How do you calculate daily noise exposures?
From experience, this is one of areas of noise assessment which is a fairly consistent cause of confusion, and not just among those who are new to it. The issue arises because the 80 and 85 dB(A) limits are calculated over an exposure time of eight hours therefore if the limit is for eight hours then surely the measurement time needs to be eight hours as well, right?
The short answer: No, each noise measurement does not have to be for eight hours.
This is how you do it, involving 'time weighting' calculations:
Yes,, the noise assessment should come out with expected exposures over a full shift but that doesn't mean you have to actually measure the noise for eight hours, or for a full shift, and some maths can come into play to sort it out. Indeed, it is even preferable that you do not measure for a full shift as full shift measurements are done with wearable meters which should be viewed with the same degree of skepticism as Donald Trump saying he is moving to Mexico.
Example calculation of daily noise exposure - same job all day
To waffle on through this by means of an example. Dave is a chap who gets to spend his days operating a chop saw that is cutting aluminium bars all day long, one every 20 seconds or so, and our Dave does this for eight hours a day (he also gets a lunch break, but manfully operates his saw solidly for his eight working hours so we can ignore his lunch break). If we stand there and watch him work like a slightly weird stalker, his work is a 20 second cycle of:
move completed piece onto exit conveyor
place new piece in place
He repeats this all day. (Dave is very bored). For a noise assessment we can stand by him with a hand-held meter and after only a few minutes the average reading will have settled on a steady figure for this complete routine, say 90 dB(A). As it's not going to change any further it makes no difference if we stand there for just that time or all eight hours; the reading is still going to be 90 dB(A). So, we take our shorter reading and say ‘OK, he will do this for eight hours, therefore his daily exposure level is also 90 dB(A)’.
Example calculation of daily noise exposure - different jobs
That one is easy, but what about where Dave is a multi-skilled chap running about like a rabbit on speed operating his chop saw but also spends time at a punch press and also waves an angle grinder around with reckless abandon for a while? No problem, we can measure all three bits of his job separately and get, for example:
Chop saw 90 dB(A) which he tells us he does for about four hours a day
Punch press 94 dB(A) which he tells us he does for about three hours a day
Angle grinder 97 dB(A) which he says he does for about one hour a day
Asking Lovely Chap Dave about his average working day and how long he spends on each is fine for the noise assessment. Sure, it may vary slightly from day to day, but as long as we are close we are OK for the purposes of the noise assessment.
We now have a choice. If we really want to punish ourselves for some evil done in a past life we can manually start dong calculations using logs and anti-logs, or we can admit it is the 21st Century and use an online tool to do the calculation for us. My favoured one is at noisemeters.co.uk on their resources page. On there we can enter our three measurements and the time for each.
As you can see, that has given us an estimated LEP,d in red by each measurement but that is for each measurement on it's own so ignore that. Further down the page is this:
And there we have it, the magic number of 93.2 dB(A), or because a point of a dB(A) is fairly trivial, a result of 93 dB(A) for Dave's total daily exposure from operating all three bits of kit. That is the eight hour time weighted average, done via three short measurements of a few minutes each. and then calculating the overall exposure from that.
This means there is no need to measure Dave for a full shift as we will just get the same answer.
What the HSE say about not measuring for a full day
This isn't some naughty short-cut by the way and is how the HSE would expect you to do it. L108 even has an exposure ready-reckoner in it which follows the same principle.
This principle applies to pretty much all workplaces - airside workers at an airport have periods of high exposures to engine noise, along with periods of quiet, and exactly the same process can be used in the assessment. So, don't worry about measuring noise levels for eight hours or for full shifts as in many cases it's not needed - just measure each part of the job and you can easily calculate a time weighted average from that. In most cases, measuring for eight hours is a bit of a waste of time and added to that, wearable meters should not be trusted with anything like the same confidence as a shorter hand-held measurement.