Noise is usually defined as unwanted sound. Sound may be defined as any pressure variation (in air, water or other medium) that the human ear can detect. When variations in atmospheric pressure occur rapidly - at least 20 times a second - they can be heard and hence are called sound.
The number of pressure variations per second is called the frequency of the sound, and is measured in Hertz (Hz) - one hertz is equivalent to one wave per second. A thousand hertz is often denoted kHz, e.g. 2 kHz = 2000 Hz.
The frequency of a sound produces its distinctive tone or musical pitch. The normal range of hearing for a healthy young person extends from approximately 20 Hz up to 20 000 Hz (or 20 kHz).
The Decibel dB
A sound's intensity, or loudness or volume is measured in bels, named after Alexander Graham Bell. A bel is a very large measurement so the loudness scale is shown in decibels, (dB, one-tenth of a bell) The ratio of sound pressures which we can hear is a ratio of 106 (one million : one). For convenience, therefore, a logarithmic measurement scale is used. The resulting parameter is called the ‘sound pressure level’ (Lp) and the associated measurement unit is the decibel (dB). As the decibel is a logarithmic ratio, the laws of logarithmic addition and subtraction apply.
Equivalent Continuous Sound Level
Another index for assessment for overall noise exposure is the equivalent continuous sound level, Leq. This is a notional steady level which would, over a given period of time, deliver the same sound energy as the actual time-varying sound over the same period. Hence fluctuating levels can be described in terms of a single figure level.
Statistical Noise Levels
For levels of noise that vary widely with time, for example road traffic noise, it is necessary to employ an index which allows for this variations. The Leq, the equivalent continuous sound level, has been adopted in this country for the assessment of road traffic noise. The L90, the level exceeded for ninety per cent of the time, has been adopted to represent the background noise level. A-weighted statistical noise levels are denoted LA10, dBLA90 and such like. The reference time period (T) is normally included, for example. dBLA10,5min or dBlA90,sh.