What a Sound Is

This section describes the sound and its propagation through various media (like the air).

A sound is a mechanical wave of pressure that oscillates at a frequency between 20 Hertz (Hz) and 20 kilohertz (kHz).

Infrasound is a low-frequency sound that oscillates at a frequency below 20 Hz. Ultrasound is a sound that oscillates at a frequency higher than 20 kHz.

A sound source creates a disturbance in a transmission medium (for example air, water, concrete, etc.) that propagates to one’s ears.

A sound propagates in every direction, but to different extents (acoustic directivity).

The speed of propagation (for example the speed of sound) depends on temperature, relative humidity, but mostly on the physical characteristics of the medium. See some examples in the following table:

Physical characteristics of the medium	Speed of propagation
Cair	340 m/s
Cwater	1450 m/s
Cconcrete	4000 m/s
Csteel	5100 m/s

The characteristic variable of sound is the acoustic pressure in Pascals (Pa).

A sound is a pressure variation around the static pressure:

Real pressure = static pressure (»1013 hPa) ± acoustic pressure (∼between 20 µPa and 20 Pa).