Fundamentals 4 min read Updated 16 Jul 2026

Frequency

Frequency is a physical property of sound. It is defined as the number of cycles of a sound wave that occur in one second.

Its unit of measurement is the Hertz (Hz), where 1 Hz equals one cycle per second.

A complete cycle of a sound wave consists of a compression and a rarefaction of the medium through which the wave travels. Therefore, frequency determines how many of these full vibrations occur within one second.

Relationship to wavelength

Frequency is intrinsically linked to the wave’s physical spacing. There is an inverse relationship between the two: the higher the frequency, the shorter the wavelength, and vice versa. This relationship is governed by the wave equation:

\[ \lambda = \frac{v}{f} \]

\( \lambda \): wavelength.
\( v \): speed of sound.
\( f \): frequency.

Difference between frequency and pitch

Although often confused, frequency is the physical phenomenon, while pitch is the auditory perception derived from it.

Lower frequencies are perceived as lower-pitched (bass) sounds, while higher frequencies are perceived as higher-pitched (treble) sounds.

A musical example is the note A4, which has a frequency of 440 Hz (the wave vibrates 440 times per second). An octave represents the interval where the final frequency is exactly double the initial one (for example, from 440 Hz to 880 Hz).

Fundamental frequency and harmonics

In practice, musical instruments do not produce a single pure frequency, but rather a spectrum of frequencies.

A note consists of a fundamental frequency, which defines the perceived pitch (e.g., 440 Hz for A4) and harmonics, additional frequencies that occur simultaneously and are integer multiples of the fundamental (2f, 3f, etc.).

For example, when playing A4 at 440 Hz, frequencies such as 880 Hz (second harmonic) and 1320 Hz (third harmonic) are also present.

The interaction and relative amplitudes of these additional frequencies allow us to distinguish the timbre of one instrument from another, even when both produce the same fundamental frequency at the same loudness.

Timbre is the characteristic of sound that enables us to differentiate between instruments, even when they play the same note at the same level.

Depending on factors such as construction materials, resonant body design or acoustic environment each instrument exhibits a unique spectral signature.

Hertz (Hz)

Hertz (Hz) is the unit of frequency in the International System of Units (SI). It quantifies how many times a sound wave vibrates within a given time interval.

Frequency range of human hearing

Under normal conditions, the human ear is sensitive to frequencies between 20 Hz and 20,000 Hz. This range is commonly divided into three primary bands:

Low frequencies (20 Hz - 250 Hz): Includes deep sounds such as a bass drum. Within this band, sub-bass (20–60 Hz) corresponds to very low vibrations often perceived through subwoofers as much as heard.
Mid frequencies (250 Hz - 4.000 Hz): Contains most speech information and the majority of musical instruments. This region is often subdivided into low mids (250–500 Hz), mids (500–2,000 Hz) critical for speech intelligibility and high mids (2,000–4,000 Hz) contributing to clarity and presence.
High frequencies (4.000 Hz - 20.000 Hz): Comprises the highest audible tones, contributing brightness and detail.

It is important to note that the ability to perceive the full frequency spectrum may diminish with age or hearing health. Additionally, some frequencies lie beyond natural human hearing:

Infrasound: Frequencies below 20 Hz, used in applications such as seismic monitoring, structural analysis, and explosion detection.
Ultrasound: Frequencies above 20,000 Hz, with practical uses in medicine (ultrasonography), navigation (sonar systems), and industrial cleaning.

Frequently asked questions

What is sound frequency?

Frequency is the number of complete cycles of a sound wave that occur per second. Its unit of measurement is the hertz (Hz), where 1 Hz equals one cycle per second.

What is the difference between frequency and pitch?

Frequency is the measurable physical phenomenon, while pitch is the auditory perception it produces. Low frequencies are perceived as bass sounds and high frequencies as treble sounds.

What are harmonics and how do they affect timbre?

Harmonics are additional frequencies that accompany the fundamental frequency and are integer multiples of it (2f, 3f, etc.). Their combination defines the timbre of an instrument, allowing us to tell a violin from a guitar even when playing the same note.

What is the frequency range the human ear can hear?

The human ear perceives frequencies between 20 Hz and 20,000 Hz under normal conditions. This range is divided into low frequencies (20–250 Hz), mid frequencies (250–4,000 Hz) and high frequencies (4,000–20,000 Hz). This range can narrow with age or hearing damage.

What are infrasound and ultrasound?

Infrasound covers frequencies below 20 Hz, used in seismic monitoring and explosion detection. Ultrasound covers frequencies above 20,000 Hz, with applications in medical imaging, sonar navigation and industrial cleaning.