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Testing the Limits

Published: 28th Nov 2012 in OSA Magazine

Noise induced hearing loss (NIHL) is the most common, permanent occupational injury in the world, according to the World Health Organization. Yet it is preventable. SATRA’s Andy Todd outlines why occupational noise assessments and fully tested and certified hearing protection products are key.

Introduction

The dangers of exposure to excessive noise are well known, yet noise induced hearing loss (NIHL) remains a problem worldwide. Asia has recently been shown to have a particularly high incidence of NIHL due, in part, to the rapid growth of manufacturing and agriculture in the region, the increasing number of workers exposed to excessive noise levels and a lack of awareness of the danger of noise by some workers and employers (OSA, September 2012).

Measuring noise levels in the workplace and finding ways to reduce this noise at 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 comes in many different forms: standard earmuff or earplug designs, known as passive devices, or more complex models incorporating electronic systems which react differently in varied noise environments, known collectively as active devices.

Wearers take it for granted that these devices are safe to use and are protecting their hearing, but how can they be sure? Hearing protection test laboratories help to ensure that products meet the specifications of demanding international and European standards and provide wearers with peace of mind.

Hearing protection

While NIHL is a growing problem in Asia, many Asian based manufacturers of hearing protection products focus much of their business on export markets such as Europe and America.

Before a hearing protection product can be placed on the market in Europe, it must be tested and certified as it falls under the scope of the Personal Protective Equipment (PPE) Directive 89/686. The testing of hearing protection equipment, or HPE, under the PPE Directive has been mandatory since June 1995.

The testing itself has been slightly modified during this time, but the majority of tests and specifications have remained constant. The testing of HPE is covered by the EN 352 series of European Standards, which is currently split into eight parts - with each part referring to a different type of device.

Part one sets out the general requirements of earmuffs while part two deals with earplugs. Part three covers helmet mounted earmuffs and the final five parts relate to active devices.

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 earmuffs, 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; however, many studies have shown excellent attenuation values in continuous noise environments, especially at frequencies below 500Hz where cancellation effects are generally more effective due to the longer acoustic wavelengths.

Another type of active device, known as the level dependent earmuff, 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 increases to dangerous levels, usually any level above 82dB(A), the loudspeaker switches off allowing the earmuff to provide its full passive attenuation.

Devices such as this are very useful in preventing overprotection. Overprotection is when the hearing protection provides too much attenuation, making it difficult to hear speech, alarms and warning signals.

The testing scheme differs for each type of device; however, all must undergo chemical, physical and acoustic testing, as well as a review of product marking and wearer information.

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 nonstaining 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.

Materials and construction assessment

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.

Sizing assessment

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 as this is the size range fitting the majority of the population; however, products can be classed as Small or Large and so must be clearly labelled 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, providing an adequate fit for the intended wearer.

When testing earmuffs, for example, 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 ensuring that there is no excessive pressure upon the head from the combination of cushions and headband.

Damage resistance

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.

Durability

The durability of headbands or standby mechanisms, which allow helmet mounted earmuffs 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

Conditioning then takes place in the form of water immersion for 24 hours. This can also be conducted with the headband under stress, by placing a parallel spacer 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.

Leakage

If earmuffs 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.

Ignitability assessment

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 HPE ignites or continues to glow after the removal of the rod then the device fails the ignition test.

Acoustic testing

In terms of assessing acoustic performance, both earmuffs and earplugs are required to undergo subjective attenuation testing, while earmuffs must also undergo insertion loss testing.

Insertion loss

Insertion loss is the algebraic difference between the sound pressure level with and without the earmuff 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 end 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 wave moving in one direction with no reflections from side walls or ends - known as a plane progressive sound wave - 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 no major variation in performance.

Subjective attenuation testing, on the other hand, uses human subjects to assess the performance of a hearing protection device, and does require a minimum attenuation value to pass the test.

Subjective attenuation testing

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 ears of the 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 across a wide range of frequencies.

Additional testing

There are additional acoustic tests which are undertaken for active devices. Level dependent earmuffs must be assessed for the criterion levels. This is done using miniature microphones which are placed into the ears of test subjects beneath the earmuff. 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 earmuffs 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. To do this 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 Simplified Noise level Reduction (SNR), also referred to as Single Number Rating; High, Medium, Low (HML); and octave band values. These are different ways to quantify the performance of a device.

SNR

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, however, does not provide any information on how much protection is provided in different frequency ranges; this is why the HML rating system is also required.

The HML method

HML bandings provide 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 narrowband noise rather than broadband noise, as it allows a more accurate assessment of the noise level at the ear. As the name suggests, narrowband noise is spread across fewer frequencies than broadband noise, and thus has a more tonal quality.

Octave banding

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.

NRR

There is also a Noise Reduction Rating (NRR) system used in America that follows American National Standards Institute (ANSI) test procedures, which is very similar to the SNR rating outlined above. The main difference is that the SNR subtracts one standard deviation from the mean attenuation result, while two standard deviations are subtracted using the NNR rating.

Occupational noise assessment

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.

In 1989, the Noise at Work Regulations were introduced in the UK to protect employees from hearing damage in the workplace. This was revised in 2005 with the Control of Noise at Work Regulations, based on a European directive on minimum health and safety requirements.

These revised regulations, which reduced the acceptable levels of noise that an employee could be exposed to by a further 5dB, reflected the concern that too little was being done to protect the hearing of workers in Europe. These regulations set out the action values and exposure limit values at which employers must act.

Exposure levels

At a daily or weekly exposure level of 80dB(A) / 135dB(C)peak or more, the employer must provide information and training to staff and make hearing protection devices available. At 85(A)dB / 137dB(C)peak and above, the employer must take reasonable action to try to reduce the noise in the workplace using engineering controls or administrative methods. If the noise cannot be reduced this way, hearing protection devices become mandatory.

There is also an exposure limit value of 87dB(A) / 140dB(C)peak, above which no worker can be exposed to. Decibel levels followed by (A) are subject to what is known as ‘A-weighting’ which mimics the human ear’s response to sound, as we do not hear all frequencies with equal sensitivity. Our hearing is much more sensitive at around 4kHz and much less so at lower frequencies.

When decibels are denoted with a (C), ‘C-weighting’ is being used, which provides a much more linear assessment but also mimics the human ear’s response to loud, impulsive noises - hence the reason it is used to measure peak values.
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.

Noise assessment

A noise assessment will normally be undertaken using a dosimeter, a device which measures a worker’s exposure to noise throughout 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.

Summary

When choosing HPE devices, a level of 70 - 80dB under the protection is recommended. This should be adequate to prevent any damage to the wearer’s hearing while avoiding overprotection.

It is also vital to ensure that the device bears the CE mark if being sold in Europe, or the marking of other regionally recognised standards and certification schemes if being sold in other parts of the world. This should provide extra confidence that the product has been properly tested before being placed on the market. Without it, the quality of a hearing protection product cannot be assured and your employees may be at risk of damaging their hearing. 

Published: 28th Nov 2012 in OSA Magazine

Author


Andy Todd


Andy Todd is an acoustic engineer at SATRA. With a background in building acoustics and several years’ experience in audio and acoustic engineering, he has been heavily involved with the installation and launch of the company’s new hearing protection test facility, and is the main point of contact at SATRA for acoustic testing.

About SATRA

As part of its range of PPE testing services SATRA offers testing and certification of hearing protection devices. The new hearing protection test facility based in the UK includes a purpose built hemi-anechoic chamber, which is used to assess the subjective performance of hearing protection devices.

SATRA also has a test facility in Dongguan, China, where it carries out some PPE testing, primarily of safety footwear. The China office is able to accept hearing protection product samples for testing in the UK.


hearingprotection@satra.co.uk
http://www.satrappeguide.com

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