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In physics, the sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid, or solid.

In human physiology and psychology, the sound is the reception of such waves and their perception by the brain. Only acoustic waves with frequencies between 20 Hz and 20 kHz, the audio frequency range, induce auditory perception in humans. In air at atmospheric pressure, they are sound waves with wavelengths ranging from 17 meters (56 feet) to 1.7 centimetres (0.67 inches). Sound waves above 20 kHz are known as ultrasounds and cannot be heard by humans. Sound waves below 20 Hz are known as infrasound. Different types of animals have different hearing ranges.

Acoustics is an interdisciplinary science that studies mechanical waves in gases, liquids and solids including vibration, sound, ultrasound and infrasound. A scientist working in the field of acoustics is an acoustician, and someone working in an acoustic engineering field might be called an acoustic engineer. On the other hand, the sound engineer deals with the recording, processing, mixing, and reproduction of sound.

Applications of acoustics can be found in almost all spheres of life in modern society; disciplines include aeroacoustics, audio signal processing, architectural acoustics, bioacoustics, electroacoustics, environmental noise, musical acoustics, noise control, psychoacoustics, speech, ultrasound, underwater acoustics, and other vibration. Sound can travel through media such as air, water and solids such as longitudinal waves and shear waves in solids. Sound waves are generated by a sound source, such as a vibrating hole in a stereo speaker. The sound source creates vibrations in the environment. When the source continues to vibrate the medium, the vibrations propagate from the source at the speed of sound, forming a sound wave at a fixed distance from the source; the pressure, velocity and displacement of the medium change over time. At a particular moment, pressure, velocity and displacement change in space. Note that particles in a medium do not move with a sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, vibrations of particles in a gas or liquid carry vibrations until the average position of the particles changes over time). During propagation, waves can be reflected, refracted or attenuated by the medium.

Sound travels through gases, plasma and liquids in the form of longitudinal waves also called compression waves. For its reproduction, an environment is needed. However, it can transmit both longitudinal and transverse waves through solid particles.