Noise exposure calculator

01What L_EX,8h actually is

A working day is rarely one steady noise. It is twenty minutes on an angle grinder, two hours near a compressor, a quiet stretch of paperwork, then a burst of something loud at the end of the shift. Each of those is a different level for a different length of time, and no single one of them describes the day.

L_EX,8h is the number that does. It is the daily noise exposure level: the steady sound level that, if it ran for a full eight hours, would deliver the same total sound energy as the real, uneven day actually did. That is what makes one shift comparable to another, and what the regulations are written against.

The subscript matters. "EX" is exposure, "8h" is the reference day. A level of 92 dB(A) for four hours and a level of 89 dB(A) for eight hours are the same L_EX,8h — the same dose, arrived at differently.

02The formula

For exposure periods with levels L_i in dB(A) lasting T_i minutes each, against a reference day T₀ of 480 minutes:

LEX,8h = 10 · log₁₀ ( Σ (Ti / T0) · 10(Li / 10) )

Read from the inside out, it is less forbidding than it looks. Decibels are logarithmic, so 10^(L/10) converts each level back into something proportional to actual sound energy. Multiplying by Ti/T0 weights that energy by the fraction of the reference day it occupied. The sum is the day's total energy. The final 10 · log₁₀ turns it back into decibels.

The consequence worth internalising: because energy is what gets added, you cannot average decibels. Two hours at 100 dB(A) and six hours at 70 dB(A) do not make a 77.5 dB(A) day. They make roughly 94 dB(A) — the loud two hours dominate almost completely, and the quiet six barely register.

03The 3 dB exchange rate

The formula above embeds what is usually called the equal-energy principle, or the 3 dB exchange rate. Because a 3 dB increase represents a doubling of sound energy, every 3 dB you go up halves the time you may spend there for the same dose.

• 85 dB(A) for 8 hours
• 88 dB(A) for 4 hours
• 91 dB(A) for 2 hours
• 94 dB(A) for 1 hour
• 97 dB(A) for 30 minutes

Every line in that list is the same daily exposure. It is the single most useful thing to know about noise on site, because it makes short loud tasks legible: five minutes with a needle gun is not a rounding error, it is a meaningful share of the day's budget.

OSHA is different. US OSHA 1910.95 uses a 5 dB exchange rate and a percentage-dose method, not the equal-energy calculation above. Selecting OSHA in this calculator changes which limit values your result is compared against — 85 and 90 dB(A) instead of 80, 85 and 87 — but the underlying number is still the equal-energy L_EX,8h. For anything that has to satisfy an OSHA inspector, use a native OSHA dose calculation. We would rather say that plainly than let the toggle imply more than it does.

04Where 80, 85 and 87 dB(A) come from

The three EU values are set by Directive 2003/10/EC, the seventeenth individual directive under the framework Directive 89/391/EEC. Each one triggers a different duty, and they escalate.

• 80 dB(A) — lower exposure action value. Hearing protection must be made available. Workers are entitled to information and training on the risk, and to hearing checks where a risk assessment shows a risk to health.
• 85 dB(A) — upper exposure action value. Hearing protection must be worn, and its use enforced. The area is marked and access restricted where practicable. A programme of technical and organisational measures to reduce exposure is required, not optional.
• 87 dB(A) — exposure limit value. This must never be exceeded. Critically, it is the only one of the three assessed taking hearing protection into account: the 80 and 85 values are measured at the ear disregarding protection, whereas the 87 limit is what actually reaches the ear. If it is exceeded, work stops until exposure is brought back below it.

Member states transpose the directive into national law, and some go further than the minimum. Check your national implementation before relying on the bare directive figures.

05What this calculator does not do

Being explicit about the edges is more useful than implying completeness.

• Peak sound pressure is not covered. Directive 2003/10/EC sets separate L_pC,peak limits — 135, 137 and 140 dB(C) — for impulsive noise. A hammer strike or a cartridge tool can breach those while the daily average looks unremarkable.
• It does not apply hearing protection attenuation. The result is exposure at the ear disregarding protectors, which is the correct basis for the 80 and 85 values, but not for comparison against the 87 limit. Subtract the protector's assumed attenuation yourself, by SNR, HML or octave-band, before judging against 87.
• It does not account for weekly averaging. The directive permits L_EX,8h to be averaged over a week in specific circumstances, with the consent of the competent authority. This calculates a single day.
• Your inputs decide the answer. A calculation is only as good as the dosimetry behind it. Levels should come from calibrated instruments and durations from observed work, not estimates.

This tool is an aid to a competent person, not a substitute for one. Nothing here is legal advice, and results should be verified against the legislation in force where you work.

06Keeping the record

A calculation you cannot produce six months later is not much use in an audit. This page deliberately keeps nothing — close the tab and the numbers are gone, because storing your measurements would mean collecting them.

Halo HSE is the same calculation on your phone, with the part this page cannot do: saved records with a change history, your name and certification on the report, and a dated PDF export to file or hand to a client. It also carries hand-arm vibration, the NIOSH lifting equation, a configurable risk matrix, and searchable GHS references.

Noise TWA and the NIOSH lifting equation are free in the app, as is PDF export. There is no account and no demo gate.