Measuring noise

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The intensity (loudness) of sound is measured in Decibels: dB(A). (A) denotes that the scale is adapted for the human hearing range. 20dB(A) is for example equivalent to a gentle breeze or a soft whisper. Sounds louder than 120dB(A), equivalent to a military aircraft taking off, provoke fear reactions and possibly pain.

The decibel scale is logarithmic, which means that a 3dB(A) increase equals a doubling of the volume of the sound. A 10dB(A) increase means the sound is 10 times louder. For example, an aircraft passing overhead is likely to be 20dB(A) – or one hundred times – louder than a normal conversation. Levels of environmental noise are often reported as averages over a sustained period
(Leq).

Other important characteristics of sound include its frequency (pitch), periodicity (continuous or intermittent) and duration. At the same volume, an intermittent sound such as one passing aircraft, is often more disturbing than a constant background noise, such as a busy road.

Noise is more disturbing at certain times of day. To allow for this the Lden (Loudness day-evening-night) measurement attributes a higher value to noise in the evening (+5dB) and night (+10dB). This means that one plane at night is seen to be as problematic as ten planes during the day.