Sound is the sensation perceived by the human ear resulting from rapid fluctuations in air pressure. These fluctuations are usually created by some vibrating object which sets up longitudinal wave motion in the air. In the case of air outlets it is caused by turbulence which creates rapid pressure pulses on the grille surfaces.

Most people have some intuitive idea of what constitutes a continuous wave, for example by observing the ripples created by a pebble striking the surface of water. Sound waves are a particular type of a general class of waves known as elastic waves, which can occur in media which possess the properties of mass and elasticity. If a particle in such a medium is displaced, then the elastic forces present will tend to pull the particle back to its original position.

The displaced particle possesses inertia and can therefore transfer momentum to a neighbouring particle, so that the initial disturbance can be propagated throughout the medium.

The major distinction between sound waves and the ripples on the surface of water, when using this as an analogy, is that water ripples are transversal, i.e. the particle velocity is at right angles to the direction of propagation, while sound in air is propagated by longitudinal waves, in which the particle velocity is in the direction of propagation.

A number of terms in common use describe the nature of propagation of a sound wave e.g. plane, diverging, spherical, progressive and standing waves.

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