Health and Safety
| Environmental noise control - stealth and efficiency? |
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| Written by Peter Wilson, Director, Industrial Noise and Vibration Centre (INVC), 2008 | |
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Innovations in the art of noise and vibration control can dramatically reduce costs. Peter Wilson of the Industrial Noise and Vibration Centre (INVC) offers a pragmatic guide to the process that has to be followed to determine what constitutes current best practice in noise control problems.
Quiet plant or efficient plant? Too often, traditional noise control measures force companies into making this choice - which is particularly disappointing given the time and resources spent on reducing carbon emissions. The rise and rise of environmental noise problems
Figure 1. Sources of noise complaints. Source: NPL research Accurate diagnosis - precisely why is it noisy?The conventional approach to environmental noise control is based on high-cost palliative measures that attempt to hide the problem e.g. converting buildings into acoustic enclosures, adding silencers, constructing barriers and local enclosures. Apart from the high capital cost, some of these techniques can also have a significant effect on maintenance (temperature and accessibility) and efficiency (increased losses from fan silencers). The only way to determine the optimum noise and vibration control measures is to carry out a detailed engineering diagnosis of the precise noise generating mechanisms within the culprit plant before deciding on the potential solution(s). This approach opens up the possibility of engineering source control techniques in addition to the more conventional solutions. The benefits include:
Only where diagnosis shows that noise and vibration control at source is not practical should you consider falling back on the traditional techniques. Common noise problems where source control techniques constitute the most cost-effective solutions (as illustrated in the case studies) include fans, chillers/compressors, burners/boilers, pumps, vibrators and many others. Psychological silencing - matching the treatment to the subjective perceptionAnother aspect of environmental noise that is often overlooked is that it is a subjective problem. Noise targets are usually crude overall level criteria that may not reflect the subjective perceptions of complainants. The classic common examples are the issues caused by low-frequency fan drones. The authorities often call for a noise reduction of perhaps 5dB(A), which may - or may not - solve the problem. Moreover, as 5dB represents a 70 per cent reduction in the overall noise (but a much smaller reduction in the subjective impression), this can be a very costly exercise. However, noise signature analysis usually shows that if you remove the fan tone at source, the complainant will be happy - despite the fact that the overall noise may have fallen by only 1-2dB. Wielding the noise control scalpel in this way to tackle the precise component of the noise causing the problem is extremely cost-effective. Counter-intuitively, it is also sometimes possible to reduce noise problems by increasing noise levels. Bland, broadband noise from a fan, for example, can mask irritating noise features from other sources (it is actually possible to remove a silencer to improve matters). Changing a control system to ramp up a noise source gradually (e.g. intermittent vibrator) increases the average overall noise, but neighbours will swear that it is substantially quieter. We have even used an electronically sculpted noise-generating system to hide pump noise. Case studiesScrap fan noise control at source High levels of tonal noise from scrap can fans were causing both occupational and environmental noise problems. Conventional noise control measures in the form of silencers, enclosures and lagging were costed at £30,000 or more per system. Detailed diagnosis showed that the fan tones were the dominant components. These were then reduced at source by fitting aerodynamic inserts inside the fan casings in a matter of hours, providing an overall noise reduction of 22dB(A) at an installation cost of c £200/fan. Unlike silencers, these noise control modifications had no effect on fan efficiency, are unaffected by the passage of scrap cans and will last the lifetime of the fans without maintenance and without the access problems associated with enclosures. Dual fuel burner noise source control The cause of complaints about environmental noise levels was traced to this dual fuel burner. The conventional palliative noise control techniques for this type of problem involve fitting large silencers into the stack and converting the boiler house into an acoustic enclosure - eye-wateringly expensive with extensive downtime that would have caused serious production problems. The company contacted us to assess whether there were alternative noise control options. First, the precise source of the problem was diagnosed as an 116Hz low-frequency 'drone' - a very common type of burner noise. After disassembling the combustion head, an alternative solution was devised in the form of aerodynamic modifications fitted inside the combustion head. These reduced the drone by 16dB, completely eliminating the problem. The cost was less than £2,000 with a downtime of a few hours, making this technology an extremely cost-effective solution to this type of burner or combustion noise problem. Roof-top chiller plant The cost of reducing the noise from a 'chiller-farm' on the roof of a large multi-storey facility to meet the requirements of the Local Authority had been estimated at over £300,000. Fitting the proposed traditional acoustic screens would also have required considerable planning (closing streets and bringing in cranes) and there was also the possibility of reduced cooling capacity. The communications company involved approached us for a second opinion. A very careful analysis of the noise sources and site layout plus some lateral thinking resulted in an elegant solution involving a combination of engineering control at source, novel local acoustic treatments and the cunning use of geometry. The result was a set of modifications that were easy to implement in convenient stages and provided a noise reduction of c 15dB(A) at a cost of c £40,000 - and with no effect on normal operation or performance. In-house noise and vibration problems The same diagnostic engineering approach is also the most effective way to tackle on-site environmental (e.g. office disturbance) or occupational (health risk) noise and vibration problems. It also applies to the development of vibration control measures to protect sensitive equipment such as servers, electronic component manufacturing plant and metrology instrumentation. Further informationPeter Wilson MSc MIOA, Director, Industrial Noise and Vibration Centre (INVC)Tel: 01753 698800 Email: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it www.invc.co.uk |
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