Human ears perceive certain modulations of the atmospheric pressure as sound. Such modulations result from a propagated vibration of an elastic medium or superposition of such vibrations. These vibrations cause an alteration in pressure, stress, particle displacement, or particle velocity of the medium. Sound propagates through the medium as a wave.
In the absence of acoustic energy in a particular space, air molecules move about in a random thermal motion. There occur frequent collisions amongst the molecules without loss of energy. The net result is an equilibrium condition with characteristic absolute temperature and pressure. On introduction of a vibrating surface, the molecules gain additional momentum by momentum transfer, and the disturbance propagates into the space with a velocity essentially equal to the thermal speed of individual molecules.

If the vibrating surface is a pulsating sphere, the sound wave spreads out spherically with a wave of single frequency. The disturbance caused by the vibrating surface spreads through the medium by momentum transfer. The disturbance induces displacement of the individual particles of the medium. The particles attain velocities in the direction of propagation. These bring changes in absolute pressure and temperature. Human ears or microphones detect this pressure change as sound. Whenever the sound is irritating, annoying, unpleasant and unwanted, it is called noise.

Effects of Noise
Excessive exposure to noise has numerous direct and indirect effects on safety and hygiene. It directly affects our sensation of hearing, disrupts sleeping, and causes masking of speech. Otologists link industrial noise as a source of numerous problems of the ear, such as noise-induced hearing loss, permanent hearing impairment, acoustic trauma, sensorineural hearing loss, tinnitus etc. Exposure to high noise levels can indirectly affect human psychology and physiology, resulting in adverse effects on general performances. A number of ailments have a causal relationship with noise exposure. .