Note → frequency (Hz)

What it's for

Each note maps to a precise frequency: standard A4 is 440 Hz. Handy for tuning instruments, sound design and understanding music theory.

Tuning 440 or 432

The international standard is 440 Hz. Some prefer 432 Hz, considered «softer»: here you can choose both.

A scale of frequencies, not of numbers

Note frequencies do not grow evenly: they double at every octave. Middle A is 440 Hz, the one above 880, the one below 220. Between one semitone and the next the ratio is always the same, the twelfth root of two, about 1.0595, and that is why the low keys of a piano are a few hertz apart and the high ones hundreds.

From which follows something surprising: the ear hears ratios, not differences. Ten hertz of error on a low A is an obvious mistuning, the same ten hertz on a high note is almost imperceptible. It is also why low notes are easier to tune by ear than high ones.

Cents, and the unit mistuning is measured in

A semitone is divided into a hundred equal parts called cents, and that is the unit every tuner speaks. The threshold at which a trained ear starts to hear a difference is around five cents, and above ten almost everyone hears it. Below that, the mistuning exists in the numbers but not in the music.

Cents also explain why the scale we use is a compromise. Equal temperament divides the octave into twelve identical parts, so that key can be changed freely; but the thirds and sixths that result sit a little outside the natural ratios, by fourteen and sixteen cents. That is the price a fretted or keyboard instrument pays for being able to play in every key.

Why the same note has different names and numbers

In international notation the number beside the letter gives the octave, and the commonest convention is the scientific one, in which the reference A is A4 and the piano's middle C is C4. But not every manufacturer follows it: some synthesisers and some libraries call that same middle C C3, and an octave of difference between two programs is the commonest cause of a MIDI track playing a scale above or below what was intended.

A second way of naming notes is the MIDI number, in which A 440 is 69 and each semitone is worth one unit: it is the language music software actually speaks, and it is useful to know when reading the contents of a track.

What knowing the frequencies is for

In mixing, to understand where an instrument's energy sits: the fundamental of a bass guitar lies roughly between 40 and 200 Hz, a male voice around 100 to 150, a female voice an octave above. Cutting or boosting a band without knowing which note lives inside it is the fastest way to hollow out an instrument.

And in tuning, where frequency is the objective reference that replaces the ear: any instrument, from a violin to a timpani, tunes to the same number. The rest is craft.

Nearby tools

To tune an instrument there is Guitar tuner, for the notes of a chord Chord notes and for distances between notes Musical intervals. For effect times in milliseconds BPM to milliseconds.