Every sound is a vibration – tiny, rapid changes in air pressure that travel outwards from a source. Frequency is how many of those pressure cycles pass a point each second, measured in hertz (Hz). A 440 Hz tone repeats 440 times per second; a 50 Hz hum, fifty.
Frequency and pitch
We perceive frequency as pitch: low frequencies sound deep, high frequencies sound shrill. But perception is logarithmic. Going from 100 Hz to 200 Hz sounds like the same musical step – an octave – as going from 1,000 Hz to 2,000 Hz. That is why the twelve notes of the Western scale are spaced by a constant ratio (the twelfth root of two, about 1.0595), not by a constant number of hertz.
Concert pitch A4 is 440 Hz. Middle C (C4) is 261.63 Hz. The lowest key on a piano, A0, is 27.5 Hz; the highest, C8, is 4,186 Hz. See every note on the note frequency chart and play any frequency with the tone generator.
Fundamentals and harmonics
Real instruments and voices do not produce a single frequency. They produce a fundamental – the frequency we hear as the pitch – plus harmonics at whole-number multiples (2×, 3×, 4×…). The balance of harmonics creates timbre, which is why a violin and a flute playing the same A4 sound different. Our brains are so good at this that we hear the pitch of a voice on the telephone even though the phone line cuts away the fundamental – the "missing fundamental" effect.
The range of human hearing
Healthy young ears hear roughly 20 Hz to 20,000 Hz – about ten octaves. We are most sensitive between about 2 and 5 kHz, helped by the natural resonance of the ear canal near 3 kHz; this is where speech consonants, alarms and babies' cries carry their energy.
- Below 20 Hz – infrasound. Felt as pressure or vibration rather than heard as pitch. Earthquakes, volcanoes, wind and elephants produce it.
- Above 20 kHz – ultrasound. Inaudible to us but used by bats, dolphins, rodents, medical scanners and parking sensors.
Many animals hear far beyond our limits: dogs up to about 45 kHz, cats to 64 kHz, some moths to 300 kHz. Compare them on the hearing range chart.
Hearing changes with age
High-frequency hearing begins to decline in early adulthood, a process called presbycusis. Many people in their twenties can hear 17 kHz; by middle age the limit is often 14–15 kHz, and noise exposure accelerates the loss. This is the basis of "mosquito" ringtones around 17.4 kHz that teenagers hear and their teachers do not. Try the hearing test – with good headphones – to estimate your own upper limit.
Wavelength
Frequency and wavelength are linked by the speed of sound: wavelength = speed ÷ frequency. In air at 20 °C (343 m/s), a 20 Hz bass note is 17 m long and a 20 kHz tone just 1.7 cm. This explains a lot of everyday acoustics: bass travels through walls and around buildings, while treble is easily blocked – which is why you hear only the thump of a neighbour's music. Calculate any wavelength with the wavelength calculator.
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