Measuring noise exposure

Simon Bull takes a look at the efficacy of using sound meters versus dosemeters for noise at work measurement and whether there is an argument for using both

To most people, a noise dosemeter and a sound meter are used for completely different applications, which can actually mean missing a trick or two.

The noise at work regulations guidance (Health and Safety Executive, 2005) says that you should use a hand-held sound meter to take short LAeq (Equivalent Continuous level - a bit like an average) measurements, which you can then use to calculate an employee's daily exposure (Lep'd) by combining the measurement from each process or task with the time that task is carried out for. This can be tricky with a calculator and pencil, but there's plenty of software that will do this for you once you have the numbers. The Health and Safety Executive (HSE) also has a useful calculator on its website that makes the job simple.

Alternatively, you can use a noise dosemeter to 'collect' the employee's noise exposure as they wear it. This can be worn for an entire shift, meaning that the number on the screen at the end of the shift is the exposure figure you can use to compare with the regulation action levels.

Dosemeters are normally used where an employee is mobile, such as forklift drivers or maintenance operatives, and where making hand-held measurements is difficult to say the least.

Discovering the differences There are, of course, problems with both of these methods. If you are using a handheld sound meter, you are only taking short measurements (it should be a minimum of at least five minutes wherever possible) and then you are estimating an eight-hour exposure figure, so the potential for error is quite large if you didn't capture the 'representative' noise levels. As for dosemeters, there can be problems with workers abusing the instruments and there are potential errors from reflections of the sound bounding off the wearer's clothing.

There are other differences that are not so obvious at first glance, according to Liz Brueck of Health and Safety laboratories (Brueck, 2012). In her 2012 study, she took multiple sets of sound meter and dosemeter readings and correlated them to determine potential differences. In the case of the LAeq readings, the dosemeter results were actually lower (not necessarily what you'd expect), which has been put down to wearers of dosemeters spending some of their time in quiet environments, whereas the sound meter operative only measured the higher events.

This shouldn't be a problem in itself as the calculation of exposure should take into account the time spent in the noise. There was also a smoothing effect, which effectively hides the detail of noisier sites or areas, so this should certainly be investigated with a sound meter.

The peak readings were the other way round, with the dosemeters reporting higher levels. The most likely cause for this is that dosemeter wearers are uncontrolled and could pick up unwanted sounds from accidental (or deliberate) tapping of the microphone, or proximity to disturbance such as air-line exhausts.

The study did find that general movement of clothing or activity by the wearer didn't significantly affect the results.

So what to do for the best? The first thing to look at is to make sure that your dosemeter readings are at least free from deliberate messing about (with a new 'toy' comes the temptation to play around with it). The best way to overcome this is to use it more! If you own a dosemeter, then it should really be in use - even if you don't note the readings all the time. With increased measurements you will increase the reliability.

Then you should consider the peaks you're getting. If the dosemeter is suggesting you don't have a problem, and bearing in mind that they tend to overreport, then you can be confident that you don't. If you're not sure, or if the results are on the borderline, then these should be checked with a hand-held sound meter.

In essence, when you look at the data, you really should be doing both; clearly, using a hand-held meter to estimate the exposure should give you a very similar result to using a dosemeter. If the results are completely different, then you may need to think again. I'm not suggesting you do this every time as you'd end up doing nothing but measuring noise.

Certainly, if you were to carry out this check once in a while, you would have a very good case for backing up the integrity of your measurements and putting the results that bit further from challenge.

Simon Bull is managing director of Castle Group, safety and environmental compliance specialists.