|
Search By Monitoring Application:
|
Control of Noise at Work
|
back| 8 Hour Dose (LAEP'd) | Peak Noise (LCpeak) | |
| Lower exposure action values (LEAV) | 80dB | 135dB |
| Upper exposure action values (UEAV) | 85dB | 137dB |
| Exposure Limit Value (ELV) | 87dB | 140dB |
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.
Figure 1.
This is why modern Sound Level Meters have an averaging feature called Leq. 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 LAeq when written down. Figure 2 shows that if noise is fluctuating (yellow line) an instrument that measures the LAeq 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.
Figure 2.
An illustration of Leq
Figure 3.  |
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 LAEP,d, (known as LEX,8h in many EU countries) 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 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 LAEP,d from the LAeq 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 LCpeak is often shortened to LCpk. Measurements of the maximum level would be incorrect. Modern instruments can measure both the LAeq and the LCpeak simultaneously, as illustrated by Figure 3. |
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 (LAEP,d).
“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”
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.
Figure 4. 
|
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 LAEP,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 problem with a Dosimeter is that it is unlikely that you are going to be present to watch the employee and what they are doing. This means that they can be left to their own devices and the novelty of having a noise monitor is too much for many to resist. For example, people shout into it and try to press buttons etc. In order to try and get good data when you first get a Dosimeter you must firstly place it on a responsible employee and explain thoroughly the importance of the measurements. Secondly, ignore the first few days worth of data that you get, and get the Dosimeter out on site every day so that the novelty will wear off. By having a Noise Dosimeter where you can see the time history of the noise, it is possible to instantly see from the downloaded graphs if there are any ridiculous spikes of noise that you know the employee could not have been exposed to and check any results you get by going out with a Sound Level Meter to verify them. It is also possible for an employee to keep a diary of his or her working day, which can be correlated to any exposure graph. This will allow you to see exactly what activities are contributing the most to an employee’s exposure and therefore concentrate any noise control on these particular activities. A typical Sound Level Meter and Noise Dosimeter can be seen in the picture below (Figure 5 & 6).
“any sound level meter used should be manufactured to IEC 61672 and noise dosimeters should meet the requirements of IEC 61252”
Figure 5. 
|
All instruments have a minimum level of accuracy required for measurements. This is primarily down to the two types of microphone that can be used are Class 2, which is the minimum required, or Class 1, a more precise instrument. Note that ‘Class’ is also referred to as ‘Type’, in certain instrument standards but uses the same numbering system. Both Dosimeters and Sound Level Meters are manufactured to a specific standard. Any Sound Level Meter used should preferably be manufactured to IEC 61672, but the older standards of IEC 60804 and IEC 60651 are still acceptable. Noise Dosimeters should meet the requirements of IEC 61252. Any noise monitoring equipment should be field calibrated prior to use, as stipulated by the regulations. This means you should use a ‘field’ calibrator prior to use. An acoustic calibrator is illustrated in Figure 7. Field calibrators produce a noise signal, normally a tone of 1KHz at 114dB, allowing you to verify if the noise monitor is measuring the same level. It is also recommended to place the calibrator on the noise monitor after the measurement as well to ensure there has been no significant deviation. As part of the Noise Regulations, it is stated that the instrument and the acoustic calibrator needs to be returned to the manufacturer for a full calibration every 2 years. This ensures both the instrument and calibrator remain within specification. |
Windshields are provided with all noise measurement equipment. These should be used always, even if inside. They not only prevent wind affecting the results, but provide some protection for the microphone from dust and moisture as well as knocks. “noise exposure is as much about how long the employee is exposed to the noise as the noise level itself” If exposed to high levels of noise, it may be necessary to calculate the effectiveness of the hearing protection provided to employees. This involves matching the frequency (Hz) of noise an employee is exposed to, with a hearing protector that provides good attenuation at those frequencies. The recommended method for doing this by the HSE is by the octave band method. This involves measuring 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: www.hse.gov.uk/noise/calculator.htm. This also has noise exposure calculators, which you may find useful. |
Figure 6. 
|
Figure 7.
|
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. |
Conclusion
The 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.
Latest news
