Home > Safety & Environmental Help and Guidance > Noise at Work Assessments > Noise – What is Dose and how is it calculated?
Dose is a measure of noise exposure and is directly related to Lex, in the UK Dose is also described as the HSE ‘Points’ system.
It is best described with the following statement:
100% Dose (or 100 points) = 85dBA for 8 hours. This is known as the ‘Criterion’ level
In other countries, Dose may be specified with a different criterion i.e. 100% Dose = 90dBA for 8 hours may also be specified.
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The tables below give examples of how DOSE is related to exposure time and Lex (Lep’d) (assuming 100% Dose = 85dBA for 8 hours.)
Each time the sound level increases by 3dB, the Dose doubles given the same exposure time. Similarly, if the sound level decreases by 3dB, the Dose will halve for the same exposure time. This number is called the exchange rate.
Doubling the exposure time doubles the Dose, and halving the exposure time halves the Dose given the same sound level.
In some countries, the exchange rate is changed to 4,5 or sometimes even 6. This means that the percentage level will change more slowly in respect of a changing sound exposure level.
The following tables show the relationship between the exchange rate, the criterion level and the noise exposure limits
There is another element to % Dose, which is the cut-off point at which the dose is accumulated. This would normally be set to 0, -5 or -10 and means the level below the 100% point where sound is counted towards the total %Dose. If the threshold is set to -10 and the criterion (100% level) is 85dB, the any sound below 75dB will not be added to the percentage dose figure.
The operation of Dose can be the mathematically described by the equation below, which relates the percentage exposure D to the integrated sound power.
Mathematical description of %Dose:
With respect to Lc, criterion levels of 90 and 85 dB(A) are most often used.
For discrete time intervals at a constant sound level, the equation above, “Mathematical description of %Dose”, can be rewritten as:
Calculate the daily (8 hr) noise exposure of a machine operator where the noise levels are constant at i)90 dB(A) and ii)100 dB(A). Use an exchange rate of 3dB and a criterion level of 90 dB(A).
Since the sound level is constant throughout the day, i.e. 90 dB(A) Equation 14: Dose Calculation from discreet time intervals, reduces to:
The Dose can be converted to an Leq by the following formula.
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