Noise & Vibration

Acoustics is the science that deals with the study and technology of sound. Generally the study of acoustics includes the generation, propagation, control and reception of mechanical waves and vibrations; their interaction with materials and their effects on the ear and on hearing.

Noise is sometimes defined as unwanted or excessive sound. Depending upon the circumstances - noise can give rise to nuisance and disturbance (environmental or community noise). Occupational exposure to noise (workplace noise) can cause noise induced hearing loss (NIHL) and tinnitus. Excessive noise at work has also been identified as a potential factor in workplace accidents.

When a surface or structure rapidly oscillates backwards or forwards the motion is referred to as vibration. The rate at which the occillation (repeated movement) occurs is known as the frequency of the vibration and this is measured in cycles per second or hertz (Hz). Sound and vibration are closely related and sound can be considered as pressure waves or air vibrations which are audible. These waves are generated by vibrating structures (e.g., loudspeakers or human vocal cords) and these pressure waves can also induce the vibration of structures. Many activities (e.g., heavy traffic, quarry blasts, etc.) have the potential to give rise to sound and vibration emissions.

Vibration within buildings is generally assessed in terms of its acceptability to the human occupants. In most instances there is a need to distinguish between the perceived risk and the actual risk and invariability we assess the level of vibration against specified criteria. Occasionally large amplitude shock and vibrations will need to be assessed with respect to their potential effects on the buildings and structures, as opposed to the building occupants.

Noise Control and Attenuation

The mechanisms and standards for controlling and regulating noise depend on the nature of the source, e.g., road traffic noise is controlled and assessed quite differently to factory noise or noise from a night club. Equally, construction noise, recreational noise and ‘neighbour noise’ may need to be assessed using different criteria and standards. Throughout industry, noise control measures are often required to protect the hearing of workers. On the other hand, environmental noise control generally gives priority to protecting Noise Sensitive Locations (NSLs) or noise sensitive receptors.

A range of factors (acoustic and non-acoustic) will affect the tolerance and or reaction of an individual to noise and the overall impact of a noise source. In assessing the likelihood of environmental noise complaints BS 4142 (1997) gives priority to the degree by which the noise exceeds the pre-existing noise levels. Some of the other pertinent factors in assessing or predicting noise impacts or nuisance complaints include:

• The absolute sound pressure level of the noise source and variations over time
• Frequency of the noise (spectral components)
• Extent of the impulsive elements and special features or characteristics of the noise
• Variations in the hearing sensitivity of individuals
• Site location and local land use
• Nature and character of the locality
• Activities underway when noise is audible
• Local attitudes to the source or sources of the noise
• The likely duration of the noise and the ability and/or willingness to control its impact.

The above list is by no means exhaustive and many social, psychological and economic factors will affect the sensitivity of an individual or a situation over time.

At some facilities the nature of the activities or the proximity of of NSLs may result in potential noise problems. In such cases, the key environmental noise issues (dominant sources, noisy production processes etc. and their impact on noise sensitive locations) need to be identified. At such facilities it is important to adopt a logical and systematic approach to noise management.

At some sites a combination of factors (e.g. inherently quiet plant, effective containment of noisy sources or remoteness of NSLs) ensures that environmental noise is a minor issue and significant noise impacts are unlikely to arise. However, some plants will require an ongoing programme of work to ensure an effective level of control over the facility’s noise emissions. In some instances this is best achieved through a structured Noise Management Programme based on a risk assessment approach.

The first step generally involves an assessment of any existing or planned noise sources and their relative contribution to ambient levels. This facilitates the establishment of target noise levels for the particular source and where necessary the degree of noise attenuation can be estimated. Having established the required reduction, the next stage is the application of noise control engineering principles. However, effective planning and management frequently involves the use of common sense and good practice as opposed to high tech engineering solutions.

Virtually all noise control problems can be represented by a simple energy flow diagram. This generally gives rise to two main options of control, i.e. to reduce the source strength or to impede the acoustic energy along its transmission path. In every situation the mechanisms by which a sound is generated and the exact part of the machinery/equipment responsible will largely dictate the treatment options.

(Sharland, I. (1972) Woods practical guide to noise control. Published by Woods Air Movement Limited)

With the exception of aerodynamic noise, noise is generally caused by a force causing a surface to vibrate. Surfaces or panels radiate sound most efficiently at or near any of their modal or resonant frequencies and some control measures involve the application of coatings or damping layers or mechanical stiffening devices which subdivide the panel so that the modal frequencies move upwards and become less problematical.

While noise control engineering is a specialist subject, in many situations the objective is to decouple the mechanical path between the source of vibration and the radiating surface. There are a wide range of noise control techniques, however, they can be loosely categorised as follows:

controlling resonance – e.g. altering the mass and/or stiffness of the panel or changing a machine’s running speed to detune it from the natural frequency of the panel;
control of stiffness – e.g. the use of resilient layers/treatments and the addition of coupling devices;
vibration isolation – e.g. the use of isolating springs to reduce the transmission of vibration to building structures or to larger machines or machine parts; and
increase in damping – i.e., a process whereby vibrational energy is converted into heat through some form of frictional mechanism (e.g. constrained or unconstrained layer damping techniques).

Many noise sources such as industrial machinery have a fixed design which may be difficult to modify without reducing its performance or efficiency, however, noise is frequently caused by the turbulent flow of gases and fluids and these types of noise sources can be modified to reduce their noise output. Common examples of these include exhausts and blow-offs of air or steam which cause jet noise. Other common examples include turbulence caused by control valves in pipelines. In most systems the noise emission is directly proportional to the degree of turbulence and many control techniques aim to reduce the velocity and pressure of the fluids and smooth out the flow.

Vibration Analysis/Predictive Maintenance

Predictive maintenance techniques help us to determine and evaluate the condition of plant and equipment in order to predict when specific maintenance tasks should be performed. This approach can help to optimise resources and provide significant cost savings over routine/time-based maintenance. The use of vibro-acoustical analysis and diagnostic systems enable Moloney & Associates Acoustic & Environmental Consultants to utilize advanced technology and condition/diagnostic monitoring methods to assist clients. These methods and technologies can be effectively applied to any sector or industry where rotating machinery is used.

Nuisance and Complaint Investigation

Generally an assessment of noise or vibration is undertaken in response to a specific complaint or a perceived problem or risk. Sometimes complaints arise as a result of a particular sensitivity or occasionally just by locating noisy activities too close to residential areas. The essence of any assessment should be to accurately and objectively characterise the noise emissions. The quality or accuracy of any assessment is only as reliable as the assessor. Identifying reasonable grounds for complaint and the likely causes and solutions, in some instances, will require a high level of expertise.

Very often background noise will need to be considered and reference to BS 4142 and other UK and international guidance documents/standards will be essential. Many situations may be aggravated by the presence of tonal or impulsive components in the noise. The presence of these components should be assessed using standardised methods (e.g., BS 4142 or ISO 1996 and ISO 9612). This is because a tonal and/or impulsive noise has the potential to be more annoying than a broad-band noise. A number of standards make provision for the attachment of a numerical penalty due to the presence of impulsive and/or tonal noise.

ISO 1996 and BS 4142 make reference to a ‘rating level’ which takes account of tonal and impulsive elements in the noise spectrum. The rating level (L_Ar,T) is calculated by adding a penalty to the measured equivalent continuous A-weighted sound pressure level (L_Aeq). The purpose of the rating level is to arrive at a better estimate of the potential community response to the measured noise.

In cases where noise nuisance is alleged, a careful examination of the relevant facts needs to be made. Rather than simply reading a noise level off a sound level analyser and comparing this with a particular limit or guideline value, a number of issues need to be considered such as:

the nature and character of the facility and its neighbourhood
the characteristics of the noise sources at the facility – e.g., is the noise broad-band, tonal and/or impulsive
the location, proximity and sensitivity of NSLs and receptors
the likelihood of noise emissions causing annoyance and/or disturbance
the ambient noise in the absence of noise from the specific facility
the presence or absence of topographical features and/or structures which may help to attenuate the noise emissions
the normal operating times of noise sources at the facility and possible variations or irregular emissions, and
the type of noise mitigation measures adopted at the facility and their success.

In some instances a particular noise limit may apply to a facility, however, we very often need to look beyond such limits. Although in a court of law, compliance with a particular noise limit can prove to be a good defence.

The EPA Act, 1992 made provision for the formal intervention of the courts to adjudicate on certain environmental noise issues. The 1994 ‘Noise Regulations’ (S.I. 179 of 1994) implement certain provisions of the EPA Act and one of the guiding principles is that noise from a facility or premises should be controlled so as to be not so loud, so continuous, so repeated, of such duration or pitch and it should not occur at such times as to give reasonable grounds for annoyance. In addition, some facilities are compelled to use Best Available Technology (BAT) to eliminate and control tonal and impulsive components and to reduce the noise impact to as low as is reasonably practicable.

Guidance on the determination of BAT and on noise control and mitigation measures is provided in an EPA publication - Environmental Protection Agency (2006) Guidance Note for Noise In Relation to Scheduled Activities, Second Edition (Click here to download document). MOLONEY & ASSOCIATES Acoustic & Environmental Consultants, have provided consultancy services to the EPA and were the principal authors of this guidance document.

MOLONEY & ASSOCIATES Acoustic & Environmental Consultants, have extensive experience of noise and vibration and have acted for many industries, local authorities, state agencies and private clients in the investigation of complaints.

In many instances our firm has been involved with quantifying the extent of any problems, diagnosing the cause/s, and developing suitable controls. Some cases lead on to court hearings and our consultants have widespread experience of court appearance.

Building Acoustics

Sometimes noise complaints arise as a result of industrial noise emissions or noisy neighbours. Particularly in apartments, semi-detached or terraced properties, the acoustic performance of the building (sound insulation) may be critical to the extent of the noise problem.

A building acoustics problem will normally require a testing programme to assess the acoustic performance of a wall or floor using international standard methods. Two types of ‘noise insulation’‘ standards arise in assessing the acoustic performance of a building. For Airborne Noise the actual levels measured are expressed in terms of the parameter D_nT,w i.e. the Weighted Standardised Level Difference. This term is defined in the International Standards Organisation methods, ISO 140 and ISO 717. D_nT,w is a single-number quantity of airborne sound insulation performance.

Impact noise in buildings is generated most often by mechanical impact excitation such as foot steps. The movement of furniture and everyday living noises can sometimes cause problems for neighbouring residents. For Impact Noise on floors, the level assessed is expressed in terms of the parameter L’_nT,w - the Weighted Standardised Impact Sound Pressure Level, which is defined in ISO 140 and ISO 717.

The Department of the Environment Building Regulations 1997, Technical Guidance, Document E, Sound, describes levels of sound insulation between new dwellings that are deemed to be ‘reasonable’ and methods for achieving these are suggested.

Workplace (Occupational) Noise

For hundreds of years mankind has known that hearing loss can result from excessive exposure to noise. A Glaswegian surgeon, Thomas Barr, is credited with one of the first controlled epidemiological studies into the effects of occupational noise with a landmark publication in 1886. While Barr noted the characteristic loss of hearing sensitivity at high frequencies, significant progress in the formulation of risk criteria was not made until after World War II. Throughout the EU a comprehensive framework was put in place to control noise at work under the provisions of Directive 86/188/EEC, ‘on the Protection of Workers from the Risks Related to Exposure to Noise at Work’. This Directive was first implemented in Ireland by a set of Regulations in 1990. Since then, a new Directive has come in to force - Directive 2003/10/EC (the ‘Physical Agents (Noise) Directive’). The 2003 Directive has been implemented by the Safety, Health and Welfare at Work (General Application) Regulations 2007 (S.I. No. 299 of 2007).

According to the World Health Organization (WHO) ‘noise-induced hearing impairment is the most prevalent irreversible occupational hazard‘. The following factors influence the response of the human ear to occupational noise: the overall sound pressure level of noise, total duration of exposure, the frequency spectrum of the noise, the sound transmission properties of the ear and individual susceptibility to noise induced hearing loss (NIHL).

European and national legislation require employers (as far as is reasonably practicable) to eliminate risks arising from noise exposure at source or to reduce them to a minimum. There are specified ‘trigger’ or action values in the regulations, i.e., values at which the employer (and employee) become subject to certain obligations.

As part of a comprehensive range of acoustical services, MOLONEY & ASSOCIATES Acoustic & Environmental Consultants, can expertly assess the noise levels in your workplace and provide the necessary guidance and expertise to ensure that you comply with all statutory requirements. For some companies this may just involve a qualified and professional assessment of the levels of noise and exposure. For managing high exposure levels the development of a Noise Control Programme and/or a Hearing Conservation Programme may be advisable. The emphasis should always be on ensuring compliance with statutory requirements, minimising the risk of damage to workers and minimising the liability of employers.

The risk assessment process should be seen as a detailed evaluation of the noise levels and noise exposure in the workplace. In particular we need to determine if and where the noise/exposure could cause harm, so that we can determine what precautions or controls are necessary. The primary intention is to prevent occupational noise exposure and protect against hearing loss. In addition, the potential interaction of noise and other hazards should be considered – e.g., contribution of noise towards accident occurrence, failure to hear warning alarms etc.

The necessary control measures which are required to adequately safeguard all employees against hearing loss must be clearly specified. This is the primary outcome of the risk assessment and all statutory obligations should be fully implemented. In specifying the control measures, the risk assessment provides the company with a practical and detailed roadmap to help manage noise exposure in the workplace.

Click here to download details on the legal obligations arising from certain exposure levels.

Where significant exposure arises, a Hearing Conservation Programme may be warranted. This provides a comprehensive mechanism for managing risks and in many cases a residual level of risk will be unavoidable.

Hearing Conservation Programme (HCP) for the control and management of workplace noise:

Whole Body Vibration (WBV) and Hand Arm Vibration

In recent years, workplace exposure to excessive vibration has become a cause for concern. Hand-arm vibration (HAV) can arise from the use of hand-held power tools and can be the cause of significant ill-health (painful and disabling disorders of the blood vessels, nerves, joints and muscles of the hands and arms).

Whole body vibration (WBV) arises principally from vibrations which are transmitted through the seat or the feet by workplace machines and/or vehicles. Drivers of certain mobile machines, including tractors, fork-lift trucks and quarrying or earth-moving machinery, may be exposed to WBV and shocks, which are associated with back pain. Other work factors, such as posture and heavy lifting, are also known to contribute to back problems for drivers and the relative importance of WBV is not entirely clear. However, research is currently underway worldwide on the specific health effects of WBV.

Similar to noise, there are clear statutory requirements provided for the management of vibration risks and exposure action values and limit values have been prescribed in national and in EU legislation. A thorough risk assessment report will include details on the following key issues:

Determine if action values and limit values are exceeded.
Is there a vibration exposure problem or issue?
How significant is the problem?
What employees are at risk?
Do we need to take immediate steps - which ones and where?
How should we prioritise our controls?
The assessment should provide a blueprint for action or a justification for inaction.
Consider the effects of exposure – direct and indirect (interactions).
Address the availability of alternative equipment or processing techniques to avoid exposure?
Address exposure beyond normal working time?
Consider health surveillance and published data.
Address the control, monitoring and health surveillance arrangements required.
Record the findings of the assessment and the effectiveness of any controls.

Within the regulations for the management of vibration exposure, it is recognised that not every situation will require on-site surveys and many companies may be able to undertake a risk assessment for vibration without doing their own measurements. In many cases, expert assistance or guidance will be required and MOLONEY & ASSOCIATES Acoustic & Environmental Consultants, have acquired a broad range of vibration assessment experience throughout industry. We have guided many clients through the risk assessment process and have developed appropriate controls, where necessary.

Assessments and applications for planning control and environmental permits and licences

At the planning stage of a development noise and vibration assessments are often necessary. The sooner an acoustics specialist can get involved the better - as often some preliminary considerations can help to identify potential problems. These can often be eliminated with good acoustical design and engineering. In addition, effective plant layout, orientation and selection can sometimes eliminate off-site impacts and issues.

Working as part of a team, MOLONEY & ASSOCIATES Acoustic & Environmental Consultants, have undertaken many assessments and have assisted developers and industries to advance their proposals in a sustainable manner. We have improved the design of facilities to eliminate problems at source and we have supported applications for planning permission and environmental permits and licences. In situations where significant impacts have been predicted, mitigation and control measures have been developed.

While predominantly acting on behalf of developers and industries, we have provided expert acoustical guidance to planning and licensing authorities throughout Ireland.