White Paper 8 Acoustic enclosures
1st edition – January 2017
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Safety of machinery – acoustic enclosures for reducing noise emissions
Jeremy Procter, a Member of international and UK standards committees ISO/TC 199/WG 6 (Safety distances and ergonomic aspects) and BSI MCE/3 (Safeguarding of machinery), and Managing Director of Procter Machine Guarding, explains how acoustic enclosures can help machine builders and end users meet their legal obligations for safeguarding workers from excessive noise.
Machinery is a major cause of workplace noise and, according to the HSE (Health and Safety Executive), around 17,000 people in the UK suffer deafness, ringing in the ears or other ear conditions caused by excessive noise at work. Damage to hearing is typically cumulative, so the person concerned is not aware of the damage and therefore does not think that the risk is significant at the time of exposure. For this reason, it is vital that noise is managed by the machine supplier and employer, rather than relying on the user to wear ear plugs, ear defenders or similar personal protective equipment (PPE). Machine builders and importers are legally obliged to manage noise emissions from the machines they supply while, at the same time, employers are obliged to manage the levels of noise to which their employees are subjected. This White Paper is intended for use by these two groups because acoustic enclosures are an extremely effective way of reducing noise emissions from both new and existing machinery.
Anyone placing machinery on the market in the EU is required to CE mark it in accordance with the requirements of the European Machinery Directive 2006/42/EC. Clearly this applies to machine builders, but it is equally applicable to importers and companies that manufacture machines in-house for their own use. Here in the UK, Statutory Instrument 2008/1597 The Supply of Machinery (Safety) Regulations 2008 came into force on 29 December 2009 and is the UK implementation of the Machinery Directive.
The Machinery Directive is intended to ensure a common standard of machinery safety throughout the EU. Excessive noise poses a serious risk to health (including hearing damage, fatigue and stress), and can also be a contributory factor in accidents – for example, due to interference with communications. The Machinery Directive therefore makes direct references to noise emissions; indeed, one of the Essential Health and Safety Requirements (EHSRs) is noise (subclause 1.5.8):
Machinery must be designed and constructed in such a way that risks resulting from the emission of airborne noise are reduced to the lowest level, taking account of technical progress and the availability of means of reducing noise, in particular at source. The level of noise emission may be assessed with reference to comparative emission data for similar machinery.
In addition, subclause 188.8.131.52 Contents of the instructions, paragraph u, requires that the sound power level must be stated in the instructions if it exceeds 80 dB(A). See below for a table of Machinery Directive harmonised standards relating to noise control and measurement methods.
Acoustic enclosures as safety components
The Machinery Directive lays down requirements not just for machinery but also for safety components. Annex V contains an indicative list of safety components and this includes noise/vibration reduction systems. As with machine guards (which are also listed in Annex V), under certain circumstances it is therefore necessary to CE mark acoustic enclosures to the Machinery Directive. For example, if an acoustic enclosure is placed independently on the market for use with a benchtop CNC router, the manufacturer of the acoustic enclosure would need to issue a Declaration of Conformity and CE mark the acoustic enclosure as a safety component. However, in most cases acoustic enclosures also perform a machine guarding role so, as far as CE marking is concerned, they are more likely to be treated as ‘guards’ (see Procter Machine Guarding White Paper CE Marking of Guards).
Managing workplace noise
Employers in the UK are obliged to comply with the Control of Noise at Work Regulations 2005 (often abbreviated to the Noise Regulations), which came into force on 6 April 2006 to implement European Council Directive 2003/10/EC (minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise)).
The Noise Regulations require employers to eliminate or reduce the risks to employees’ health and safety arising from noise at work. In essence, the Regulations give average (daily or weekly) and peak exposure limits:
- Lower Exposure Limits are the points at which employers are required to assess the risks and provide employees with information and training;
- When the Upper Exposure Limits are reached the employer must provide hearing protection and define the areas where this must be worn;
- The Exposure Limit Value is the noise level to which employees should not be subjected.
The HSE has published guidance on its website and in documents (most of which can be freely downloaded as PDFs) to help employers decide whether or not they have a problem with noise. For example, action may well be required if noise is intrusive for most of the working day, if voices have to be raised so that a conversation can be held from 2m away, or if noisy powered tools or machines are used for more than half an hour each day. If it is thought that noise might be a problem, then a risk assessment must be undertaken to identify where the risks are, who might be affected and employees’ exposure levels. In addition, employers need to identify what actions need to be taken, whether any employees need to be provided with health surveillance and if any employees are at particular risk.
Noise levels can be estimated from published information (eg industry guidelines), information from similar workplaces and data from machine suppliers. However, the more accurate alternative is to measure the noise in the workplace. This can be done using suitable calibrated instruments that are operated properly and the readings interpreted correctly (NB a ‘sound meter app’ on a mobile phone will not be sufficiently accurate), or specialists can be contracted to undertake noise surveys. Most importantly, the noise must be measured when the machine is operating normally and/or when it is at its noisiest; for example, the noise emitted by a milling machine will be significantly higher when it is cutting metal than when it is idling.
Once the risk assessment has been completed and the noise either estimated or measured, individual machines (or parts of machines) that produce excessive noise can be addressed.
How to reduce noise emissions from machinery
When a machine is being designed, the hierarchical approach to be adopted is the same as for other machinery safety issues:
- Design and construct the machine to reduce the noise emissions at source (ie make the machine operate quieter);
- Use integrated measures to reduce noise emissions (eg install an acoustic enclosure); and
- Inform users about residual risks (eg place warnings in the instruction manual and install warning signs on the machine so users are aware they need to wear hearing protection), provide training and suitable PPE.
With new machinery, it is common practice to incorporate noise attenuation when designing the guarding.
For an existing machine, there may be limited scope for making modifications to reduce noise generated at source, though improvements can be made by measures such as: undertaking maintenance and replacing worn parts; fitting silencers and/or piping pneumatic exhausts away from operatives; upgrading fluid power systems to servo electric control; installing antivibration mounts and flexible shaft couplings; replacing chain drives with timing belts; adding sound-deadening materials to flat panels; and redesigning ejection chutes to minimise drop heights and avoid metal-to-metal contact.
Often, however, existing machines benefit most from being retrofitted with acoustic enclosures either totally or partially. Sometimes the existing guarding is supplemented with an additional acoustic enclosure, while at other times the existing guarding is replaced by new components with integral noise attenuation. Depending on the circumstances, it might be appropriate to install acoustic barriers between the noisy machine and the area where people may be present. If the machine is large and access is not required during operation, a fully enclosed soundproof room can be erected around the entire machine.
When HSE inspectors visit workplaces where noise is potentially a problem, they will look to see whether there are known solutions that are ‘reasonably’ practicable’; if there are, employers will be expected to have implemented them.
Guidelines for designing acoustic enclosures
Most acoustic enclosures for machinery are manufactured from sheet metal with the addition of sound-deadening and soundabsorbent materials to reduce radiated and reflected noise. These materials will depend on the application, whether there are liquids or mists present, and the location indoors or outdoors. One way to reduce radiated noise is to apply self-adhesive sheet steel to the flat steel panels; alternatively, entire panels can be manufactured from proprietary metal-polymer-metal sandwich materials that can be processed and finished the same as mild steel. Where reflected noise needs to be reduced, materials such as foams, fibreglass and mineral wool are popular, usually sandwiched between sheet steel on the outside and perforated sheet steel on the inside. Sometimes plain panels are adequate for reducing noise, such as for enclosing conveyors, but bear in mind that polycarbonate can outperform steel or aluminium, with the added benefit of providing visibility.
Fixed panels should be designed and installed so that they seal against adjacent panels and structures. This may require a stiffer panel design than might normally be adopted, and sufficient fasteners should be installed at appropriate centre distances, possibly supplemented by a gasket or sealant. Alternatively, the panel can be welded or bonded in place.
Where doors or removable panels are installed, care must be taken to provide adequate sealing. Rubber or foam gaskets work well, provided they are specified and applied so as to bridge the gap without being over-compressed. For opening panels or doors, sufficient latches or fixings must be installed to ensure that the gasket is compressed along its entire length. Because noise is considered to be a hazard in the same way as moving machinery, access doors are usually fitted with safety interlocks. If the acoustic enclosure is also performing a machine guarding role, then safety interlocks are essential. Where acoustic enclosures are also preventing access to dangerous parts of the machine, more information can be found in Procter Machine Guarding White Papers on Machinery guarding standards and Machinery guarding for PUWER (see below).
Often machines require clear viewing panels. These can be double-glazed if necessary, but it is important to ensure the viewing panels are properly sealed, usually by means of rubber sealing gaskets. If apertures are required for entry and exist of materials for processing on the machine, the aperture should be as small as possible. ‘Acoustic tunnels’ can reduce noise leakage from apertures or, in extreme cases, powered hatches can be installed that only open when materials need to pass.
Other sound-reduction measures might include exhaust gas removal for internal combustion engines and filtered air inlets.
Acoustic enclosures can trap heat, so it may be necessary to install passive or forced-air ventilation to prevent machinery from overheating. If so, care must be taken to ensure that the ventilation ducting does not become a new source of noise.
Acoustic enclosures are a key point of interaction between the machine and personnel for both operation and maintenance, so it is essential that acoustic enclosures are, as far as possible, designed so they do not impede access.
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