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With an appropriate selection and use of hearing protectors, employees can do their work in noisy areas without danger of hearing damage. According to the EC Noise at Work Directive (2003/10/EC), hearing protectors have to reduce the noise exposure level at the ear to 87 dB(A) with respect to an eight hour shift.
That means that the sound attenuation of the used hearing protector must be high enough for specific working conditions. The determination of the daily noise exposure level according to the EC directive is the basis for the selection.
For the selection of the hearing protectors it is important that:
- Different types have different sound attenuation
- Real world sound attenuation of a hearing protector (in practical use) is lower, as determined within the EC type examination testing
Both these factors are first topics that have to be considered in the selection of hearing protectors.
On the other hand, the sound attenuation should not be too high. Sound isolation or missing of warning signals is possible.
The frequency response of the hearing protectors is the next criterion that has to be considered.
It is important to hear warning signals and to have sufficient speech intelligibility, but also to be able to recognise machine sound. If the selection fulfills these boundary conditions, it is possible that the hearing conditions at the workplace are better by using hearing protectors than without.
The last point is the acceptance of use and the influence of the wearing time. The hearing protector should be selected with a view of the ergonomics of use (e.g. head band force).
The correlation of short non-using time of the hearing protector during a working shift and the rapidly increasing danger of a hearing damage is the final topic of prevention by use of hearing protectors.
Regulations
The EC Noise at Work Directive (2003/10/EC) contains two lower and two upper action levels.
The intention of action levels is to encourage the persons in charge to take appropriate measures, like technical noise reduction, sound isolation or medical surveillance.
In addition, there are two exposure limit values within the noise directive. If the exposure action values are exceeded different actions should be taken.
If the lower exposure action values are exceeded, medical surveillance, controlling and information are required. If the upper exposure action values are exceeded, the employer should establish and implement a programme of technical and organisational measures intended?to reduce the exposure to noise.
If all technical and organisational methods are exclusive of sufficient results, HPDs have to be used by the individuals. In that case the appropriate selection and use of HPD is necessary to protect the workers. HPDs should be available to workers up from the lower action values and should be used when the upper action values are exceeded.
When applying the exposure limit values, the determination of the worker’s effective exposure should take account of the attenuation provided by the individual hearing protectors worn by the worker. As a result of this fixing, the selection of an appropriate HPD is crucial for the compliance with the directive.
With an appropriate selection of hearing protectors the employees can continue their work in noisy areas without the danger of hearing damage. According to the Directive (2003/10/EC), hearing protectors have to reduce the noise exposure level at the ear to less than 87 dB(A) with respect to an eight hour shift. That means that the sound attenuation of the used each hearing protector must be high enough for the specific working conditions. The determination of the daily noise exposure level according to the EC directive is the basis for the selection.
Selection of HPD
Determination of the sound exposure
For selecting an appropriate HPD it is necessary to determine the sound exposure. A good way is to use a sound level meter to measure the equivalent continuous level LAeq (the ‘A’-frequency weighted average sound pressure level over the measurement period), and the peak sound pressure exposure Lpeak (the ‘C’-frequency weighted instantaneous noise pressure). The microphone should be positioned near to the ear of the worker, so the daily noise exposure level (LEX,8h), the maximum peak sound pressure for compliance with the lower and upper exposure action values, and the exposure limit values can be determined.
Selection of the appropriate type of HPD
According to results of the measurements an appropriate HPD is to be selected. That requires choosing the type and the sound attenuation of the HPD.
Selection of the type
Ear-muffs and banded ear-plugs are especially recommended in cases where the HPD must be taken off frequently due to repeated short term noise exposure, in cases where the user has narrow ear canals and in cases of inflammation of the ear canal.
Ear-plugs are recommended at workplaces with continuous noise, in case of extreme sweating under ear-muffs, in case of simultaneous wearing of spectacles, goggles or if other personal protective equipment is worn.
Banded ear-plugs are suitable in changing and not very high noise situations, where they must be worn and taken off frequently. They shouldn’t be worn if sound pressure peaks can occur due to contact of the band – at a welder’s screen, for example.
Custom moulded ear-plugs are individually fitted, offer a high level of wearer’s comfort and a high level of safety if the function (fitting) is checked when the plugs are delivered, and after that at regular intervals. They are often used if the worker has the beginnings of hearing damage, to ensure a reliable sound attenuation.
Ear-muffs and ear-plugs with electro-acoustical equipment (level dependent HPD) are good to use in intermittent noise situations in the workplace if a good speech perception is required. Communication ear-muffs or ear-plugs can promote or allow communication in noisy areas.
Ear-muffs with integrated entertainment facilities like FM radio are suitable particularly for workplaces with monotonous tasks in noisy areas. A check for the intelligibility of warning signals at the work place is necessary in each case.
Acoustical selection 1. Sound attenuation
At first the acoustical selection criteria have the aim to comply with the regulations of the EU Directive 2003/10/EC for the daily sound exposure level and the peak sound pressure. From the ergonomic point of view (e.g. feeling of isolation) and considering the importance of hearing warning signals and informative sounds, the sound attenuation should not be higher than necessary. The sound attenuation of a HPD is good if the sound level effective to the ear is between 75 and 80 dB (A) and is acceptable between 80 and 85, or between 70 and 75 dB(A) (see Tab.1).
For sound pressure levels beneath 70 dB(A), the sound attenuation may be too high and speech intelligibility could be hindered, so please check for communication and acoustical isolation. That is called an overprotection due to the HPD.
It is possible to calculate the sound level at the ear. If this is calculated with the information given on the packaging, it is necessary to take into account that this information is based on a laboratory testing. A check for exceeding the exposure limit values (ELV) of the EU Directive 2003/10/EC is easily possible with the following method:
The calculated sound pressure level at the ear should be:
LEX,8h – M/L-value of the HPD < ELV for high/middle (M-value) or low frequency (L-value) noise at the workplace.
The real world sound attenuation is mostly lower, which means that the protection effect will be overestimated. So it is necessary to take into account the practical situation, which is possible with derating factor (KS). So the calculated sound pressure level at the ear should be:
LEX,8h – M/L-Value + KS < ELV
In Germany the following derating factors (KS) are used:
- Ear-plugs formable Ks = 9 dB
- Ear-plugs pre-formed Ks = 5 dB
- Banded ear-plugs Ks = 5 dB
- Ear-muffs Ks = 5 dB
- Custom moulded ear-plugs Ks = 6 dB
- Custom moulded ear-plugs with fitting check Ks = 3 dB
- Combination of ear-muffs with formable ear-plugs Ks = 9 dB
In principle, it is also possible to determine the sound level beneath the HPD with measuring methods for the individual. Measuring systems are under development, or will shortly be on the market. But the transfer functions of these systems are still unknown, so that the comparison with the ELV is difficult.
2. Frequency response and speech intelligibility
Beside the sound attenuation the frequency response of HPD is the most important factor.
In the majority of cases the working sound is structured in a way that HPD with nearly constant sound attenuation in the dominating frequency range (between 125 Hz to 4 kHz) will be the best solution (HPD with flat attenuation response, Fig. 1). A good criterion to judge the suitability of a HPD for communication is the slope of the attenuation values. In Germany there are two criteria used, depending on the target group of users. Persons with normal hearing have good speech intelligibility with a slope of the used HPD not larger than 3.6 dB/octave. For persons with hearing damage that may not be flat enough. If they need speech intelligibility while wearing an HPD, a smaller slope of sound attenuation (e.g. 2 dB/octave) is an appropriate selection criterion.
Only in cases where the machine has a high frequency dominated sound is it useful to select a HPD with strong rise of sound attenuation with the increasing frequencies.
3. Warning signals and HPD
It is known that hearing protectors can decrease perception or diminish acoustic perception, which is especially important for acoustic warning signals. That can create a dangerous situation e.g. with fork lifters or trucks in work traffic or in public road traffic.
The reason is that the majority of HPD change the spectral composition of the transmitted sound. Therefore, in areas where signal audibility is important, only hearing protectors with a suitable attenuation value and frequency curve should be used. In Germany, a listing of appropriate HPD on the basis of an IFA database is published, that can be used in the selection process of HPD with respect to warning signals. Another way in Germany is to use a PC programme for selection based on EN 458. Because the use of HPD in public transport on the road is generally not allowed, a special quality criterion has been developed. The HPD which fulfil the requirements of these methods are listed, and marked with the sign ‘V’ for warning signals in road traffic. An additional sign ‘W’ exists for good signal audibility in general, which means informative operating sound, speech intelligibility and general warning signals.
4. Machinery sound
Another question is the problem of perception of informative operating sound, which in the main concerns information on machinery parameters.
In cases where informative operating sound is important, hearing protectors should be fitted to the frequency spectra and sound pressure level of the machinery the operator needs to recognise. For these purposes, in each case (independent from the frequency spectra of the machinery) it is important to choose a HPD with flat frequency response.
Ergonomics and acceptance
Environmental criteria for selection
In addition to these selection criteria, the environmental influence has to be considered for selecting an appropriate HPD. This means that different factors like temperature, high dust exposure, substances at work, moving machinery parts, vibrations in the workplace, exposure to continuous noise or repeated short term noise, informative sound of the working process, localisation of the noise source and the daily wearing time of the HPD – or the personal incompatibility of the user – should affect the selection of the HPD type before a specific model is chosen. Table 2 summarises the environmental factors important for the selection and use. It gives advice for the selection of HPD with respect to criteria like very high temperatures, high dust exposure and short term noise exposure.
- Appropriate with sweat absorbing ear-muffs cushions
- Ear-plugs without grip should be inserted only with clean hands
- Dust can accumulate at the HPD and can irritate the skin. (Typical activities with high dust exposure: grinding in containers or fettling)
– Generally not suitable
+ Generally suitable
+/- In particular cases suitable or not
Acceptance
The acceptance of HPD covers aspects of the hearing protector type, the participation of the wearer in the selection of the HPD and objective criteria like sweating, and pressure on the ear canal or the skull.
It is well known that custom molded ear-plugs are seen as being objects of private ownership, and are therefore often used more carefully. Ear-muffs are often experienced as too big and unwieldy in comparison with ear-plugs. Sweating in hot environments reduces their acceptance. In general an ear-muff causes a stronger sweating effect than an ear-plug does. Within the group of ear-plugs, devices with rough and porous surfaces have lower sweating effects. They are mostly better accepted.
Fig. 2 shows results from a study that demonstrates the described effect.
Correct use
The worker as the user of HPD has an important influence on effectiveness of the protection effect. He can use the HPD or not. And he can put on ear-muff or insert an ear-plug with a wanted leakage, to the aim of better communication or ventilation, or he can use it in a correct way. The manner to use the HPD with leakage has the effect of lowering the sound attenuation of the HPD. Beside these wanted reductions of the sound attenuation, which is a misuse of the HPD, there are additional reasons for a lower sound attenuation in its practical use.
For the different hearing protector types, the following items have been found to be the main causes of reduced sound attenuation in the field (differing from that attained in the type examination).
For ear-muffs we know, for example, that the band can be aged, the sealing cushions can aged or damaged, or the user wears glasses or protective goggles. Earplugs often fail to fill the ear canal, an incorrect size is selected or, after inserting the earplugs into the ear canal, the user does not position them sufficiently far enough into the ear with their finger to allow them to expand fully. (Fig. 3).
A special problem is known for custom moulded earplugs. The new product is individually produced and can exhibit a leakage, or, when used over long periods of time, the ear canal can widen. Therefore leakage tests are necessary. Particular problems arise with earplugs which must be shaped prior to use. If they are not properly rolled and compressed, they cannot be inserted sufficiently far enough into the ear canal, and may then slip out again, partly or completely, with the result that the sound-attenuation level measured in the type examination is not attained.2
Another important factor for the correct usage of HPD is a time-related problem, when the HPD is not used during the whole working shift. In situations where communication is required, the wearer often takes off the HPD for some time. The relation between wearing time and risk of hearing damage is not linear, but logarithmic, which is mostly not known. As a result of this the hearing damaging effect of a short, non-usage time is underestimated. For example, in the case of a noise exposure of LEX,8h equal to 105 dB in the workplace, a hearing protector which gives an attenuation of 30 dB is used. If it is worn for the entire eight hours, the noise level effective to the ear is L’EX,8h = 75 dB. If the hearing protector is not used for 30 minutes out of the eight hour day, the L’EX,8h = 93 dB. As result, in this situation despite the use of hearing protection, there is the risk of noise-induced hearing loss.
The most important influence is short term non-usage (e.g. nearly 30 minutes due to phone calls). It reduces the effective protection of the HPD quickly from 30 dB to between 15 and 10 dB.
Qualified use by qualified information and training
The consequence of non-use of HPD is an increase in hearing damage, despite the use of HPD. It is difficult to detect the situations of non-usage, or the lifting of the ear caps. The best way is to educate the workers about the influence of a small leakage of the HPD while wearing and the strong influence of short times with no usage of the HPD and train them in the right way to insert ear plugs. Information, special instructions and training should be given prior to the first use and afterwards repeatedly. They should include practical exercises like shown in Figure 3.
References:
1 P. Sickert, Sicher ist sicher-Arbeitsschutz aktuell 10-2009 2 BG-rule 194 Use of hearing protectors
Author:
Peter Sickert Berufsgenossenschaft Metall Nord Süd
Peter Sickert studied physics and received his diploma degree from the University of Halle. Since 1990 he is employed at the German Statutory Accident Insurance of the metal industry as a Technical Inspector and as specialist for noise at work. He is head of technical board for hearing protection of the German Social Accident Insurance Institution for the public sector and in this function member of the national expert committee for “Personal Protective Equipment”. He is head of the German standardisation committee ‘Hearing Protection’ (DIN) as well as member and convener of a working group of the European Technical Committee TC 159 ‘Hearing Protection’ (CEN). For 15 years he has a lectureship at the University of Applied Sciences ‘Wuerzburg – Schweinfurt’ in the department of Mechanical Engineering.
Berufsgenossenschaft Metall Nord Süd Weinmarkt 9/11, 90403 Nuremberg, Germany
T: +49-911-2347-19129 E: [email protected]
Published: 10th Jul 2010 in Health and Safety International
