.

• Sine: the fundamental only. It is the purest tone a loudspeaker can make.
• Square: odd harmonics only (1, 3, 5 and so on). It sounds hollow and buzzy.
• Sawtooth: all harmonics. It sounds bright and rich, and is the classic synthesiser wave.
• Triangle: odd harmonics only, like the square, but the higher ones are much weaker, so it sounds softer and closer to a sine.

Put a square wave and a sine at the same frequency side by side and the pitch will match while the square sounds more edgy. That difference is the harmonics, not a change of note.

#A4, cents and the reference pitch

A4 is the note above middle C, and the common reference for tuning is A4 = 440 Hz. ISO 16 gives 440 Hz as the standard pitch, but orchestras and instruments in practice tune anywhere from about 415 Hz to 445 Hz, and 442 Hz is common. The reference selector changes which frequency counts as A4, and every note name and cents reading follows from it.

Notes are spaced by equal ratios. Each semitone is a frequency ratio of 2 to the power one twelfth, and a cent is one hundredth of a semitone, so an octave is 1200 cents. The readout gives the nearest equal-tempered note and the difference in cents. A tone at 432 Hz, read against A4 = 440 Hz, is about 32 cents flat of A4. A difference of that size is easy to hear when the two tones sound together, because the beating makes the mismatch obvious.

#Beating between close frequencies

Two tones that are close together interfere with each other. Their loudness rises and falls at the difference between their frequencies. Play 440 Hz and 443 Hz together and you hear three slow swells per second, because the two waves reinforce and then cancel each other three times a second. Set the second tone offset to any value from 0 to 20 Hz to hear this. Tuning two strings to unison works the same way: the beats slow down to nothing when the frequencies match.

#Checking speakers, rooms and tuning

A sweep moves through the range so you can hear where a speaker or a room changes level. In a room, standing waves make some frequencies louder in one place and quieter in another, so the level can change a lot when you move your head by a few centimetres. A single fixed tone shows whether one frequency rings. Small speakers, laptops and phones reproduce little below their design range, so a missing bass tone there is a property of the device, not a fault in the tool.

For tuning, the generator gives a steady reference you can match by ear. Set the reference first, then the note, then listen for beating. It is a reference, not a replacement for a tuner in a quiet room.

#Hearing safety

Level and time both matter, and hearing damage from sound is permanent. Keep the level low when you start, and never test a high frequency at full volume through headphones. Children and pets may hear above 17 kHz, so a tone that seems silent to you may still be heard by them. Do not leave a tone running unattended. This tool is not a hearing test and cannot tell you anything about your hearing or your health.

#Why the first press makes sound

Browsers keep audio silent until the page has had a user gesture, such as a click or a key press. This stops websites from making noise without being asked. The audio engine therefore starts with the first press of Play. Start and stop each fade over 40 ms, which avoids the click a waveform makes when it begins or ends at a non-zero level. Changing the frequency while the tone plays glides to the new value over about 10 ms, so the pitch moves without a jump in level.

#Limits

The 40 % ceiling is a digital figure, a fraction of full scale. It is not a sound-pressure level. How loud the tone is in the room depends on the device, its own volume control and the speakers. The generator does not calibrate anything and does not measure sound. A tone that seems quiet here may be loud on another device, so start low on every new device.