Printed on the packaging of a CE approved hearing protector, you may have seen a quantity known as the Single Number Rating (SNR) denoted in decibels and wondered “what is it, and how is it of any use”?

The SNR is a single attenuation value which is quite often sufficient enough and can be manipulated easily to estimate the protection provided by a hearing protector against exposure to noise. It is a convenient figure to use particularly if your sound level meter (which must be an integrating one) does not have the facility to take octave band readings, but incorporates a ‘C’ weighting filter.

From the octave band attenuation performance of the hearing protector (derived from a test defined in a BS EN standard), the SNR performance can be calculated (by a method prescribed using a BS EN ISO standard) and be published by the manufacturer.

To predict the noise level at the ear of a person exposed to noise and who is wearing hearing protection, you must first determine through measurement the equivalent continuous ‘C’ weighted sound pressure level of the offending noise. The result is then processed as follows:

L_Aeq’ = L_Ceq – SNR

where L_Aeq’ = predicted equivalent continuous ‘A’ weighted sound pressure level (in dB) at the ear with the hearing protector in place (this quantity allows you to determine whether the hearing protector provides adequate protection).

L_Ceq = the equivalent continuous ‘C’ weighted sound pressure level of the offending noise field (in dB) as measured by your sound level meter.

and SNR = Single Number Rating of the hearing protector (manufacturer’s data) (in dB)

The Health and Safety Executive (HSE) recommend that a ‘Real World’ factor of +4dB should be added to the predicted result due to factors such as poor fitting, the wearing of spectacles and interference by other forms of personal protective equipment or headwear. As a result, the noise level at the ear is predicted to be 4dB higher than the computed result.

E.g. If L_Ceq is measured as 108 dB, and the SNR of the hearing protector is 30dB, then L_Aeq’ would equate to 78dB (i.e. 108dB – 30dB). To account for the ‘Real World’ factors explained in the above paragraph, the noise level at the ear would increase by 4dB to 82dBA.

The HSE also recommend that the hearing protector should reduce the noise level at the ear to below 85dBA. In the case of the above example, the hearing protector of SNR 30dB provides adequate protection at 82dBA.

Note: The LCeq measured quantity is different to an LCpeak one also seen on sound level meters and should not be confused with one another. The latter is a ‘C’ weighted absolute peak measurement where high peak levels may present a risk to hearing from a physical damage point of view. The selection of appropriate hearing protection against such damage uses a prediction method that is different to the one described in this posting.

Although the method used to arrive at the noise level prediction is most accurate when using the manufacturer’s octave band attenuation data of the hearing protector, using the SNR figure to predict the noise level at the ear when wearing a hearing protector gives a close enough approximation in most cases. The exception to this is when the offending noise is rich in high and low frequency tonality, and caution must therefore be exercised when this situation arises. Under such circumstances it would be advisable to use the octave band attenuation data for the prediction, along with an octave band analysis of the sound source.

Still need help with this? Then Essel Acoustics has the skills, experience and qualifications to carry out a noise risk assessment which will include recommending the appropriate hearing protection for staff working in your environment, to fulfil regulatory requirements.

Click here to download a free guide in how to conduct a competent noise risk assessment (which also includes an article on hearing protector overprotection).

You have a situation where you believe employees are exposed to high noise levels, and without carrying out a proper noise risk assessment (if you do carry one out), to be extra cautious you go in overkill mode and issue staff with hearing protectors that provide a higher than necessary level of attenuation. Job done, you reckon.

Beware of what lurks! It might surprise you to realise that you may be overprotecting staff where the noise levels at the ear are suppressed to such an extent that it results in an inability to communicate between co-workers.

There is also the risk of warning signals (i.e. emergency sirens, vehicular alarms and even speech) not being audible enough which could compromise safety possibly endangering life and limb. With overprotection comes a sense of isolation, and workers are tempted to remove their hearing protection or wear them incorrectly thus making them vulnerable to the loud sounds which they were exposed to in the first place, risking hearing damage. So, in effect you are back to ‘square one.’

HSE recommends that with the wearing of hearing protectors, to avoid overprotection the noise level at the ear should not be less than 70 dBA. An ideal choice of hearing protector is one that reduces noise levels to between 70 dBA and 80 dBA at the ear.

Click here to receive your FREE guide in how overprotection may be recognised in your workplace and also what constitutes a good noise risk assessment.

The Assumed Protection Value (APV) of a hearing protector – what is it? How is it derived, and how does it relate to the ‘real world’?

You may have seen the APV printed on the packaging of hearing protectors and probably thought “what does this actually mean?”

The APV data is typically represented as shown in the table below:

The APV is the minimum attenuation protection provided by the hearing protector (in each of the octave frequency bands) for a percentage of the population. It is the APV that is used to predict the noise levels ‘in the ear’ with the hearing protector in place.

So, the noise level ‘in the ear’ (dB) = the noise level external to the hearing protector (dB) – APV(dB), in each of the octave frequency bands.

NB: to arrive at the overall noise level ‘in the ear’ (in dBA), for assessing the possibility of hearing damage, you’ll need to convert each of the spectral band noise levels into equivalent ‘A-weighted noise levels, and logarithmically sum them all up. That’s another exercise.

Deriving the APV

The APV is derived from the mean attenuation obtained of a hearing protector tested under controlled conditions in a certified laboratory. The test, in accordance with an EN352 standard, follows a subjective based method where the hearing threshold of subjects are measured with and without the hearing protection in place. The noise attenuation is simply the difference between these two measurements.

The test conditions include trained and motivated wearers (receiving supervision, if required) fitting the hearing protection in accordance with the manufacturer’s instructions. This test method provides the best assurance that the attenuation data obtained will be repeatable.

The test data is collected across a combination of a specified number of wearers and hearing protectors, and by processing this data, the mean value of the attenuation and the standard deviation (which depends on the spread of the data) may be calculated at each of the frequency bands in the above table for the hearing protector type under test. By definition, we have:

APV (dB) = mean attenuation (dB) – p x standard deviation (dB)

where p is a constant.

With p = 1, which is often chosen to be the case (in UK and Europe), statistically, 84% of the population (if tested under laboratory conditions) will be assumed to achieve a minimum attenuation which is the APV. Therefore 16% of the population will NOT achieve the minimum attenuation. So not everyone will be equally protected.

The statistical spread of test results in the laboratory is down to, amongst other things, anthropometric differences in head shapes between the test subjects and any manufacturer’s tolerances across the build of the hearing protectors tested.

APV and the ‘real world’

In the ‘real world’, however, user conditions are highly uncontrolled compared to a laboratory type environment, due to factors such as incorrect fitting, poor maintenance, the wearing of spectacles and jewelry, plus interference by other personal protective equipment. For these reasons, it’s highly likely that the manufacturers’ specified APV performance will suffer some degree of degradation. The HSE recommends that a derating factor of 4dB be applied to your calculations (i.e. add 4dB to the result) when predicting the noise level at the ear with the hearing protector in place.

Although it is highly unlikely that the APV performance of a hearing protector can be replicated in the ‘real world’, with the provision of instruction and training (which is a legal requirement in high risk noisy areas), staff can be taught how to properly wear, maintain and look after hearing protection. The outcome would more than likely maximise the effectiveness of the hearing protection and therefore minimise the chances of noise induced hearing damage.

Confused? Then leave it to us. Essel Acoustics has the skills, experience, and qualifications to correctly prescribe appropriate hearing protection. We can also provide training in noise awareness and instruction in the correct use of hearing protection at both, operator and supervisor/manager levels.

Click here to receive a free guide in what constitutes a good noise risk assessment. This guide also includes an article on hearing protector overprotection.

See www.esselacoustics.com

The above are just some examples of huge fines (excluding costs) imposed on businesses for failing to protect workers from over exposure to vibration emissions through extended use of power tools. This resulted in staff developing Hand Arm Vibration Syndrome (HAVS).

In many cases, there was evidence of the symptoms of HAVS being ignored and also insufficient information, instruction and training awareness of HAVS within the business, inadequate health surveillance, and either a poor or zero risk assessment conducted. Here are quotes from HSE inspectors regarding some of the above cases:

“All employers need to do the right thing to protect workers’ health.”

“This case shows there is no excuse for not putting in place a management system which includes risk assessment, control measures, health surveillance and information and training to reduce these risks to as low a level as is reasonably practicable.”

Some examples to consider of the further consequences of failure to protect staff are:

• absence costs
• decreased productivity
• increased insurance premiums
• …..and most importantly the HUMAN cost.

As an employer, where power tools associated with HAVS are used regularly, you are obliged to carry out a competent risk assessment to reduce the risks to as low as reasonably practicable (ALARP) and provide staff with training in awareness to the symptoms and causes of HAVS. This illness is entirely preventable providing it is detected and controlled at an early stage.

Essel Acoustics has the relevant skills, training, qualifications and experience to carry out competent risk assessments in accordance with industry best practice methods. With this in-mind, we have prepared a FREE booklet to help you to find out more about hand-arm vibration syndrome.

Download our FREE booklet here: A guide to hand-arm vibration syndrome