Sound is a mechanical wave produced by pressure and density variations in a material medium.

In gases and liquids, sound propagates mainly as a longitudinal wave: compressions and rarefactions move through the medium while the particles oscillate around equilibrium.

Frequency and pitch

The frequency of a sound wave is set by the source. For audible sound, frequency is perceived mainly as pitch: higher frequencies correspond to higher pitches.

Human hearing has a limited frequency range, while other animals can detect frequencies outside it.

Amplitude and intensity

Larger pressure amplitudes generally correspond to greater sound intensity. Perceived loudness depends on intensity but also on frequency and the response of the ear.

The decibel scale expresses sound levels logarithmically:

$$L=10\log_{10}\left(\frac{I}{I_0}\right).$$

Speed of sound

Sound speed depends on the elastic and inertial properties of the medium. It changes with material and, in gases, with conditions such as temperature.

Sound as a wave

Reflection produces echoes, superposition produces interference, and confined systems can support standing acoustic modes. Sound therefore provides a concrete example of the general wave principles already developed.