Hearing, sound frequencies and why 440 Hz is the reference pitch

What 440 Hz means

Hertz is the number of cycles per second. A 440 Hz tone makes the air pressure wave repeat 440 times each second, and each cycle lasts about 2.27 milliseconds.

In music, 440 Hz is the A just above the piano’s middle C, also called A4. It is the value electronic tuners commonly adopt as their default.

Why 440? What the standard says (and what we do not claim)

The verifiable answer is short. ISO 16:1975, “Acoustics: Standard tuning frequency (Standard musical pitch)”, specifies that the frequency of the note A in the treble stave shall be 440 Hz.

The standard also requires that tuning and retuning be done with instruments producing the tone within an accuracy of 0.5 Hz. The ISO catalogue shows it as confirmed on 30 September 2022.

That answers “what is the standard?”, but not “why this number?”. A standard is a convention: 440 works because many people agreed to use it, not because nature chose it.

The same goes for 432 Hz or 528 Hz. ISO 16 fixes a reference, it does not rank pitches by benefit, and the sources used here do not support healing claims tied to a specific frequency.

From the reference note to the others: equal temperament

In equal temperament, the octave (from one note to the same note higher up) doubles the frequency and is split into 12 equal semitones. Each semitone multiplies the frequency by 2 to the power 1/12, about 1.0595.

Example: middle C sits 9 semitones below A, so f = 440 x 2^(-9/12) = 261.63 Hz. Go up an octave and the value doubles: 440 becomes 880 Hz.

You can check these values in the tone generator, which accepts a number in hertz and produces sine, square, triangle or sawtooth waves.

What the ear actually perceives

Two tones with the same sound pressure do not seem equally loud if their frequencies differ. ISO 226:2023 handles this with equal-loudness contours for pure tones, between 20 Hz and 12.5 kHz, measured in otologically normal listeners aged 18 to 25.

ISO 7029:2017 does the other side: it describes how hearing thresholds are distributed by age and sex, from 18 to 80 years, between 125 Hz and 8 kHz. Above 8 kHz, up to 12.5 kHz, the medians are informative only.

That gap above 8 kHz explains why a study of 51 young people becomes a provisional reference. The authors tested 9 to 18 kHz, in 1 kHz steps, and write that there is still no consensus on normal standards for those frequencies.

The most sensitive point was at 14 kHz, and the threshold climbs sharply up to 18 kHz. The paper uses the label “dB NA” in the text and “dB HL” in the summary, and measured everyone on a single audiometer, so treat the numbers as a sample, not a rule for the population.

Using the tools safely

The hearing test shows the highest frequency you can hear in each ear, and the generator lets you explore tones freely. Both depend on your headphones and sound card, so they are for curiosity, not diagnosis.

• Start with the system volume very low and raise it slowly.
• Never raise the volume to hear a faint tone: stop and rest.
• Use headphones and a quiet room to compare both ears.
• Phone speakers reproduce very low tones poorly; use headphones to compare.
• If you notice loss, ringing or discomfort, see a hearing professional.

To understand how loudness is measured in audio and streaming, see the guide on LUFS and audio normalization.