How Noise Is Measured, and Why the Decibel Scale Is Not Linear

In one line

Noise is measured in A-weighted decibels averaged over time, the scale is logarithmic, and a lawyer who does not know that cannot argue about a limit.

In exam wording: the ambient air quality standards in respect of noise in the Schedule to the Noise Pollution (Regulation and Control) Rules 2000 are expressed in dB(A) Leq, where a decibel is the unit in which noise is measured, "A" denotes the frequency weighting corresponding to the response characteristics of the human ear, and Leq is the energy mean of the noise level over a specified period.

Why a law student needs this

Because every operative provision in this module is a number, and the numbers are not on a scale anybody's intuition handles.

Rule 5(4) permits a loudspeaker at a public place to exceed the ambient standard by 10 dB(A). Rule 5(5) permits a private sound system to exceed it by 5 dB(A). Rule 7 lets a person complain where the level exceeds the standard by 10 dB(A) or more. A residential area's night limit is 45 and a silence zone's is 40.

A reader who thinks those are small differences has misunderstood every one of them.

The three parts of "dB(A) Leq", from the Rules' own note

The Schedule's note defines all three, and it is worth quoting because it is the statute defining its own units.

"A decibel is a unit in which noise is measured."

"A", in dB(A) Leq, denotes the frequency weighting in the measurement of noise and corresponds to frequency response characteristics of the human ear.

"Leq: It is an energy mean of the noise level over a specific period."

The decibel, and why the scale is not linear

The decibel is a logarithmic unit. That single fact carries the practical consequences, and there are three worth learning.

An increase of about 10 dB is roughly a doubling of perceived loudness. So the 10 dB(A) allowance in rule 5(4) is permission for a loudspeaker to be about twice as loud as the ambient standard, not a marginal indulgence.

An increase of 3 dB is a doubling of sound energy. Two identical machines make about 3 dB more noise than one, not twice the number of decibels. This is why controlling one source in a street of many achieves less than it looks.

And you cannot add decibels arithmetically. 60 dB plus 60 dB is about 63 dB, not 120. A submission that three sources of 55 dB together produce 165 dB is nonsense, and it is a submission that gets made.

For orientation: a quiet library is about 40, ordinary conversation about 60, a busy road about 80, and a firecracker at close range well above 120. The difference between a silence zone limit of 40 at night and a commercial limit of 65 by day is therefore very large indeed.