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A decibel is a ratio against a reference about as quiet as the faintest sound a young ear can hear: 20 micropascals. Three unfamiliar ideas at once.
Unrelated sounds add by energy, not pressure, and doubling energy is 3 dB: 60 dB plus 60 dB is 63, not 120.
6 dB per doubling from a point source, which spreads over a sphere; 3 dB from a line of traffic, which spreads over a cylinder.
85 dB(A) for eight hours under ISO and the EU, halving every 3 dB. The US OSHA rule halves every 5.
A source 10 dB below another adds 0.4 dB to it; 20 dB below adds 0.04. In any room, fixing anything other than the loudest thing changes a number nobody can hear: the single most useful consequence of the unit, and why silencing the second-loudest machine on a factory floor changes almost nothing.
A level without a distance is meaningless, and most quoted figures quietly assume one metre. “A chainsaw is 110 dB” means 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 more than the last few hundred near a road. Ten times the distance is 20 dB from a point source and 10 from a line.
About 10 dB sounds twice as loud, a perceptual rule of thumb, 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 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, 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.
They make 63. Unrelated sounds do not line their pressure peaks up, so their energies add rather than their pressures, and doubling the energy is always 3 dB. It follows that a sound 10 dB quieter than another adds 0.4 dB to it — in any room the loudest thing is very nearly the whole answer.
6 dB from a point source, because the sound spreads over a sphere. Only 3 dB from a road, because a line of traffic spreads over a cylinder instead. That is the commonest error in everyday noise arithmetic, and it is why moving twice as far from a motorway disappoints.
Fifteen minutes under the ISO and EU rule — 85 dB(A) for eight hours, halving for every 3 dB above it, because 3 dB is twice the energy. Two hours under the US OSHA rule, which halves every 5 dB instead. The physics did not change between them; the policy did.
About that, perceptually — and it 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.
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