noise (physics)

Unwanted sound. Permanent, incurable loss of hearing can be caused by prolonged exposure to high noise levels (above 85 decibels). Over 55 decibels on a daily outdoor basis is regarded as an unacceptable level. In scientific and engineering terms, a noise is any random, unpredictable signal.

Noise pollution
Noise is a recognized form of pollution, but is difficult to measure because the annoyance or discomfort caused varies between individuals. If the noise is in a narrow frequency band, temporary hearing loss can occur even though the level is below 85 decibels or exposure is only for short periods. Lower levels of noise are an irritant, but seem not to increase fatigue or affect efficiency to any great extent. Loss of hearing is a common complaint of people working on factory production lines or in the construction and road industry. Minor psychiatric disease, stress-related ailments including high blood pressure, and disturbed sleep patterns are regularly linked to noise, although the causal links are in most cases hard to establish. Loud noise is a major pollutant in towns and cities. The European Commission announced in 1998 that every EU city with a population of more than 250,000 will have to produce ‘noise maps’ by 2002, as part of its directive to reduce noise pollution. An estimated 80 million people suffer daily through excessive noise in the EU. Workers in noisy places should wear ear defenders. Barriers such as lines of trees, or walls, can be used as barriers to reduce noise levels near sources such as motorways.

Electronic noise
Electronic noise takes the form of unwanted signals generated in electronic circuits and in recording processes by stray electrical or magnetic fields, or by temperature variations. In electronic recording and communication systems, white noise frequently appears in the form of high frequencies, or hiss. The main advantages of digital systems are their relative freedom from such noise and their ability to recover and improve noise-affected signals.