Sound in fluids involves time-dependent variations of pressure. In atmospheric air, the acoustic pressure variations are superimposed on the local ‘static’ atmospheric pressure. Even at ‘high’ noise levels, for example at 200 m from a large passenger aircraft taking off, the maximum sound pressures are only about one-hundred-thousandth of the atmospheric pressure. Pressure is only one of the quantities that varies in time in a fluid medium carrying a sound wave, the others being density and temperature, together with particle position, velocity and acceleration. Sound pressure is the most commonly measured of these quantities because it is the easiest to measure and, most importantly, it is the physical agent that activates the human auditory system.