Frequency is measured in hertz (Hz), where 1 Hz means 1 cycle per second, 440 Hz means 440 cycles per second etc. Periodic sounds above about 20 Hz are perceiv

Frequency is measured in hertz (Hz), where 1 Hz means 1 cycle per second, 440 Hz means 440 cycles per second etc. Periodic sounds above about 20 Hz are perceived is being `pitched'. Our perception of pitch is logarithmic. I.e. if you keep multiplying the frequency of a sound by the same number we perceive the pitch as going up in equal steps. For example, to go up (an equally-tempered) semitone (aka half step or half tone) — which is usually the smallest pitch distance used in music — you multiply the frequency by 2^(1/12) (roughly 1.059463). To go up an octave in pitch, you multiply the frequency by 2. But going from the pitch A3 (220 Hz) to the note an octave higher, A4 (440 Hz), sounds like the same `size' of increase in pitch as going up an octave from A4 (440 Hz) to A5 (880 Hz), even though you are now going up 440 Hz instead of 220 Hz for the previous octave.