About 450 million people - 65% of the European population - are exposed to noise intensities above 55 dB, a level high enough to cause annoyance, aggressive behaviour and sleep disturbance.
(European Environment Agency Report, 1995)
Acoustic waves can be defined as pressure fluctuations in an elastic medium. Sound is the auditory sensation produced by these oscillations. In air, sound consists of oscillations about the ambient atmospheric pressure (Figure 11.1). Vibrating surfaces or turbulent fluid flow can act as the source, propagating alternately high-and low-pressure areas. The amplitude of the acoustic wave is expressed in Newtons per square metre or in pascals (1Pa = 1N/m2). The threshold of hearing (lowest amplitude of pressure oscillations in air detectable by the ear) is 0.00002 N/m2 at a frequency of 1000 Hz.
Figure 11.1 Sound is propagated in air as pressure oscillations above and below the ambient pressure.