The Exchange Rate

The exchange rate is the least discussed number in noise exposure and the one that decides everything above 90 decibels.

When employers review noise dose alongside working patterns, employee attendance tracking software can provide operational context, while exposure limits still need to be assessed from sound level and duration.

What it is

The increase in level that halves the permitted exposure time.

A 3 dB exchange rate means every 3 dB doubles the dose, so the allowable time halves. This follows directly from the physics — 3 dB is twice the sound energy.

For additional technical context, see Safe Work Australia noise guidance.

A 5 dB exchange rate means the time halves every 5 dB, which is more permissive at every level above the starting point.

Why they diverge so fast

Because the effect compounds.

Starting from 85 dBA at 8 hours with a 3 dB rate: 88 gives 4 hours, 91 gives 2, 94 gives 1, 97 gives 30 minutes, 100 gives 15.

Starting from 90 dBA at 8 hours with a 5 dB rate: 95 gives 4 hours, 100 gives 2, 105 gives 1.

15 min vs 2 hpermitted exposure at 100 dBA under the 3 dB and 5 dB exchange ratesNIOSH recommended exposure limit and OSHA permissible exposure limit

Two rules, one physical situation, an eight-fold difference in the answer.

Which one is right

The 3 dB rate reflects the energy relationship and is used by most national standards outside the US, by ACGIH, and by NIOSH.

The 5 dB rate reflects an older view that the ear recovers somewhat between exposures, and it survives in US regulation because changing a regulation is slower than changing an understanding.

Use 3 dB. It is the more protective and the better supported, and where a workplace uses 5 dB the additional exposure is real regardless of its legality.

Where it bites in ordinary life

Impulse and short exposures.

A single hour at 94 dBA is a full day's dose under the 3 dB rule. A concert, a power tool, a motorcycle commute.

And it means the loudest ten minutes of a day can dominate the total, which is not how people account for it — the day is usually assessed by what it mostly sounded like.

The arithmetic, once

Permitted time = 8 hours ÷ 2^((level − 85) ÷ 3), for the NIOSH rule in dBA.

At 85 that gives 8 hours. At 100 it gives 8 ÷ 2^5, which is a quarter of an hour.

Worth running once on the level you are actually exposed to, because the result is usually shorter than expected and the calculation is a single line.

The short version

  • The exchange rate is the level increase that halves permitted exposure time
  • 3 dB follows from the physics: 3 dB is twice the energy, so half the time
  • 5 dB is more permissive at every level and survives in US regulation from an older understanding
  • At 100 dBA the two rules give 15 minutes and 2 hours — an eight-fold difference for one physical situation
  • One hour at 94 dBA is a full day's dose, so the loudest ten minutes can dominate a day nobody thought was loud
  • The formula is 8 hours divided by 2 to the power of (level minus 85, over 3), and it is worth running once