Sound is defined as mechanical compression or longitudinal waves that propagate through a medium (solid, liquid or gas). Most sounds are combinations of signals, but a theoretical pure sound can be described as having a given speed of oscillation or frequency measured in hertz (Hz) and amplitude or energy with measures such as decibel level. The range of sound audible to the human ear falls roughly between 20 Hz and 20 kHz at typical amplitudes with wide variations in response curves. Above and below this range are ultrasound and infrasound, respectively.
Humans and several animals perceive sounds with the sense of hearing with their ears, but low frequency sounds can also be felt by other parts of the body. Sounds are used in several ways, most notably for communication through speech or, for example, music. Sound perception can also be used for acquiring information about the surrounding environment in properties such as spatial characterics and presence of other animals or objects. For example, bats use one sort of echo-location for flying. Ships and submarines use sonar. Humans acquire and use spatial information perceived in sounds.
The perceived "amount" of sound (energy of pressure wave) is defined as loudness and measured in sones. The human ear is most sensitive to sound in the middle of the audible frequency range. The amplitude of a sound wave is measured in pascals, and the intensity of a sound is measured in decibels relative to a stated scale.
Scientific studies of sound are generally classified under the field of acoustics. Perception of sounds, i.e. hearing, are studied in psychoacoustics, which also implies the close relation between perception and psychology. Modelling the sounds that can be heard (and leaving out the inaudible sounds) is discussed in the psychoacoustic model. Further, the class of techniques and methods involving sound processing are usually called audio signal processing methods.