Sound consisting of a mixture of airborne vibrations which is completely irregular with regard to sound intensity, frequency and phase.

Noise is usually regarded as a nuisance, if the sound intensities involved are very high, cause damage to the organs of hearing. Such objectionable noise is, for example, causes when a piece of sheet metal is struck with a hammer (Fig 1a). When the sound intensity is plotted as a function of time, the curve obtained presents a very irregular shape with jagged peaks and valleys (Fig 1b).

An important function of modern structural engineering is to provide protection against noise. For example, in offices in which the rattle of typewriters would produce unbearable conditions if it were reflected back from the walls, the ceilings and, if need be, the walls can be lined with sound-absorbing panels (Fig.2). The actual sound-absorbing material, e.g., glass wool or rock wool, is interposed between the ceiling or wall itself and an inner skin formed of perforated plates secured at a certain distance from the wall face. The sound waves which pass through the perforations are absorbed by the glass wool or similar absorbing material. The result is a remarkable reduction of noise.

According to the Weber-Fechner law of hearing, the apparent loudness of a sound i.e., the intensity of the acoustic perception is approximately proportional to the logarithm of the sound intensity. The unit of objective loudness or sound level is the phon. The loudness, in phons, of a sound is equal to the intensity in decibels of a sound of frequency 1000 cycles/sec, which seems as loud to the ear as the given sound.

Two sound intensities, P1 and P2, can be said to differ by n decibels when n=10 log (P2/P1), where P2 is the intensity of the sound under consideration and P1 is the intensity of the reference sound level.

The following table gives typical loudness values, in phons, for a number of sound sources:

Loudness values in excess of 130 phons produce a sensation of acute discomfort. Continuous noise in excess of 100 phons may cause hearing damage. The sound level scale from 1 to 140 phons comprises the intensity ratio of 1:1014 . Objective measurement of sound intensity can be carried out by means of a Rayleigh disc (Fig.4). This is a small thin disc which is suspended from a fine thread of glass or quartz and is placed at an angle of 45o to the direction of propagation of the sound waves. The disc experiences a torque and strives to place itself at right angles to the waves. The amount of rotation it undergoes is a measure of the intensity.