Covering multiple shifts in noise assessments
This is related to the issue of time weighting and how long a measurement needs to be done for. There is often a train of thought in many people looking at having noise assessments done that they have to have assessments over all their differing shifts. I understand where this comes from but it is not always true.
The problem for many employers arises as they don't have a nice simple day shift and then everyone goes home, but instead they are running 24 hours a day, usually with one shift on 6am-2pm, then 2pm to 10pm, and then 10pm to 6am. Other employers may run a rotating continental shift so one group of people working 6am to 6pm and the second group working 6pm to 6am, and often this is on a four day rotating pattern. Add to that short shifts for some people, and permanent nights for those who just prefer it, and it all gets very messy very quickly.
Whether measurements are needed on multiple shifts is influenced by the type of work so here are a number of scenarios and the noise measurement recommendation.
Scenario 1 - shifts all do the same stuff.
If the night shift are doing the same job as the day shift, with the same number of machines running and doing the same work, and the noise from the machines is fairly fixed rather than being influenced by the operator then it is fine, measure on days only.
Examples of this would be CNC operators, workers in big food factories, people on production lines, bottling plants, etc.
Scenario 2 - night shifts are lighter than day shifts
If the night shift uses fewer machines, or has fewer people in, or maybe is predominantly a cleaning shift with a small volume of production work going on only, then it should be treated separately and separate measurements made. Don't forget, see the duration of noise measurements page as that doesn't mean measuring for the full night shift, just long enough to get measurements or confirm the differences, if any, to the day shift.
Examples of this would be large factories where say 500 people are on the day shift and then only 200 working on a reduced night shift. As their noise generation is likely to be different some measurements should be made to see what the changes are.
Scenario 3 - noise generation is very individual-based
The previous two both deal with employees using fixed machinery or doing the same job as other people but in a type of work where how individual operators act has no impact on their noise generation. But what about those where it does, for example joiners, welders, etc. In these cases the individual involved can have a large influence on the noise they generator or expose others to so separate measurements are needed.
Examples would be large joinery workshops, maybe metal fabrication shops with a lot of hand-tool use, ship building, etc.
Don't forget though, if the shifts are rotating, in this scenario you can always catch everyone when they rotate onto days and that is fine.
Summary for measuring noise in shifts
In a nutshell, big factories are likely to be OK with day-shift-only measurements. Smaller night shifts should have some basic verification checks done to see what the differences are, if any, while sites where noise is very much individual based probably should be measured on all shifts.
As a small diversion, even if you decide specific noise measurements are not needed for a night shift, the employer should still check that night shift workers are complying with things like use of hearing protection. Why flag this up? From experience night shift people can be right buggers with thing like hearing protection - all the senior managers are home tucked up in bed and over time I have seen that compliance with their use often drops on night shifts. As part of a noise assessment also includes looking how well hearing protection is being used, this part still needs to be done but can easily be done in-house.