How accurate do noise assessment measurements need to be?
The HSE in their benevolence have been very carefully realistic in the wording within the Noise Regs about this. The Regs say:
"The employer shall have a suitable and sufficient assessment". This is then defined in the accompanying guidance as containing "a reliable estimate of your employees' noise exposures".
The regulation says specifically that the employer shall have a measurement of the level of noise to which his employees are likely to be exposed.
So the Regs don't say you will absolutely know to the precise decibel what the noise exposures are for every single employee in every single circumstance, but do say that they shall be a 'reasonable estimate', and that you know the noise to which employees are likely to be exposed.
This is a good common sense approach as exposures will change over time and no noise assessment is always going to be 100% accurate. Also, what people do on one day may vary just slightly on another day and that may have a small change in the actual noise exposure, but if the noise assessment is done correctly then this change won't be significant and therefore is perfectly fine.
If your measurements are slightly below the limits then treat them as exceeded
The HSE do say that because noise levels vary so much in many workplaces, that if a noise assessment comes in slightly below the 80 or 85 dB(A) limits, you should treat them as having met or exceeded them as the gap between the measurement and limit is well within the natural daily variation or accuracy of the measurement. (Guidance 5(2), paragraph 44).