High noise levels endanger the hearing of many people in a variety of circumstances. The protection provided, therefore, must be effective.
Hearing protection is vital for many people in a wide range of industries around the world. In fact, in many situations it is critical in protecting an individual’s hearing. Worldwide, approximately 900 million people suffer with some form of hearing loss. Many of these cases could have been prevented with the correct use of adequate hearing protection.
Hearing protection can be found in many different forms, whether standard ear-muff or ear-plug designs (known as ‘passive devices’) or more complex models incorporating electronic systems which react differently in varied noise environments, known collectively as ‘active devices’. The design, construction and materials selected for a particular product will contribute to the level of protection offered to the wearer and manufacturers may offer a number of similar products within a ‘family’ which offer a range of levels of protection across different frequency ranges.
With a wide range of products offering different levels of protection it is important that wearers assess the hazard to which they are being exposed so as they may select the most appropriate product to provide the necessary protection.
Passive and active devices
The performance of models in their passive state is dependent on several factors. The depths of the cups which enclose the ears, the headband force and the acoustic absorption of liners are just a few of the variables contributing to the performance of earmuffs, while size, fit and construction material generally govern the effectiveness of earplugs. However, the protection provided by any device will be compromised if it is poorly fitted or incorrectly worn – even the slightest break in a seal around an earplug or the cushion of an earmuff will reduce the ‘attenuation’ (noise reduction) provided by the hearing protection. Because of this, it is important that wearers are shown how to correctly fit and use their hearing protection.
“even the slightest break in a seal around an earplug or the cushion of an earmuff will reduce the attenuation”
Passive devices can still provide different levels of protection at specific frequencies, active devices can use electronic circuits to provide additional effects. This helps in conditions where high levels of protection, easy communication and situational awareness are needed (for instance when engaging in railway work).
One option is to use speakers inside the earmuffs to relay external sounds, such as a conversation with a co-worker. When levels of sound are low, they are reproduced at an attenuated level for the wearer, whereas high (damaging) levels of sound are not replicated at full amplitude. Another option is to use the circuit to produce the same sound, but with an opposite phase. These noise-cancelling systems provide additional protection.
In all cases, the general structure of the test is to use microphones near the ear and to determine at what levels of external sound the ear will be exposed to combined sound levels (from sound passing through the earmuff and from recreated sound) that approaches dangerous levels. Such an assessment is also used to ensure that internal speakers do not produce harmful levels of noise.
Testing and certification
Before a hearing protection product can be placed on the market in Europe, it must be tested and certified. This is because it falls under the scope of the Personal Protective Equipment Regulation 425/2016. Furthermore, the regulation categorises hearing protection PPE as complex or ‘Category III’ which means that following a type examination, bulk production must be checked by a notified body at regular intervals, usually annually using either the module C2 or module D route.
The wide range of hearing protection types available is reflected by the number of European Standards governing this testing. The EN 352 series of standards is currently comprised of eight parts, which cover the general requirements of each type of hearing protector. For example, parts one and two relate to passive earmuffs and earplugs respectively. Part three sets out the requirements of earmuffs attached to industrial safety helmets, and the next five parts cover active devices – including level-dependent, active noise reduction and audio input devices. A number of these standards are currently under review and are at the approval stage.
Two further standards EN 352-9 for earplugs with electrical audio input and EN 352-10 for ear plugs with entertainment audio input will join this series with publication expected in June 2020. During the drafting of these standards a critical development was the drafting of a third draft document, FprEN 13819-3:2018, which defines the test procedures for products covered by parts 9 and 10.
“microphones near the ear determine the levels of external sounds that approaches dangerous levels”
The testing scheme differs for each type of device. However, each of them must undergo physical and acoustics testing plus testing to ensure the innocuousness of the materials used, as well as a review of product marking and wearer information.
The type examination of PPE requires notified bodies to inspect the marking of the device and the information provided to wearers. This involves a review of the supplied user manuals and an examination of the final product to ensure that the correct markings are present, and that the required information is supplied to users, as specified in the relevant European standard.
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.
A range of physical and mechanical tests are carried out to assess the ear protector’s basic performance. Initial testing begins with assessing if the product can fit on the claimed head size range, and whether or not it has appropriate adjustment. The headband force is measured, following which the hearing protectors are subjected to a drop test (or optionally, a ‘low temperature drop’ test), a flexing test, water immersion or an optional water immersion with the headband under stress. After this, the headband force is re-measured and the change in headband force is calculated.
One of the first physical tests undertaken in the testing scheme is a materials and construction assessment. This confirms that the device is free from sharp edges, is 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, to ensure that the product is suitable for the range of head sizes designated by the manufacturer. The majority of devices are classified as ‘medium’ size range, which should fit the vast majority of the population. However, 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 headforms and size gauges are used to ensure that the products can meet specified test dimensions and so provide an adequate fit for the consumer.
For earmuffs, cup rotation is assessed to confirm that the cups can be rotated sufficiently so that the wearer can find the most comfortable position. Headband force and cushion pressure are also measured to ensure that there is no excessive pressure upon the head from the combination of cushions and headband.
“a steel rod heated, to around 650°C is applied to the device. If any part ignites or continues to glow after the removal of the rod, the device fails the ignition test”
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, the device will fail the test, and will most probably require redesign and resubmission for testing. This testing can also optionally be conducted at -20°C for devices which are designed for use in colder environments.
The durability of headbands or standby mechanisms, which allows helmet-mounted earmuffs to be returned to the position which they occupy while not in use, are also tested if they are incorporated into 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. The process continues for 1,000 cycles to replicate 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. As an option, this can be conducted with the headband under stress which is applied using a parallel spacer placed between the cushions of the device. Once complete, the change in headband force is measured for a second time, with a maximum deviation between the two measurements providing the pass criterion.
If earmuffs with fluid filled cushions are under test, resistance to leakage must be assessed. A vertical load of 28±1 Newtons is applied to the cushion for 15 minutes, and any leakage caused 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 around 650°C is applied to the device. If any part ignites or continues to glow after the removal of the rod, the device fails the ignition test.
Both earmuffs and earplugs are required to undertake ‘subjective attenuation’ testing (the level of noise reduction noted by the wearer), while only earmuffs are subject to ‘insertion loss testing’. These tests establish whether harmful levels of noise are reduced to acceptable levels at the ear.
The ‘insertion loss’ acoustic test is conducted by an electro-acoustic method. The test evaluates the levels of noise received by microphones placed in a fixture representing the head. This compares the levels of noise received, both with and without the hearing protector in place. The insertion loss test is carried out to ensure that the hearing protection provided by the set of samples assessed is at a consistent level. 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 generally 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 earmuff model, in order to ensure that there is not a major variation in performance.
The sound attenuation test is a subjective assessment using at least 16 human wearers who indicate the threshold sound level – the lowest sound pressure level perceivable by the ear –with and without the hearing protection worn. As the subjective attenuation test uses human subjects to assess the performance of a hearing protection device, and does require a minimum attenuation value to pass the test. The difference (in dB) is the level of protection the protectors provide at the test frequency. Attenuation ratings awarded to hearing protection devices are denoted using ‘Simplified Noise level Reduction’ (also referred to as ‘Single Number Rating’, or SNR), ‘High-Medium-Low’ (HML) and ‘octave band values’. These are different ways to quantify the performance of the device in question. SNR provides a single attenuation value based on the subjective attenuation tests. Theoretically, this value can be subtracted from measured external noise levels in order to estimate the noise level at the ear, beneath the hearing protection. However, it should be noted that this method does not provide any information as to how much protection is provided in different frequency ranges, which is why the HMLrating system is also required.
Assessing a subject’s threshold of hearing requires extremely low background noise levels. These noise levels are so low that they are expressed in negative decibels. To achieve such a quiet environment, a specially-designed location is required, such as:
• An audiometry booth (an isolated booth used to measure hearing or assess hearing protection)
• An anechoic chamber (with walls, floor and ceiling which absorb acoustic energy – sound – inside, resulting in a lack of echoes)
• A ‘hemi-anechoic chamber’ (which has a solid floor and a top hemisphere to absorb sound)
These chambers are designed to insulate against sound travelling through their walls, and use a heavy-duty construction of two independent wall structures separated by a cavity and acoustically-absorbent insulation. ‘Anechoic’ means that an extremely high percentage of sound inside the chamber is absorbed by the walls and ceiling, which are covered with foam wedges that absorb sound. SATRA has a large hemi-anechoic chamber in which the acoustic tests are carried out.
Marking of the device and the information provided to wearers also needs to be inspected. This involves a review of the supplied user manuals and an examination of the final product to ensure that the correct markings are present, and that the required information is supplied to users, as specified in the relevant European standard.
Chemical testing Any materials used in the manufacture of the device which will come into contact with the skin must be confirmed as being non-staining, and not likely to cause skin irritation, allergic reaction or any other adverse effect on health. Textile parts in contact with the skin should be checked to ensure that their pH is generally neutral and that no banned azo dyes are present. Rubbers and polymers in contact with the skin should be tested to ensure that there are no polycyclic aromatic hydrocarbons (PAHs). Metal components in contact with the skin should be free from nickel. Manufacturers which require an EU type-examination will need test data to prove the safety of such materials.