What is sound? Waves, speed and wavelength

Sound is a small, rapid variation of pressure that travels through a medium such as air. A loudspeaker cone pushes the air in front of it, the compression moves outwards, and a moment later the cone pulls back and the air thins out. A microphone or an eardrum feels this sequence of high and low pressure.

Longitudinal waves

In air, particles move back and forth along the direction the wave travels. This is a longitudinal wave, unlike a water wave, where the motion is up and down. The pressure variations are tiny: normal speech produces about 0.02 Pa, one five-millionth of atmospheric pressure.

Frequency, period and wavelength

Four quantities describe a simple tone:

At 20 °C, 100 Hz has a wavelength of 3.4 m, 1 kHz of 34 cm and 10 kHz of 3.4 cm. Wavelength relative to the size of objects decides how sound is diffracted, absorbed and reflected, see Reflection, absorption and diffraction.

Speed of sound

Speed depends on the medium and its temperature, but not on frequency or loudness in ordinary conditions:

In air, the speed rises about 0.6 m/s per °C, and humidity adds a little, see the speed of sound tool.

Amplitude

How strongly the pressure varies is the amplitude. It can be described in several ways, such as the peak value or the RMS value, see Peak, RMS and average. Levels in decibels compare them with a reference, see Decibels explained.

Real sounds are mixtures

A sine wave is the simplest sound. Real sounds consist of many sine waves at different frequencies, each with its own amplitude, together with noise and a changing envelope. Their combination is the spectrum. See Harmonics, partials and overtones.

Why it matters

Everything else on this site builds on these relationships: delay times follow from the speed of sound, filter and room behaviour from wavelength, and levels from amplitudes. Try the frequency and wavelength tool to feel the scale of different frequencies.

Draft: written by an AI assistant, awaiting technical review.