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Exposure to excessive noise is one of the main causes of permanent hearing loss worldwide, and a significant occupational hazard.
Andy Todd, acoustic engineer at SATRA, explains why occupational noise assessments and fully tested and certified hearing protection products are a must.
According to the World Health Organization (WHO), noise-induced hearing loss (NIHL) is the most common, permanent occupational injury in the world, yet it can be prevented. The boom in recent decades in Middle Eastern construction and petrochemical industries – where noise can be a significant hazard – means that many workers are at risk of damaging their hearing unless adequate preventative measures and awareness are in place.
While legislation on noise in the workplace has been in place in many Middle Eastern countries for decades, it was not universally enforced. Today it is recognised as a serious cause for concern. For example, in 2010 the United Arab Emirates’ Environment Agency commissioned a study to estimate the burden of disease attributable to exposure to noise – part of an effort to strengthen occupational safety and health programmes in the country (American Journal of Industrial Medicine, 2012).
Measuring noise levels in the workplace and finding ways to reduce the generation of noise at its source are the most effective ways of preventing occupational NIHL, but in workplaces where noise limits are exceeded and cannot be reduced, hearing protection is required.
Hearing protection can take the form of passive devices such as ear muffs and ear plugs, or active devices where electronic systems react to provide different levels of protection in varied noise environments.
Wearers tend to assume these devices are safe to use and are protecting their hearing, but how can an employer or worker be sure that adequate protection is given? Hearing protection test laboratories such as SATRA help to ensure that products meet the specifications of demanding international standards and provide wearers with peace of mind.
Hearing protection
Before a hearing protection product can be placed on the market it must be rigorously tested. Different test standards are in place and specify the testing required in order to place a product on the market in a particular part of the world.
In Europe, the testing of hearing protection devices is covered by the EN 352 series of European Standards, which is currently split into eight parts– each part refers to a different type of device.
Part one sets out the general requirements of ear muffs, part two deals with ear plugs, and part three covers helmet-mounted ear muffs. The final five parts relate to active devices.
Active devices provide not only passive attenuation but additional attenuation thanks to electronic circuitry which adapts the way in which the device reacts in varied noise environments. Active noise reduction ear muffs, for example, utilise a microphone on the outer shell of the cup, and a loudspeaker within the liner of the device.
An inversion circuit between the microphone and speaker allows the noise present at the microphone to be inverted 180° out of phase with the original signal. In simple terms, this inverted signal is the exact opposite of the original with positive pressures replaced by negative pressures and vice versa. The summing of the original noise and the inverted noise within the cup causes a cancellation effect at the wearer’s ear as the positive and negative pressures cancel each other out. Results of these systems can be varied, but many studies have shown excellent attenuation values in continuous noise environments, especially at frequencies below 500Hz where cancellation effects are generally more pronounced due to the longer acoustic wavelengths.
Another type of active device, known as the level-dependent ear muff, again works on the basis of a microphone and loudspeaker combination. These devices effectively ‘listen’ to the level of the external noise via a microphone mounted on the outer skin of the cup. If the volume of the noise is at a safe level, the loudspeaker within the cup reproduces the noise without any attenuation. When the noise level increases to dangerous levels, usually any level above 82dB(A), the loudspeaker switches off, allowing the ear muff to provide its full passive attenuation. Devices such as these are very useful in preventing overprotection.
Testing to international standards
Different international standards demand different levels of testing. The European EN 352 testing scheme differs slightly for each type of device, but all must undergo chemical, physical and acoustics testing, as well as a review of product marking and wearer information. The acoustic testing carried out follows international ISO standards (see Box 1).
In the United States, the marketing and sale of hearing protection devices is governed by the American Environmental Protection Agency which specifies testing to the ANSI S3.19:1974 standard. It is anticipated that the requirement will soon change to the ANSI S12.6:2008 standard. The American standard only sets out acoustic testing to assess the attenuation, or sound reduction, offered by the hearing protector.
Australia and New Zealand use the AS/NZS 1270:2002 standard and, like Europe, this demands acoustic, chemical and physical testing to be carried out. An overview of the different types of testing is given below.
Chemical testing
Chemical testing ensures that the materials used in the manufacture of the device and which will come into contact with the skin are non-staining and not likely to cause skin irritation, allergic reaction or any other adverse effect on health.
Physical testing
The physical testing schedule is designed to replicate the day to day physical demands which will be put upon the hearing protection, and to ensure that the device is fit for purpose. One of the first physical tests which is undertaken in the testing scheme is a materials and construction assessment, confirming that the device is free from sharp edges, safe for use and that any cleaning and disinfection methods specified cause no damage or impairment to the hearing protection.
A sizing assessment is also required, ensuring that the product is suitable for the range of head or ear sizes designated by the manufacturer. The majority of devices are classified as ‘Medium’ – the size range fitting the majority of the population, although products can be classed as ‘Small’ or ‘Large’ and must be clearly labelled so before they are placed on sale.
During this test, a range of fitting rigs, moulded head forms and size gauges are used to ensure that the products can meet specified test dimensions, providing an adequate fit for the intended wearer.
For ear muffs, cup rotation, headband force and cushion pressure are assessed to confirm that the cups can be rotated sufficiently, allowing wearers to adjust the device for the best fit and ensure that there is no excessive pressure upon the head from the combination of cushions and headband.
Resistance to damage is evaluated by dropping the hearing protection from a specified height onto a solid steel plate. If any part of the sample cracks or breaks then the device will fail the test and is likely to require redesign and resubmission for testing. For devices which are designed for use in colder environments, this testing can also optionally be conducted at -20° C.
The durability of headbands or standby mechanisms, which allows helmet-mounted ear muffs to be returned to the position which they occupy while not in use, is also tested if they are incorporated in the device. This is gauged by placing the cups of the product onto a pair of plates which oscillate between a minimum and maximum separation distance. This process continues for 1,000 cycles, replicating the action of a wearer fitting and removing the device or activating the standby mechanism.
Conditioning then takes place in the form of water immersion for 24 hours. This can also be conducted with the headband under stress, with a parallel spacer placed between the cushions of the device. Once complete, the headband force is measured for a second time. This is compared with the value recorded before the headband flexing and water immersion, with a maximum deviation between the two measurements providing the pass criterion.
If ear muffs with fluid filled cushions are under test, then the cushions’ resistance to leakage must be assessed. A vertical load of 28 newtons is applied to the cushion for 15 minutes and any leakage will constitute a test failure.
The final physical test which is undertaken for all types of hearing protection is an ignitability assessment. A steel rod heated to 650° C is applied to the device. If any part of the hearing protector ignites or continues to glow after the removal of the rod then the device fails the ignition test.
Exclusively to meet the Australian and New Zealand standards, a dry heat test is also required. This involves placing the device under test into a conditioning chamber which is at room temperature. Over a period of one to two hours, the conditions will change to 50° C and a relative humidity of between 5% and 15%. The device will then be left in these conditions for at least 16 hours.
Acoustic testing
In terms of assessing acoustic performance, both ear muffs and ear plugs are required to undertake subjective attenuation testing, while ear muffs must also undergo insertion loss testing.
Insertion loss is the algebraic difference between the sound pressure level with and without the ear muff fitted to a test fixture. This test does not use human test subjects; instead, it uses an acoustic test fixture which simulates the approximate dimensions of the human head. Microphones are housed in cavities in the sides of the fixture to replicate the position of the ears. The testing is normally conducted in an acoustic tunnel, with a loudspeaker at one end, and acoustically absorbent foam at the other, and along the length of the tunnel.
This creates an ‘anechoic’ effect, meaning that sound waves striking the sides and the end of the tunnel are absorbed rather than reflected, thus allowing a ‘plane progressive sound wave’ (moving in one direction only, with no reflections from side walls or ends) to propagate along the tunnel.
It is worth noting that this test sets no limit on the minimum attenuation which should be achieved. It is designed to assess the difference in the attenuation values between ten samples of the same ear muff model, in order to ensure that there is no major variation in performance.
The subjective attenuation testing uses human subjects to assess the performance of a hearing protection device, and does require a minimum attenuation value to pass the test. The results of these tests are published for the model when it is placed on sale and will be supplied to wearers. This test measures the ‘threshold of hearing’ – the lowest sound pressure level perceivable by the ear – of 16 human test subjects with and without the hearing protection worn. The performance of the model is calculated from these values.
The test requires a human test subject to sit in a chamber surrounded by a loudspeaker array. The subject will then be presented with test signals at known decibel levels and frequencies. If the subject hears the signal, he or she presses a hand switch and the signal level is reduced by 10dB. This process continues until the subject can no longer hear the signal, at which point the signal level increases in 2dB steps until it is eventually heard again. The process is repeated until the exact level at which the subject first perceives the signal is pinpointed – this is the hearing threshold.
Assessing a subject’s threshold of hearing requires extremely low background noise levels. These noise levels are so low they are expressed in negative decibels. To achieve such a quiet environment, a specially designed audiometry booth or an ‘anechoic’ or ‘hemi-anechoic’ chamber is required.
Anechoic means that an extremely high percentage of sound inside the chamber is absorbed by the walls and ceiling – they are covered with wedges, usually made from foam, which absorb acoustic energy over a wide range of frequencies.
Additional testing
There are additional acoustic tests which are undertaken for active devices. Level-dependent ear muffs must be assessed for the criterion levels. This is done using miniature microphones which are placed into the ears of test subjects beneath the ear muff. With the device worn, the sound pressure level is increased until the microphone measures a level of 85dB(A) at the ear. This external noise level is known as the criterion level.
Active noise reduction ear muffs require two additional tests. Active attenuation levels are measured by placing the microphones into the test subject’s ears and measuring the level at the ear which corresponds to an external sound pressure level of between 85dB and 95dB. There is also a requirement to assess the maximum sound pressure level at which linear operation ceases – again, in-ear microphones are used, and the point at which a 5dB increase in external sound pressure does not equate to a 5dB increase at the ear is calculated.
Attenuation ratings
Attenuation ratings awarded to hearing protection devices are denoted using SNR, HML and octave band values. These are different ways to quantify the performance of a device. Single Number Rating (SNR) provides a single attenuation value based on the subjective attenuation tests.
Theoretically, this value can be subtracted from measured external noise levels to estimate the noise level at the ear, beneath the hearing protection. This method does not provide any information on how much protection is provided in different frequency ranges, however, which is why the HML rating system is also required.
HML, or ‘high – medium – low’, provides further detail allowing the attenuation provided to be assessed across high, medium and low frequency ranges. This is particularly useful if a person is subjected to narrow band noise rather than broad band noise, as it allows a more accurate assessment of the noise level at the ear. As the name suggests, narrow band noise is spread over fewer frequencies than broad band noise, and thus has a more tonal quality.
Octave band attenuation data will also be provided by the manufacturer. This is usually shown in octave bands from 125Hz upwards, although there is an optional test band of 63Hz for devices designed for low frequency attenuation. Within this octave band analysis, users should be provided with the mean attenuation of each band, calculated using 16 test subjects, the standard deviation and the assumed protection. The assumed protection value is calculated by subtracting one standard deviation away from the mean attenuation of each octave band.
There is also an NRR rating system (noise reduction rating) in the United States following ANSI test procedures which is very similar to the SNR rating outlined above.
The Australian and New Zealand standard offers a ‘class’ rating to quantify the attenuation offered by the device. This works on a scale from Class 1 to Class 5, with class 5 offering the highest amount of protection.
Occupational noise assessments
When coupled with an accurate occupational noise assessment, these rating values can help to provide the correct amount of noise reduction while simultaneously avoiding overprotection. As a general rule of thumb, if you need to raise your voice to be heard by someone one metre away, the noise level is potentially dangerous and should be investigated with a noise assessment.
A noise assessment will normally be undertaken using a dosimeter, a device which measures a worker’s exposure to noise over an average working day. It may also be necessary to use a high quality sound level meter which allows spectral analysis, meaning that the level of noise in individual frequency bands can be investigated.
If it is possible to look at the noise measurements in octave bands, an estimate can be made as to how much noise will be reaching the wearer’s ear by simply subtracting the octave band attenuation values provided by the manufacturer of the device, as calculated during the subjective attenuation testing.
What to bear in mind
When choosing hearing protection equipment (HPE), ideally a level of 70 – 80dB under the protection is recommended. This should be adequate to prevent any damage to the wearer’s hearing while also allowing the wearer to hear warnings, speech and machinery. In many situations, overprotection can be just as dangerous as under protection, but for different reasons.
It is vital to ensure the device bears the CE mark if being sold in Europe, or the marking of other recognised standards and certification schemes if being sold in other parts of the world. Without it, the quality of a hearing protection product cannot be guaranteed and employees may be at risk of hearing damage. Professionally tested and certified HPE will give employers confidence that they are doing what they can to protect their workforce.
Published: 22nd Jul 2013 in Health and Safety Middle East
