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A source 10 dB below another adds 0.4 dB to it; 20 dB below adds 0.04. So in any room, fixing anything other than the loudest thing changes a number nobody can hear. It is the single most useful consequence of the unit.
A level without a distance is meaningless, and most quoted figures quietly assume one metre. “A chainsaw is 110 dB” means 110 dB at the handle; at ten metres it is 90, and across a field it is conversation.
Doubling your distance from a point buys 6 dB; from a line of traffic it buys 3. That is why moving twice as far from a motorway is so disappointing, and why the last few metres near a drill matter so much more than the last few hundred near a road.
About 10 dB sounds twice as loud, and 10 dB is a factor of ten in sound power. Halving the perceived loudness of a machine means removing ninety per cent of what it emits — which is why quiet is expensive and why 3 dB improvements are the ones actually on offer.
In a lab, on a steady tone, about 1 dB is detectable. In a real room, 3 dB is the practical figure — and that is exactly what you get from switching off one of two identical machines.
ISO and the EU halve the allowed time every 3 dB, because 3 dB is twice the energy. US OSHA halves it every 5. At 100 dB that is fifteen minutes against two hours, for the same sound.
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A decibel is a ratio, on a logarithmic scale, against a reference so quiet it is roughly the faintest sound a young ear can hear — 20 micropascals of pressure. Three unfamiliar ideas at once, which is why the unit misleads so reliably.
Two sounds of 60 dB together make 63 dB, not 120. Unrelated sounds do not line their pressure peaks up, so their energies add rather than their pressures — and doubling the energy is 3 dB, always.
The consequence is worth keeping: a sound 10 dB quieter than another adds 0.4 dB to it, and one 20 dB quieter adds 0.04. In any room the loudest thing is very nearly the whole answer, which is why silencing the second-loudest machine on a factory floor changes almost nothing.
A point source spreads over the surface of a sphere, and that surface grows as the square of the radius. Twice as far means a quarter of the intensity, which is −6 dB. Ten times as far is −20 dB.
A road is not a point. A line of traffic spreads over a cylinder instead, so it falls off as 1/r rather than 1/r², and doubling your distance from it buys only 3 dB. This is the commonest error in everyday noise arithmetic, because a road is the thing people most often want to reason about — and it is why moving twice as far from a motorway disappoints.
Roughly 10 dB sounds about twice as loud. That is a perceptual rule of thumb rather than a physical identity, and it is the one number that connects the scale to experience. It also explains why quiet is expensive: ten decibels is a factor of ten in energy, so halving the perceived loudness of a machine means removing ninety per cent of its sound power.
About 3 dB is the smallest change most people notice in ordinary conditions — which is exactly what you get from switching off one of two identical machines.
The ISO and EU rule is 85 dB(A) for eight hours, and the allowance halves for every 3 dB above it, because 3 dB is twice the energy.
| Level | Safe for | Something like |
|---|---|---|
| 85 dB | 8 hours | heavy traffic from the pavement |
| 88 dB | 4 hours | a food blender at arm's length |
| 91 dB | 2 hours | a petrol lawnmower |
| 94 dB | 1 hour | a busy nightclub |
| 100 dB | 15 minutes | a chainsaw, or the front of a gig |
| 112 dB | under a minute | a siren at close range |
The American OSHA rule uses a 5 dB exchange instead of 3, which is markedly more permissive at high levels — the same 100 dB buys two hours rather than fifteen minutes. The physics did not change; the rule did.