PHYSICS OF SOUND

Noise can be defined as "disagreeable or undesired sound" or other disturbance. From the acoustics point of view, sound and noise constitute the same phenomenon of atmospheric pressure fluctuations about the mean atmospheric pressure; the differentiation is greatly subjective. What is sound to one person can very well be noise to somebody else. The recognition of noise as a serious health hazard is a development of modern times. With modern industry the multitude of sources has accelerated noise-induced hearing loss; amplified music also takes its toll. While amplified music may be considered as sound (not noise) and to give
pleasure to many, the excessive noise of much of modern industry probably gives pleasure to very few, or none at all. Sound (or noise) is the result of pressure variations, or oscillations, in an elastic medium(e.g., air, water, solids), generated by a vibrating surface, or turbulent fluid flow.
Soundpropagates in the form of longitudinal (as opposed to transverse) waves, involving a succession of compressions and rarefactions in the elastic medium, as illustrated by Figure 1.1(a). When a sound wave propagates in air (which is the medium considered in this document), the oscillations in pressure are above and below the ambient atmospheric pressure.