Frequency in Music: Hz, A440 & Concert Pitch

A plain-language guide to what "Hz" means, and why A is usually 440.

Short answer: frequency is how many times a sound wave repeats each second, measured in hertz (Hz), where one hertz is one cycle per second. The faster the vibration, the higher the pitch we hear. The musical note A above middle C (A4) is defined as 440 Hz, the international reference that lets instruments everywhere agree on where "in tune" sits.

What frequency actually is

Sound travels as a pressure wave that pushes and pulls the air. Frequency counts how many complete push-pull cycles happen in one second, and the unit for cycles per second is the hertz, named after the physicist Heinrich Hertz. A tone at 220 Hz completes 220 cycles every second; a tone at 880 Hz completes 880. Two numbers describe the same wave from different angles: frequency, the cycles per second, and its period, the time for a single cycle, which is simply one divided by the frequency. A 100 Hz tone therefore has a period of one hundredth of a second. Frequency is an objective, physical quantity you can measure with an instrument, whereas the pitch we perceive is the brain's read-out of that frequency.

From frequency to the notes we name

Musical notes are simply agreed-upon frequencies, and the pattern that connects them is doubling. Every time you double a frequency you rise one octave, landing on a note that shares its name. Starting from concert A at 440 Hz, the A an octave below is 220 Hz and the A an octave above is 880 Hz. The same doubling explains why the audible span from 20 Hz to 20,000 Hz covers only about ten octaves, even though it stretches across a thousandfold change in raw hertz. Because pitch follows ratios, equal musical steps look like multiplication on the frequency scale rather than addition, which is exactly why a fixed reference point is so useful.

Why A4 is 440 Hz

The note A4 sits near the middle of the treble range, and by convention it anchors the whole tuning system. Fixing it at 440 Hz is an international agreement rather than a fact of nature. The value was endorsed at a London conference in 1939, carried into an ISO recommendation in 1955, and codified as the standard ISO 16 in 1975. That standard even specifies a tolerance of about half a hertz, so tuners across the world share a precise target. Before this agreement, pitch standards drifted widely between cities and eras, and older instruments were often tuned noticeably lower or higher than a modern ear expects.

Concert pitch, and the "440" that isn't always 440

"Concert pitch" is the reference an ensemble tunes to before it plays. Although 440 Hz is the global benchmark, real orchestras vary. Many continental European ensembles tune their A4 a little higher, commonly in the 440 to 444 Hz band, which lends a brighter, more brilliant character. Because every note is defined relative to A, nudging the reference up or down shifts the entire scale with it, without changing the musical relationships between the notes. A whole orchestra can play in perfect internal tune at 443 Hz and still sound slightly bright next to a recording made at 440.

The 432 Hz question, kept neutral

You may see A4 = 432 Hz presented as an alternative tuning. It is exactly that: a different choice of reference that lowers the whole scale slightly compared with 440. Some musicians prefer it for tradition or taste. It is a preference, not a measurable property of the sound itself, and no special claim needs to be attached to it. The 440 and 432 references are simply two points among several used across history and genres, and the ear adapts to whichever one an ensemble settles on.

Hear the difference for yourself

Related reading

• What is pitch? Pitch vs frequency: how objective hertz becomes the pitch you perceive.
• What is an octave?: why doubling the frequency lands you on the "same" note.
• What are cents in music?: the fine unit for the frequency ratios between notes.

Sources: Brian C. J. Moore, An Introduction to the Psychology of Hearing (Brill); Wikipedia, A440 (pitch standard) and Concert pitch; International Organization for Standardization, ISO 16:1975, Acoustics – Standard tuning frequency.