Sound is a disturbance of mechanical energy that propagates through matter as a wave (through fluids as a compression wave, and through solids as both compression and shear waves). Sound is further characterized by the generic properties of waves, which are frequency, wavelength, period, amplitude, speed, and direction (sometimes speed and direction are combined as a velocity vector, or wavelength and direction are combined as a wave vector).

Humans perceive sound by the sense of hearing. By sound, we commonly mean the vibrations that travel through air and are audible to people. However, scientists and engineers use a wider definition of sound that includes low and high frequency vibrations in the air that cannot be heard by humans, and vibrations that travel through all forms of matter, gases, liquids, solids, and plasmas.

The matter that supports the sound is called the medium. Sound propagates as waves of alternating pressure, causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate. The scientific study of the absorption and reflection of sound waves is called acoustics.

Noise is often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal.

Speed of sound

Decibel, sone, mel, phon, hertz

Sound pressure level

Particle velocity, acoustic velocity

Particle displacement, particle amplitude, particle acceleration

Sound power, acoustic power, sound power level

Sound energy flux

Sound intensity, acoustic intensity, sound intensity level

Acoustic impedance, sound impedance, characteristic impedance

Speed of sound, sound velocity, amplitude

• See also Template:Sound measurements