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Sound Counsel

Published: 27th Nov 2013 in OSA Magazine

Noise management in the workplace

Human ears are precious assets. These diminutive organs are one of our primary defences, helping us to detect danger even when we are asleep, but this sensitivity is a double-edged sword. While being finely attuned to hearing useful sounds, the ear is also highly vulnerable to noise trauma that can result in hearing loss.

It is self-evident that employers should avoid exposing staff to such noise trauma, for the sake of their ear health – but there are other benefits to noise management. For example, the comfort and concentration of workers can be affected by high noise levels, and excessive noise can hinder communication – which could potentially result in dangerous accidents or costly mistakes.

The term ‘occupational noise’ refers to any sound heard by a person at work. Sound is transmitted as a pressure wave through air, water and solids, and it can be either re-radiated as noise or felt as vibration. Occupational vibration brings its own particular hazards, and specific guidelines for this exist separately from occupational noise hazards.

Mechanical and fluid noise

There are different types of noise source, which should be considered in the workplace. The first is mechanical noise, which results from two components striking or rubbing each other – either incidentally or as part of a process essential to the work at hand. For example, using a tool to press or cut materials might create a noise.

The second noise type is fluid noise, which is caused by turbulence in liquids or gases. The most common form of fluid noise is turbulence in air, typically from fans associated with extraction and ventilation. Fluid noise can also be produced in an industrial setting by the flow of water, oil and other liquids.

Both mechanical and fluid noise are inevitable in a typical industrial workplace, and so the challenge is to confine them to safe levels. Each country will have its own specific regulations defining acceptable noise levels, but most regulations will be based on guidelines issued by the World Health Organization or International Labour Organization.

Recommendations will typically be based on the daily or weekly noise dose imposed on a worker, which is in turn derived from average noise energy levels.

A workplace noise risk assessment is advisable, and will usually be a requirement. A rough indicator that a workplace noise risk assessment should be undertaken is if two people have to raise their voices to speak to each other when two metres apart, as this indicates that noise levels may be excessive. 

The purpose of a workplace noise risk assessment is to identify persons who may be exposed above noise action levels. This will depend on the level of the noise and the period of exposure.

Noise dose is measured on a decibel (dB) scale which rises exponentially. Exposure increased by 3dB when the exposure period is doubled, and conversely it reduces by 3dB when exposure is halved. So, for example, if a one hour working shift had an exposure level of 74dB, then two hours would result in 77dB exposure, four hours in 80dB, eight hours in 83dB, 16 hours in 86dB, and so on.

Noise exposure is the same regardless of the type or quality of sound, and so acceptable working limits do not vary according to the noise source. Nor do they vary according to the purpose or utility of the sound. This means that if the sounds made by a machine help a worker to judge whether its parts are in good condition or whether it is working correctly, this does not exempt it from noise limits. The same applies for warning sounds and alarms – and even for musicians, where the creation of sound is the purpose of the job.

Classifying noise sources

In many countries, information about the noise level produced by high-noise equipment must be made available by the manufacturer, and is usually found in product specifications, operational manual or certification. 

This is usually provided in terms of sound power level (SWL, Lw) and the overall frequency weighted level is given in A-weighted decibels (dBA). The A-weighting is a frequency curve that approximates the sensitivity of typical hearing at medium sound levels. 

Sometimes it is useful to look at the noise levels at certain frequencies; for example, if the noise is a tone, hum or whine. The A-weighting can be applied to individual frequencies, or the levels can be stated without the weighting. 

In addition to SWL, sound pressure level (SPL, Lp) may be stated. This is the sound level measured at a particular position. When relating to a noise source, an SPL should always state the distance from the source to which the level relates; for example, a typical SPL could read ‘84 dBA SPL @ 3m’ – as sound levels generally reduce as the distance from a source increases.

Tackling noise at the source

If workers’ noise exposure is found to be above acceptable levels, there are a number of mitigating courses available. The preferred methods of mitigation are those that reduce the noise at source, as these benefit many people at once and help to avoid noise pollution, which is particularly important on sites close to residential areas. Reducing the noise at source is also often the most economical solution as it negates the need to purchase additional equipment, and may even bring about more efficient ways of working or reduce the amount of energy consumed.

The most obvious way to reduce noise at source is to eliminate the source, but if it is part of an essential process the task becomes one of modifying the process. Noise reduction should be a key component of process set-up and revisions, as well as in the choice of plant.

Take the example of hammering in a nail to secure a piece of wood. If this process is only to be carried out a few times there may be no way to reduce the noise, but if it is to be repeatedmany times then quieter alternatives could be preferable – for example, using a screw in place of a nail to reduce the need for hammering, or employing a mechanical or pneumatic nail gun to reduce the number of blows needed.

Another example is when noise is created by a part dropping into a bin. A reduction in the length of the drop could reduce the noise level, or the provision of a soft landing could have the same effect while also reducing the risk of damage to the part.

Noise reduction methods could also help to protect machinery; for example, using lubricant on a cutter to reduce the noise of its operation could also improve the quality of cutting and extend the life of the cutter.

Where fluid noise is concerned, making the flow smoother and reducing the flow velocity will help to reduce noise. As with mechanical noise, there may be energy saving and efficiency improvements associated with reducing flow rates and turbulence. For example, replacing fixed rate fans with variable speed fans allows lower speeds to be used when higher speeds are unnecessary.

As well as amending processes and plant, reduction of noise at source can also be achieved by changing work methods to reduce and manage the period of time over which the noise is created. Work schedules can be designed so that an individual only uses noisy equipment, or works in a noisy area, for a limited period each day. This tactic requires diligent management as it must be ensured that the operative’s noise exposure during the ‘quiet’ periods of the day is kept to a minimum, to avoid the total exposure exceeding daily or weekly limits.

Alternative protection options

If it is not possible to reduce noise at the source in order to bring noise levels within safe limits, the next most preferable course of mitigating action is to obstruct or limit the transmission path between the noise source and the worker. 

A simple way to achieve this is by moving the source further away from people who are at risk of exposure. For example, a compressor or generator may be located further away from an operative, or the operating station for a large machine may be located further away from the noise-producing elements. Noise exposure typically reduces rapidly with distance – although this may not be the case in an enclosed space with hard surfaces, such as a plant room, waterproof or hygienically sensitive area. 

If increasing the distance is not possible, barriers can come into play. Enclosing the noise source can effectively obscure the noise transmission route, interrupting the signal path between the source and the worker. Many manufacturers of potentially noisy equipment also produce optional noise-reducing enclosures for those machines. This strategy is sometimes impossible or unhelpful, however; for example, fully enclosing a fan will prevent the useful movement of air, and enclosing a press or other machine may act like an amplifier and cause the noise to resonate.

Alternatively, the workers could be enclosed instead of the noise source. A cab, room or enclosure can contain machine operatives, observers, supervisors or visitors, and prevent them being excessively exposed to the noise being created. A permanent or temporary barrier could also be used instead of a fully enclosing structure. An effective temporary barrier could be formed from another existing structure, such as a well stocked materials store on a construction site. Of course, the use of enclosures or barriers is only suitable in instances where they will not diminish workers’ ability to carry out the task effectively and efficiently. 

The use of personal hearing protection is perhaps the most obvious form of noise mitigation.

This can be a complementary measure in addition to other strategies, but using personal hearing protection alone should only be a last resort. This form of protection can come in various forms, and there are various considerations to be taken into account when using them (see box).

Proper use of personal hearing protection

Where personal hearing protection is provided for employees – whether on a mandatory or optional basis – it is important to ensure that it provides suitable protection. 

The unprotected noise dose must first be assessed, and the hearing protection must then be confirmed as providing at least the minimum level of protection. This can be accomplished as a ‘desktop’ assessment if all the relevant information is available or if the plant is being newly designed and is not yet available; otherwise, a site survey will be required to gather the necessary data on the plant, its installation and the exposure patterns created by it.

There is a choice to make between in-ear or over-ear protection. Sometimes this decision can be to the operative’s personal preference, but in some circumstances one type may be ruled out as inappropriate;

for example, over-ear protection could interfere with head protection. In some circumstances where hygiene and contamination control are major concerns, individual foam ear plugs may not be acceptable – such as in a food preparation environment, or other controlled workplaces. 

Personal hearing protection carries a higher risk of improper use and human error, which is one of the reasons it should only be used as a last resort or complementary measure. Training is essential to ensure that the equipment is correctly used and maintained. There should also be clear communication of the hearing protection zones where personal hearing protection must be worn – which need to be actively enforced.

Finding a safe balance

Whichever noise mitigation methods are chosen, the priority should be achieving the optimal balance of safety and capability. Mitigation tactics should be carefully designed so that they do not impede workers’ ability to carry out their work correctly and safely – too much hearing protection could prevent safety communication and reduce situational awareness.

Specialist personal hearing protection is available where needed. For example, musicians and sound engineers – known as ‘critical listeners’ – can use custom moulded ear plugs fitted with a choice of inserts to ensure the correct protection level, and workers who need to communicate in noisy environments can be supplied with over ear or custom moulded protectors fed with a communications signal. 

Where worker comfort and concentration are concerned, it is also worth having an understanding of how people are affected by different noise qualities. A steady noise environment is less distracting than an environment with changes of noise, and gradual changes in noise levels are less distracting than sudden ones. Tonal or impulsive noises, buzzes, clicks and whining are often more irritating to people than the same level of steady noise without any strong character.

Whatever the working situation, the noise exposure levels, the plant being used and the working requirements of the individual, a solution can be found to ensure that normal working operations can progress as required without resulting in hearing loss.

The crucial requirement is that the necessary data is available, the risk assessment is competently completed, and the mitigation measures are skilfully designed.

Published: 27th Nov 2013 in OSA Magazine

Author


Simon Kahn


Simon Kahn is the technical director of acoustics at Mott MacDonald; chairman of the Institute of Acoustics’ Education Committee; a member of the British Standards Institute panel reviewing acoustic standards; and a guest lecturer at UK universities.

Simon has provided acoustic advice for a wide variety of buildings, environmental projects and workplaces, from art galleries, theatres and music festivals, through to rail projects and workplaces.

Simon has also provided workplace noise risk assessment advice for oil installations, water treatment plants, factories, theatres, art workshops and orchestral musicians.

About Mott MacDonald

Mott MacDonald is a uniquely diverse £1.1 billion global consultancy working across 13 core business areas: transport, buildings, water, environment, mining, power, oil and gas, international development, urban development, industry, communications, education, and health.

The firm is one of the world's largest employee-owned companies, with 15,000 staff, 168 offices in 50 countries and projects in 140. The consultancy is driven to achieve innovative, efficient, sustainable solutions that are easier to deliver, work better and provide greater value for money for its clients. With the strength of the worldwide group, Mott MacDonald is able to provide local service to world class standards.

Mott MacDonald is an industry leader in health and safety and works with its customers, national and local governments, safety regulators and enforcers, industry peers and professional bodies to share best practise, shape attitudes to safety and develop guidance. In this the consultancy continues to raise standards of health and safety globally.

Health and safety forms part of Mott MacDonald’s core training for all staff, covering all work related activities – in the office, in the field or travelling. 


Simon Kahn

Website:
http://www.mottmac.com


http://www.mottmac.com

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