Noise assessments look at volume and duration

The noise regs talk specifically about daily noise exposures and while it can sound like splitting hairs to non-noise people, there is a difference between a ‘noise level’ and a ‘daily noise exposure level’.

A noise level could be say 92 dB(A) for someone using a tool - i.e. the tool generates 92 dB(A) of noise, but that may not be the daily exposure. The daily exposure is the noise level and also how long they are exposed to it in a day and that changes the result entirely.

Try my free daily noise exposure calculator to work out how exposure duration impacts the total noise level for a day in a noise risk assessment.

Noise risk assessment and exposure duration

The noise exposure limits used in a noise assessment are detailed in the Noise Regs and these word it very carefully as:

  • A daily or weekly lower limit of 80 dB(A) and 135 dB(C).

  • A daily or weekly upper limit of 85 dB(A) and 137 dB(C).

What they don’t say is there is an absolute noise limit of 85 dB(A) at work, the Regs say it is a daily or weekly limit of 85 dB(A) and that is done very deliberately.

The Regs do not say that if a machine is over 85 dB(A) then hearing protection is needed, along with hearing testing, etc. They say these things are needed if the daily noise exposure reaches that.

Impact of duration on noise risk assessment

Bear with me on this as it is not as complicated as it appears at first, but the way noise is measured is not entirely common-sense. A 3dB change in the noise level equals a doubling of the noise risk, so 88 dB(A) is twice as dangerous as 85 dB(A), even though the numbers are quite close.

What that also means therefore is that if something is twice as dangerous then you can only be exposed to it for half the time.

Think of it like this - the louder a noise is the faster you are receiving chunks of noise, and once your ears have dealt with all the chunks they can safely handle in a day any excess ones start to cause damage. You can have those chunks spread out over an entire day, or ram them all in within 30 minutes.

So for every 3dB noise levels go up the risk doubles meaning the amount of time you can safely be there halves.

The limit in a noise assessment is 85 dB(A) for eight hours. If you have 88 dB(A) then it is twice as dangerous so the safe time halves to four hours.

85 dB(A) for eight hours is the same number of chunks of noise as 88 dB(A) over four hours.

And on it goes, 91 dB(A) is safe for two hours or less, 94 dB(A) for one hour or less - you are still getting the same total amount of noise, just faster, so any excess damages hearing.

That is why exposure time is so critical in a noise assessment.

Example of the impact of exposure duration

Let’s say a machine generates a noise level of 90 dB(A), that is how loud it is, but that is not the daily exposure and for that we also need to know how long the person is exposed to it.

If someone is exposed to 90 dB(A) for one hour and has no other significant noise in the day then their average daily exposure is 81 dB(A). That actually means they do not meet the 85 dB(A) daily exposure limit in the Noise Regs so don’t need mandatory hearing protection or hearing testing.

Conversely, if that same person is exposed to the 90 dB(A) noise for six hours in a day, their daily noise exposure is then 89 dB(A), well over the limits so hearing protection must be worn and hearing testing is needed.

Same noise level at source, entirely different daily exposure levels.

You can test this out for yourself on our Noise Calculator, simply enter a noise level and the exposure duration and it will calculate the daily noise exposure. Leave the noise level the same but change the exposure duration and the daily noise exposure level changes.

The daily exposure limits in a noise assessment

All these are the same total dose of noise.

  • A noise level of 85 dB(A) for eight hours (how the Regs define it)

  • A noise level of 88 dB(A) for four hours

  • A noise level of 91 dB(A) for two hours

  • A noise level of 94 dB(A) for one hour

  • A noise level of 97 dB(A) for 30 minutes

  • A noise level of 100 dB(A) for 15 minutes

  • A noise level of 103 dB(A) for 7.5 minutes

  • A noise level of 106 dB(A) for 3.75 minutes

dB(A) vs dB(C)

All these talk in terms of dB(A) which is what the Noise Regs set as the 85 dB(A) limit. There is also the limit of 137 dB(C) - exposure time doesn’t matter there, a one-off event is powerful enough to damage hearing.