Noise

Definition

Noise is a class of sounds that are considered as unwanted. Sound is produced by vibratory movement of molecules in solid bodies and fluids, causing a series of pressure changes.

Basic Characteristics

Physical Aspects of Noise

Sound frequency is defined as the number of sound pressure changes per time unit. It is expressed in Hertz units (1 Hz = one pressure change per second). Sound level is defined as the logarithmic ratio between the actual sound pressure (P) and the sound pressure at hearing threshold (P ₀), according to the following formula:

$$ L = 20 \log_{10} P / P $$

Sound level is expressed in decibel units (dB). Concerning the time characteristics of sound, it can be continuous or intermittent (pauses longer than 1 second), while a sudden change of sound pressure during a period shorter than one second is referred to as an impulse.

Public Health Importance

Noise is a ubiquitous noxious factor in living and occupational environments. Globally, some 120 million...