Regulations, equipment and guidelines on use
With the Control of Noise at Work Regulations 2005, many more employers will be looking to perform noise measurements because of the reduced action levels. This article reviews the different instruments that can be used for noise measurements within the workplace and offers guidelines on how to use them correctly.
Before undertaking any noise survey within the workplace it is important to understand what needs to be measured for the relevant action levels to ensure the correct results are taken, so firstly let us examine what needs to be measured.
The action levels for noise within the workplace have been reduced compared to the previous regulations. The action levels for noise come in two forms. Firstly, those based on an average exposure over a working day. This is to protect employees from damage to hearing over their working lives. This means making a measurement of the average exposure to noise over a working day. Secondly, instantaneous damage 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. To assess this we must measure this impulsive noise, if you believe there are likely to be any present with the workplace. The action levels are summarised in the table below.
On Sound Level Meters the settings of the measurement instrument needs to be different depending on which type of noise is to be assessed. The sound pressure level (L) in dB (decibel) needs to be weighted according to how the ear hears the noise. A microphone hears noise at different frequencies fairly evenly, whereas the ear does not. Hence a Sound Level Meter ‘weights’ the frequencies to represent this as illustrated in Figure 1. The ‘A’ weighting represents how the ear responds 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.
Old Sound Level Meters or very simple units 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 3dB. A 3dB increase is a 100% increase in noise energy, meaning that large errors can be made when assessing noise dose.
|8 Hour Dose (L AEP,d )||Peak Noise (L Cpeak )|
|Lower exposure action values (LEAV)||80dB||135dB|
|Upper exposure action values (UEAV)||85dB||137dB|
|Exposure Limit Value (ELV)||87dB||140dB|
This is why modern Sound Level Meters have an averaging feature called L eq . This is called the equivalent continuous sound energy level and is illustrated in Figure 2. As this value uses the ‘A’ weighting, this is normally displayed as L Aeq when written down. Figure 2 shows that if noise is fluctuating (yellow line) an instrument that measures the L Aeq would give a value as illustrated by the black line. Note that because this is a logarithmic average, the line is towards the top of the fluctuations, because then the average best represents that most of the noise energy is held within the higher points of the variations.
In order 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 L AEP,d , the ‘daily personal noise exposure’. In order to take the measured L Aeq values to calculate the LAEP,d , other considerations need to be taken. When measuring the L Aeq it is important to measure for a period of time that is representative of the employees’ exposure and what is the exposure time for that noise. If an employee performs different jobs (and hence differing noise levels) it will be necessary to repeat this for each different job noting the exposure time for each. Within the Control of Noise at Work Regulations 2005 a very simple to use ‘Exposure Point’ system has been introduced which will enable the calculation of L AEP,d from the L Aeq values and exposure times without the need for complex calculations.
If people are likely to be exposed to high levels of impulsive noise, such as noise emitted from heavy pressing operations or sheet metal working, then the peak noise must be measured. It is important to note that on a noise monitor the ‘peak’ and the ‘maximum’ are two different things. Essentially the 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, with the L Cpeak is often incorrect. Modern instruments can measure both the Land the Lsimultaneously, as illustrated by Figure 3.shortened to L Cpk . Measurements of the maximum level would be Aeq Cpeak
Sound level meter and dosimeter
When performing a noise survey, an accurate representation of a person’s exposure to noise with a Sound Level Meter or Dosimeter is required. A Sound Level Meter is a hand-held device, which enables measurements to be taken at the ear (within 10-15cm) with the instrument pointing at the noise source, measuring both ears. By repeating this exercise for all the operations that an employee performs during the day, it is then possible to calculate his or her daily exposure (L AEP,d ).
It is always important to inform the employee so you do not surprise them by holding something close to the ear without their prior knowledge. However, it may not be possible to do this in certain situations (e.g. fork lift truck drivers) or for individuals that have a complex work pattern and are therefore exposed to many different noise levels. In this situation it is best to use a Noise Dosimeter that is worn on the person, preferably for their entire shift.
A Sound Level Meter is primarily designed as a hand held device, whereas the Noise Dosimeter is worn by an employee for his or her working shift. The question is, which one is best for your application? A common misconception is that if you have to measure noise dose, you use a Dosimeter. In fact the preferred method of measurement for noise surveys is to use a Sound Level Meter. There are several reasons for this, primarily because when performing a noise survey with a Sound Level Meter the operator is present and measuring the noise of the employee. This ensures there are good quality measurements that are repeatable.
With a Sound Level Meter, a representative measurement is made for each job function and the exposure time for each, and from there the 8 hour exposure can be calculated. The more complex the work pattern of an employee becomes, the difficulty to assess the noise dose using a Sound Level Meter is increased because more and more measurements would need to be taken in order to calculate noise dose. It may at times not be possible to measure the noise with a Sound Level Meter because it is not practical or safe, mainly for people such as fork lift truck drivers or individuals with complex work patterns. This is when a Noise Dosimeter is the best way of getting a noise dose.
Where the selection of hearing protection is required it may be necessary to measure the frequencies of the noise that are present (octave bands) to ensure the hearing protection is giving the adequate attenuation.
A Noise Dosimeter consists of a microphone on a cable, which can be clipped to the lapel or collar. The microphone cable is then passed under the clothing to the unit itself, which is small enough to be located in a pocket or clipped to a belt. The Dosimeter can then be started at the beginning of the shift. If it runs until the end of the working day, then the L AEP,d can be directly read from the instrument or downloaded without the need for calculations.
Another useful feature of Noise Dosimeters is that they will ‘log’ the noise data so that, when downloaded to a PC, the time history of the noise can be viewed, as illustrated in Figure 4. This gives the ability to analyse when and where high noise exposures occur. This can be even more useful when the Dosimeter can be placed on an employee who is prepared to make a diary of what times and jobs he or she was performing throughout the day. This will give the employer the ability to directly see which operations most need noise control in order to reduce noise exposure.
When it is applicable to use a Noise Dosimeter, it is important to know the pitfalls in order to get reasonable results. The inherent the frequencies of noise that a person is exposed to using an octave band Sound Level Meter. A calculator to then determine how effective the hearing protection provided can be found on the HSE website: http://www.hse.gov.uk/noise/calculator.htm. This also has noise exposure calculators, which you may find useful.
A noise survey involves much more than taking measurements. Noise exposure is as much about how long the employee is exposed to the noise as the noise level itself. Therefore, it is just as important to look and ask questions of what employees are doing and how long for, as well as their managers, in order to get an accurate idea of an employees’ exposure time. However, performing a good noise survey is as much about using your eyes and ears and knowing employees’ work patterns and responsibilities as it is about using the instrument.
For example, someone whose job role it is to drill holes in metal plates may be performing that job role all day. So if you ask him or her ‘How long every day to you spend doing this job?’ you are likely to get the response ‘All day’ from the employee. Hence, if you used the noise level of when the drill is on in your exposure calculation then an incorrect dose would be calculated. What needs to be done instead is to look at the operation as a whole and analyse how long the drill spends on and off. By measuring the noise with the drill off as well as on, you will be able to use both these values and times, together with their quiet times during breaks to achieve an accurate dose calculation. It is most important to remember that the LAEP,d is a personal value for each employee, not an area value. Just measuring noise at points around a site is inadequate.
The new Control of Noise at Work Regulations 2005 calls for the use of ‘expert intermediaries’ meaning an employer should rely on competent third parties where they themselves are not competent for the relevant aspect of noise in the workplace. However, it will no longer be the case that an employer needs a ‘competent person’ certificate to perform noise surveys. This means with some knowledge and understanding of noise measurements and terminology it is possible to achieve a good quality noise survey without the need to rely on a consultant. ?
Tim Turney is Product Manager for Noise Instrumentation at Casella CEL and since graduating as an engineer from Queen Mary and Westfield in London, he has been involved in the acoustics industry. Tim deals regularly with all aspects of industry for ‘Noise at Work’ and is an affiliate member of the Institute of Acoustics.
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Published: 10th Apr 2006 in Health and Safety International