Hand-held versus wearable meters
Well this is a right old can of noise-worms, but here goes. If you are new to this it may help to have a read through the Time Weighting page as that has some relevant information on how a daily exposure can be calculated from short-term hand-held measurements rather than wearable meters worn all day. It's good stuff, honest.
Noise meters generally come in two forms, hand-held and wearable. (I will ignore light-up signs and stuff for now as they're not overly useful in a noise assessment which is the subject thrilling you at the moment). I used dosebadges as wearables so tend to refer to all wearables as dosebadges but that will also apply to 'personal dosimeters' and anything else wearable. (Why do we use dosebadges particularly? Because there are no buttons for the wearer to fiddle with, no cables to get caught and generally fewer ways for the wearer to interfere with it).
Here is a clear unambiguous statement on how the two fit together, and the rest of the text afterwards is all waffle explaining this:
Wherever possible use a hand-held meter. Dosebadges and personal meters are mainly for situations where you physically cannot use a hand-held noise meter.
That's as clear as I can be - hand-held trumps personal dose badge every time and there are many good reasons for this. Dosebadges are beloved of noise assessment consultants though as you can stick 20 meters on people and disappear to the pub for a nice long leisurely lunch, then appear a couple of hours later and have 20 sets of lovely graphs to put in the report, which if it is one of those reports where the quality is measured by thickness and weight must only be a good thing.
There are at best only a few occasions where wearable noise meters are either the only way to get a measurement or are the best way of getting a measurement.
Reasons why hand-held is best
If you are doing a noise assessment and pop a wearable meter like a dosebadge on someone and send them on their merry way, the second they walk away you have lost control of that assessment. You have no idea if their mates are shouting down it (they inevitably will be) or whether it has moved slightly and their collar is now rubbing against the microphone which will cause a massive spike in the results.
Identifying noise sources
The potential interference is not always deliberate interference and once they walk away you have no idea what noise sources are causing any high exposures or any intermittent peaks. A noise assessment is not just about measuring a number of decibels but also seeing what can be done to control noise and if you don't know what caused it in the first place then identifying potential controls is impossible.
You also have no idea what job they were doing. They may supposed to be on a particular machine but you don't know if they've had a breakdown, gone off to help a mate out, moved to another machine for a bit, or whatever.
Basically, once they walk away all you get back is a dB(A) and dB(C) exposure pattern with little confidence in how it was obtained.
With a hand-held meter you know what caused the noises you are measuring and know that there was no interference, deliberate or accidental.
One of the esteemed and rarified beings who teaches Institute of Acoustics courses once told me that personal wearable meters should never be used for less than three weeks for any one measurement. Within that, you do one week of measurements and then throw those results away as the individuals concerned are certain to have fiddled with the meter or their mates pratted about with it, usually by shouting at it. Once those results are binned then use it for another two weeks to iron out any other errors from their work routine. He is a little extreme in his dislike of personal dose meters but the point stands, they are very unreliable.
Reasons why personal meters do have some use
So, that's pretty well established that dosebadges and similar equipment are a big pile of unreliable pants, so why use them at all? Well, they do have some positives:
A hand-held meter is all well and good, but not when you are measuring a highly mobile worker like a fork lift truck driver. I don't know about you, but I am more likely to sit in a corner and have a quiet heart attack than get a good result if asked to run about after a fork lift driver. Sticking a dose badge on mobile workers may have inherent negatives but it can be the only way of getting a good result that tracks them for a while.
Exposure patterns in noise assessments
Aside from mobile workers, the other main reason I use them is to identify patterns of noise exposure which I can then measure in more detail with a hand-held meter. My normal routine on a noise assessment is to look at the site and identify where looks noise and stick a load of dosebadges on people. I then get on with some hand-held measurements and then after an hour or so get the dosebadges back and look at the data. From that I can see who has had any particularly high exposures and then go and spend time measuring them with the hand-held meter, while using the dosebadges on other people. That way I use the personal results as a guide to where to focus the efforts with the hand-held meter but try not to always use the personal dose badge result as the only result.
So there you have it, hand-held is best. And if you are now thoroughly confused how a hand-held measurement can help when the Regs set limits for eight hour periods, have a gander at the Time Weighting page of the noise measurements section as that has a walk-through of how this works.