Plugging the knowledge gap in noise attenuation
Hearing protection is an urgent priority across Europe and the USA, where legislative changes have increased the need for simple measuring technology and consistent hearing protection products. This article looks at the Law, and the formula for staying on it’s right side.
Ears are clever things. When subjected to constant noise for a short period of time the hearing adjusts, and the perceived level of noise reduces considerably. As a result a person might barely notice a 3dB increase in volume, even though a 3dB change doubles the sound energy being produced. Two pieces of machinery which each produce 85dB will produce 88dB when running together, and although perceived as only a small increase in volume this will double its propensity to damage hearing.
Control of noise at work Regulations 2005
Advice in the United Kingdom from the Health and Safety Executive on assessing workplace noise levels is that if you have to shout to be understood by someone two metres away, the ambient noise levels are at around 85dB. It advises that if the noise is intrusive but normal conversation possible, the level is likely to be 80dB. What is clear is that casual noise assessments are fraught with difficulty, not least of which being the body’s own accommodating mechanisms. Less accommodating are the new Control of Noise at Work Regulations 2005, which came into effect in the UK in 2006 and brought the Lower Exposure Action Value (LEAV) down to 80 dB.
The LEAV is now set at an 8-hour average noise exposure level (or daily personal noise exposure level, L EX,8h ) of 80 dB, and at this level the employer must provide information and training for their staff, and make hearing protection available; the value has been reduced by such an extent that a significant number of additional employees will need to wear hearing protection.
The Upper Exposure Action Value (UEAV) is set at a L EX,8h of 85 dB and above at this point the employer is required to take reasonable, practicable measures such as engineering controls or other technical or organisational measures to reduce noise exposure, and the use of hearing protection is mandatory if noise cannot be mitigated.
There are many considerations involved in compliance with the EU Physical Agents Noise Directive and the Control of Noise at Work Regulations which empower the Directive in the UK. From the initial assessment of noise levels using the HSE (Health & Safety Executive) guidelines, to the risk assessment and the planning of noise control measures, it is likely that at some point within the process it will be necessary to issue PPE (Personal Protective Equipment) to at least some employees.
The regulations are designed to protect workers from on-going noise that can progressively damage hearing over the course of their working lives, as well as from acoustic shock, or explosive noise, which can create instantaneous hearing damage. If risk assessment has shown that employees are likely to be exposed to noise above the second action levels, then measuring the noise exposure for these individuals is essential. Small errors in noise level estimates can lead to large errors in exposure calculations. These can lead in turn to hearing damage for workers and the risk of prosecution for employers, or the unnecessarily expense of undertaking noise reduction or exposure limiting measures on the basis of inaccurate data.
Essential assessment equipment
Two pieces of equipment essential for the assessment are the sound level meter, primarily designed as a hand held device used by an operator, and the noise dosimeter, which is worn by the staff member for his or her working shift. A common misconception is that if you have to measure noise doses you use a dosimeter, but in fact the preferred method of measurement for noise surveys is a sound level meter, which when an operator is present ensures that high quality measurements which can be replicated are taken.
Using a sound level meter, a representative measurement is made for each task and with the exposure time recorded for each the 8 hour exposure is calculated. The more complex the work pattern of an employee, carrying out different tasks in different locations, the more difficult and complex it is to calculate the dose using sound level meter readings. It may not be possible at all in some situations where individuals have extremely complex and diverse working patterns, in which case a noise dosimeter such as our CEL-350 dBadge is the best equipment for assessing a noise dose. The CEL-350 incorporates an exposure alarm that uses an ultra-bright LED to give an early visual indication if an individual will exceed the action values based on the current noise exposure.
Casella CEL has recently launched the 620 series of pocket sized meters which incorporate the high resolution colour display into one of the easiest to use sound level meters on the market. The Casella CEL-620 is now the smallest octave band analyser in the world and its colour readout is in a format that is impossible to misinterpret. Simplifying its operation was a key objective in the development of this instrument, and has resulted in a simple point and shoot tool for the production of full industrial noise assessments.
The all digital technology can measure up to 140dB and dismisses the need for range adjustment and reduces the risk of reading errors. It also measures all occupational noise parameters simultaneously, and so there is no instrument setup to change and you simply select the noise parameters you wish to see, knowing that all parameters can be viewed later even if not selected.
Whatever equipment is used for this task needs to be simple to operate and reliable. Expectations for electronic devices have been driven by products like the iPod and mobile phones, and consumers are comfortable with equipment that is small, simple to use and with clear colour displays. Casella’s new CEL-600 range of sound level meters fits this, and all three have an intuitive user interface with colour coded measurement parameters and displays. Simple icons replace a complex menu system and allow users to pick up the unit without using a manual. The product demonstrates that using a sound level meter need not be a specialist task.
If the assessment establishes that there is a problem and noise levels are likely to be at the Upper Exposure Action Values, personal hearing protection should be used immediately while other more permanent solutions are put in place. Personal hearing protection should be considered as the main solution only when all other options have been exhausted. Many things that can be done to reduce average daily doses, like physically separating staff from the noisiest areas or rotating jobs and shifts to spread individual exposures.
Measurement and calculation
The two main types of exposure to be measured or calculated are the L EX,8h , which is the daily noise exposure, and the L Cpeak , which addresses shock exposure. A brief summary of when hearing protection needs to be worn relative to these action levels is: Below the first action levels: Below an L EX,8h of 80dB(A) or L Cpeak of 135dB(C) hearing protection does not have to be worn. Between the first and second action levels: With an L EX,8h of between 80dB(A) and 85dB(A) and an L Cpeak of 135dB(C) and 137dB(C) hearing protection should be made available to employees who ask for them but it is not compulsory to wear them. Above the second action levels: With and L EX,8h over 85dB(A) or an L Cpeak of 137dB(C) employees must wear the hearing protection provided and employers will need to provide training in their correct use, until such time as noise at source can be reduced to an acceptable level.
Selecting the correct hearing protection
Selecting the correct hearing protection is essential and there are many factors to be considered, including the basic question: will the PPE be used worn! There is no point buying the most expensive earmuff that attenuates the noise by, say, 30dB(A) if the employee has to remove the muffs for two hours of an eight hour shift because they are uncomfortable. That will only reduce the daily noise exposure by 6dB(A) instead of 30dB(A)!
Many workers do not like to wear hearing protection because it is uncomfortable, although finding a comfortable hearing protector is largely common sense. As an example, earmuffs would generally not be supplied in a hot and humid environment, as they will make the ear sweat uncomfortably.
It can be a great bonus if employees are able to choose the type of PPE themselves from a suitable selection - a worker is more likely to wear the protection they have chosen as the most comfortable for them. Often employees working in the same environment will have different preferences: some prefer plugs, some prefer ear muffs and others will go for inserts.
The ultimate for ‘individualisation’ in hearing protection is custom-made earplugs made from silicone and moulded especially for the person’s ear. Although initially expensive, they have several advantages: they are more likely to give the required fit and therefore the required protection, and the employee is more likely to look after his or her ‘own’ unique plug.
The level of motivation and training an employee has in the use of their hearing protection has a considerable influence on the effective protection offered by the product. In the USA all hearing protection devices are currently sold with a Noise Reduction Rating (NRR) printed on the packaging, and for years safety professionals have enjoyed the apparent simplicity of a single number that can be used to differentiate products during the purchasing process.
From early next year the single number will now become a two number range; the higher number will indicate the amount of protection which about one in five motivated and trained wearers can attain or exceed, and the lower number will indicate the protection which the remaining four out of five users can expect. The range shows how consistently the hearing protectors provided protection within the same group. By subtracting the lower number from the higher one and comparing the difference to the same calculation with other products, the purchaser will get a good idea of the uniformity of protection given; the smaller the difference between the low and high figures the more consistent the efficiency of the products amongst workers.
The change in the US will bring about a new appreciation that the protection levels offered by a hearing protector varies, and no single figure can adequately describe the changing levels of protection offered between one wearer and another. It may show that selecting and specifying hearing protection might come down to issues that have in the past been considered secondary such as comfort and ease of use.
More than one form of PPE
The interaction of hearing protection with other PPE is another consideration: an employee wearing prescription or safety glasses will not obtain an adequate fit from a standard ear muff so plugs or perhaps semi-inserts become the better option. The use of hardhats with earmuffs can still be an issue. If the employee constantly needs to wear them both together a hard hat with built in hearing defenders should be considered.
Communication and attenuation
Communication can be a big question with PPE and it all comes down to the attenuation. If a protector with too little attenuation is used then not enough protection will be given. However, too much reduction of the noise can create a feeling of isolation that is detrimental, and an employee may need to remove their PPE in order to communicate. The subsequent exposure to noise will therefore far outweigh the benefits that a high attenuation hearing protector would have provided
Another problem with over-attenuation of a hearing protector is that of safety. There is an inherent danger in providing too much protection which cuts out the safety warnings of fire alarms and sirens from reversing vehicles. A general rule of thumb is to not providing so much protection as to reduce the level to below 75dB(A). Other individual preferences such as hair and jewellery affect the choice of hearing protection. Long hair that flows over the ears will cause an inadequate fit of an earmuff and hence a significant reduction in the effectiveness of the protection and earrings will cause a comfort problem.
The working environment also has a bearing on the choice of protector; hot humid conditions make earmuffs uncomfortable to wear and dusty environments cause problems with hygiene, especially with plugs. In this case it is important to keep the hands clean when inserting the plugs to avoid ear infection. It is advisable to ascertain from an employee any history of ear problems such as irritation or earache, in which case the use of earmuffs that fit over the outer ear is preferable.
Calculating the effectiveness of hearing protection
There are three ISO methods for predicting the overall attenuation that a hearing protector will give and each rises slightly in the amount of mathematics required, these are defined within ISO 4869. This is where we enter into the dreaded world of logarithms!
The methods for calculating the effectiveness of hearing protection are:
- SNR (Single Number Rating)
- HML (Standing for High, Medium, Low)
- Octave band method
In terms of accuracy in predicting the attenuation, the SNR method is the least accurate and the octave band is the most accurate, and usually preferred. However, it does require a little more calculation!
This data is for a standard disposable earplug. Protection is better at higher frequencies, which is the case with most hearing protection. The effectiveness of a protector at various frequencies varies between PPE, so it is necessary to match the frequency of the noise produced to the noise the employee is exposed to.
Generally, earmuffs provide better protection at higher frequencies above about 250Hz, but earplugs are on average slightly better at lower frequencies. All the three methods describe ‘frequency weightings’ of either ‘C’ or ‘A’. This is the way a sound level meter converts the decibel value it measures which is all the frequencies of the noise evenly, and converts them into a value to represent how the human ear hears the noise.
Essentially the ear does not hear low or high frequencies very well, but actually exaggerates noise roughly between 1000Hz to 4000Hz. The ear does this because most of our speech takes place at these frequencies. The ‘A’ weighted scale is shown below in the ‘Octave band method’ description.
|Any catalogue with hearing protection will list values corresponding to these three methods, which should look something like this:|
|H=31dB M=25dB L=22dB SNR=28dB|
|Frequency (Hz)||Mean Attenuation (dB)||Standard Deviation (dB)||Assumed Protection Value (APV in dB)|
The SNR method
The Single Number Rating (SNR) method is the simplest form of calculation but does not take into account the frequency content of the noise in any depth. To do this simply take the SNR value quoted for the hearing protector away from the ‘C’ weighted sound pressure level which needs to be measured for the employee in question.
The dB value at the ear (L A ’) is calculated by:
L A ’= L C – SNR
The value of L C is a measured value of the sound pressure level. It can be difficult to get an idea of this fluctuating value on the screen of a sound level meter so it is highly recommended that you get an instrument that provides an averaged result call Leq.
So if the L C was measured and found to be 105dB(C) our worked example would be:
L A ’= 105 – 28 = 77dB(A)
This value is rounded to the nearest whole number, which is the recommended way for decibel values once the calculation is complete. This results means this protector would be adequate if worn in the correct way.
The HML method
This method requires a sound level meter that will measure both the ‘A’ weighted and the ‘C’ weighted sound pressure level or average level (Leq). Take a ‘C’ weighted measurement of the sound pressure and an ‘A’ weighted measurement also. Then if:
L C – L A is less than or equal to than 2:
PNR = M – (H – L)/4 x (L C – L A –2)
The LC – LA is greater than 2:
PNR = M – (M – L)/8 x (L C – L A –2)
Where PNR is the predicted noise reduction provided by the hearing protector and H, M and L are the values provided by the PPE catalogue. The HML values stand for high, medium and low. This is therefore the value for the attenuation at high, medium and low frequencies.
There are two formulas because by taking two values of the sound pressure with different frequency weightings (LC and LA) then this is giving an indication of the frequencies of noise the employee is exposed to. That is why in the two formulas different values are used from the high, medium and low combination.
For example, for measured values of L C = 105dB(C) and an L A = 102dB(A), from our example earplug the results would be:
L C – L A is greater than 2 so:
PNR = 25 – (25 – 22)/8 x (105 – 102 –2) = 24.6dB = 25dB
This would make the value at the ear:
L A ’= 102 – 25 = 77dB(A)
The result is exactly the same as for the SNR method.
The Octave Band method
This method requires the use of an octave band sound level meter to take the measurements. To do the final calculations a calculator with the Log function is required. The use of this method is considered the most accurate way of measuring effectiveness of hearing protectors because it is looking at the actual frequencies of noise that are present for the employee.
The A-weighting factors are standard as shown above. By taking these values from each other will give the A-weighted value at the ear for each frequency. For example at the 63Hz octave band.
L A ’ = 92 – 13.8 – 26.2 = 52dB(A)
(see table below)
|The table shows an example measurement taken from a sound level meter for a different employee.|
|Octave band centre frequency (Hz)||63||125||250||500||1000||2000||4000||8000|
|Sound pressure level (dB)||92||93||95||95||96||98||96||94|
|So for the hearing protector in question:|
|Octave band centre frequency (Hz)||63||125||250||500||1000||2000||4000||8000|
|Assumed protection Value APV (dB)||13.8||16.5||20.2||23.1||23.8||31.0||35.9||34.5|
In order to add these together a formula is required to work out the overall SPL at the ear taking into account of the hearing protection.This formula is:
Sound Pressure Level (SPL) = 10 log (10 L1/10 + 10 L2/10 +....10 Ln/10 )
Simply insert the values for each frequency in to obtain the result:
L A ’= 10 log (10 5.2 +10 6.04 +10 6.62 +10 6.87 +10 7.22 +10 6.82 +10 6.11 +10 5.84 ) = 75.8dB(A) = 76dB(A)
Although this was for a different noise source than for the first two examples it still shows that the protector would be adequate. Under recent UK changes to legislation this protection is taken further in that the assumed protection is further reduced by 4dB under all three of the above methods. This would make the dB value at the ear 80dB(A) instead of 76dB(A). Hearing protection should be selected so that it reduces exposure to at least below 85dB(A) but ideally between 75 and 80dB(A).
There are many aspects to consider when implementing a plan for noise within the workplace, including noise control, training and audiometry. It must be remembered that hearing protection is a ‘band aid’ measure until it is possible to reduce the noise exposure to below the first action level by other means. However, with a little application of knowledge the effectiveness of hearing protection can be dramatically increased.
For more information on Hearing Protection visit http://www.osedirectory.com/product.php?type=health&product_id=6
Published: 10th Aug 2008 in Health and Safety Middle East