Control of noise in the workplace

An employer must ensure that appropriate risk control measures are taken when noise levels in the workplace exceed an eight-hour noise level equivalent of 85 dB or peak noise at more than 140 dB. So what does it really mean?

Before undertaking any noise survey within the workplace, it’s important to understand what needs to be measured for the relevant action levels to ensure the correct results are taken, so firstly let’s examine what needs to be measured.

The levels for noise come in two forms:

• Average exposure over a working day that can damage employees’ hearing over their working lives. Measurements are made of the average exposure to noise over a working day.
• Instantaneous damage that can occur to hearing from high levels of impulsive noise, such as from heavy pressing operations. Such damage may cause ‘ringing’ in the ears, for example. Impulsive noise must be measured to assess this. A good rule of thumb is that if someone with normal hearing can’t hold a conversation at one-metre distance, then there is a potential noise level problem and action needs to be taken.

Measuring

The settings on sound level meters (SLMs) need to be different depending on what type of noise is to be assessed. The sound pressure level (L) in dB (decibel) needs to be weighted according to how ears hear the noise. A microphone hears noise at different frequencies fairly evenly, whereas the ear doesn’t. Hence an SLM ‘weights’ the frequencies to represent this. The ‘A’ weighting represents how ears respond to frequency at lower levels of noise. A similar effect is also seen at high frequencies. A curious feature of the ear is that as the dB level that the ear is exposed to increases, the ear hears low and high frequencies of noise easier, so the response curve flattens out. Hence, when we are measuring high levels to check for impulsive noise that can cause instant hearing damage, we use the ‘C’ weighting.

Older SLMs give only an instantaneous level of noise in dB, which makes it difficult to look at the value on the display and assess an average value. This problem is compounded by the fact that machine noise, which sounds ‘steady’ to the ear, can vary by 3 dB. A 3 dB increase is a 100% increase in noise energy, meaning that large errors can be made when assessing noise dose.

This is why modern SLMs have an averaging feature called Leq - the equivalent continuous-sound energy level. As this value uses the ‘A’ weighting, this is normally displayed as LAeq when written.

The levels of noise for differing effects on the ear are shown below:
 

To compare different industries and shift times in a way that the same action levels can be used, a noise exposure is standardised to an ‘8’ value called LAEP,d,. In Australia, we commonly use LAeq,8h - the ‘daily personal noise exposure’. In order to take the measured LAeq values to calculate the LAEP,d, other considerations need to be taken. When measuring the LAeq, it’s important to measure for a time period representative of the employees’ exposure and the exposure time for that noise. If an employee performs different jobs (and is exposed to differing noise levels), it will be necessary to repeat this for each job, noting the exposure time for each. AS 1269.1:1998 stipulates LAeq,8h as 85 dB and Lcpeak as 140 dB.

If people are likely to be exposed to high levels of impulsive noise, then the peak noise must be measured. It’s important to note that on a noise monitor, ‘peak’ and ‘maximum’ are two different things. Essentially, peak removes some time-averaging effects that are normally measured when reading the maximum, so the peak gives a true value for the highest level the noise has gone up to. The peak should be measured because maximum level measurements would be incorrect. Modern instruments can measure both the LAeq and the LCpeak simultaneously.

Part 2 will analyse the differences between sound level meters and noise dosimeters.