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Noise Monitoring [Dec 2010]

Published: 01st Mar 2011 in OSA Magazine

This article explores the background to health and safety at work with respect to hearing, an often under estimated problem, not only because it is not immediately,apparent when we put our hearing at risk – the symptoms can occur years later – but also because of the consequences hearing loss can have upon our lives.

Introduction

Since the industrial revolution the noise climate in which people live and work has changed, bringing regular exposure to higher levels of noise. However, human hearing has not developed to perform optimally or protect itself within the more noisy environments this change has brought, which is where the potential for damage arises.

In recognition of the potential effects of human exposure to noise, the World Health Organization has described noise-induced hearing loss as ‘the most
prevalent irreversible industrial disease’1.

Occupational hearing damage will be explored from the perspective of understanding sound and how it is quantified, typical legislative methods of prevention, how potential risks to our hearing may be identified and what can be done about it. This article has referenced criteria adopted in the European Union which are similar to those adopted elsewhere. However, the interested reader is advised to seek out Regulations specific to their own country.

Physiology

The human ear is made up of three main sections as shown in Figure 1:

• The outer ear (pinna) made up of cartilage and skin on the side of the head leading to the ear canal up to the ear drum

• The middle ear containing the auditory ossicles (bones)

• The inner ear (cochlea) containing fluid and hair cells connected to the auditory nerve. It is used for both hearing and balance

Noise in air creates fluctuations in air pressure received in the ear canal which causes the ear drum to vibrate. The ossicles attached to the ear drum amplify
and transmit vibration to the fluid of the cochclea and hair cells which send impulses to the auditory nerve.

Muscles within the ear can contract to protect it from large vibrations due to noise. However, vibration can overstimulate the hair cells from large or frequent vibrations until the nerve cells die resulting in noise-induced hearing loss.

Damage can occur progressively through frequent exposure to high levels of noise and it may also occur as a result of a very high peak in sound pressure such as gunfire.

The effects may be experienced as a temporary shift in the hearing threshold which subsides after a few hours.

However, prolonged exposure to noisy environments can result in permanent damage to the hair cells and a permanent threshold shift, which is currently incurable.

Hearing loss affects an individual’s response to high frequencies and is perceived as a muffling of sound and a reduction in loudness. This impairs the
recognition of consonants in speech. Generally, vowel sounds have lower frequency characteristics and are less affected although more speech
frequencies become affected through progressive deterioration.

Hearing loss is also a symptom of the ageing process (presbyacusis). This also affects the response at high frequencies and when this is combined with noise induced hearing loss the problems are more acute.

Exposure to high levels of noise can also cause a perceived constant shrill whistling or buzzing called tinnitus. The mechanism for this ringing is not fully understood.

Psychological effects may develop as a result of hearing damage including distress, disturbed sleep, problems with communication, concentration and social isolation.

Sound

There are various mechanisms by which sound is produced, but probably the most often encountered in environmental noise is radiation by vibrating surfaces, essentially acting like loudspeakers, and aerodynamic noise caused by turbulence and air flow. Examples of aerodynamic sound sources might be fans, exhausts and combustion while examples of mechanical sound sources might be presses, punches, shearing machines, piling rigs and engines.

Acoustic pressure oscillates about the mean or atmospheric pressure and in the audible pressure range this is between 0.00002Pa and 200Pa. In order to compress the scale (for meaningful use) and because the ear responds to sound pressure waves in a non linear, logarithmic way the decibel scale is applied to sound measurements. The decibel describes the logarithm of a power or intensity ratio, and when applied to acoustics considers the ratio of the sound pressure being measured to that of the threshold of hearing (po) thus: dB = 20 log(p / po ) = 10 log(pÇ / poÇ)

The dB scale exhibits some convenient properties. A doubling of source strength (such as two pneumatic drills instead of one) generates an increase of 3dB,
whatever the starting level, thus two sounds each of 60dB will add to give 63dB. Where one sound is dominant over another, the increase will be small, for
example 60dB + 50 dB = 60.4dB.

Subjective loudness also follows non-linear rules; for example an increase of 1dB would be just noticeable under ideal conditions, an increase of 3dB is generally considered to be noticeable while a 10dB increase is required to sound subjectively twice as loud.

In addition to the non-linear response to sound amplitude, the ear also responds in a non-linear way to frequency (Hertz). The lower limit of human hearing is 20Hz and the upper limit is 20000Hz (20kHz), but the sensitivity of human hearing varies with frequency.

Equal loudness contours relate perceived loudness to frequency, with reference to that at 1kHz. From the original equal loudness curves in Figure 2, known as Fletcher–Munson curves 4, it’s clear humans are most sensitive to frequencies between 1kHz and 5kHz and less sensitive to frequencies below this range. Hence a filter circuit is applied to the output of measuring instruments to simulate this response. This characteristic which is now generally used for environmental noise is known as an ‘A’ weighting, and is generally referred to with the suffix dB(A). The ‘A’ weighting curve resembles the inverse of the 40 phon contour in Figure 2.

Typical noise sources may be measured and examined in terms of both time history and frequency distribution. Frequency distribution may be shown as
octave bands or one-third octave bands where the audible frequency range divided into 8 or 24 frequency bands as shown in Figure 3 5. Alternatively finer resolutions are readily available down to resolutions approaching 1Hz. Full spectrum analysis is particularly useful for picking out tonal components.

The way sound is perceived is affected by the amplitude and rate or frequency of the fluctuations in air pressure. For example, a fan rotating slowly creates
low frequency pressure waves. Therefore, energy in the resulting sound is also emitted at a low frequency. The rate that this energy is emitted can increase
in proportion to the speed of rotation/ frequency. Where the mechanism of noise generation concentrates the energy in a very small frequency range (for example a ringing bell) then a clear tone is audible.Within typical industrial environments, the generation and characteristics of noise are complex and the energy content varies over a range of frequencies and with time.

Under the UK’s Control of Noise at Work Regulations (CNAWR) 6, the assessment of exposure to high peak noise levels is based on the exposure to noise expressed as ‘C’ weighted values. At higher levels of noise the relative acuity in the 1 to 4 kHz range diminishes and the sensitivity is more uniform across the audible range, which is reflected in the ‘C’ weighting network. The ‘C’ weighting curve resembles the inverse the 100 phon contour in Figure 1.

Legislation

In the UK, research into auditory health effects due to industrial noise was advanced by the work of Burns and Robinson in the 1970s 7. They demonstrated that the degree of hearing loss was proportional to the levels of noise energy and duration of exposure. This allowed limits of acceptability in terms of exposure to be defined. Based on this, the Department of Employment issued a Code of Practice that sought to use this in preventing industrial hearing loss. The adoption of EEC directives in the 1980s led to the Health and Safety Executive’s introduction of the Noise at Work Regulations which came in to force in 1990.

In 1997, the Institute of Noise Control Engineering carried out a review of limits for occupational noise that have been adopted by various countries. This
was published by the World Health Organization 8 and showed that for energy averaged exposure a warning limit of 85dB(A) and a danger limit value of 90dB(A) were consistently adopted by the majority of countries. A limit on peak pressures of 140dB(C) was also a common criterion.

The European Council Directive Physical Agents (Noise) 2003/10/EC 9 lowered these limits which were referred to as ‘exposure action and limit values’. This was implemented in the UK by the CNAWR which places duties on employers to minimise the risks from the effects of noise in the workplace. Employers are
required to:

• Assess the risks to employees from noise

• Take action to minimise the exposure to noise

• Provide hearing protection where the exposure cannot be adequately reduced

• Ensure that the exposure limit values are not exceeded

• Provide employees with information and training

• Carry out hearing surveillance if appropriate

The Regulations also place a legal duty on employees to comply with instructions given by their employer on ensuring protection and in meeting their obligations.

Employers are required to take action where the daily or weekly personal exposure of employees to noise, and the peak levels in noise, are found to exceed levels described as ‘exposure action values’. Exposure is usually determined through the measurement of noise levels that employees are exposed to throughout the working day.

The degree of action required depends on whether the lower or upper exposure action values are exceeded. ‘Exposure limit values’ are also defined that specify the level of personal exposure that must not be exceeded allowing for any reduction in noise through the use of hearing protection. Table 2 presents the exposure action and limit values for daily or weekly exposure and peak sound pressure.

Table 2. Exposure action and limit values set out in CNAWR

Undertaking an assessment

Noise assessments need only consider employees who may be exposed to harmful levels of noise. The CNAWR provides the following guide to indicate
whether this is likely:

• At 90dB(A) it is necessary to shout to speak with someone 1 metre away

• At 85dB(A) it is necessary to shout to speak with someone 2 metres away

• At 80dB(A) noise is intrusive but conversation is possible

Where employees may be exposed to noise levels of 80dB(A) and above then a formal, documented assessment should be undertaken by a qualified practitioner.

Most assessments are based on the direct measurement of the noise levels received at the ear position of employees. This can be carried out using a personal noise dosemeter where the employee wears a microphone connected to small logging sound level meter (dosemeter) which is carried throughout a normal working day. The employee’s activities during the survey are noted so that the recorded levels can be associated with individual tasks. This helps to identify activities that present the greatest risks so that measures to minimise exposure are focused on these tasks.

The Health and Safety Executive publish guidance on applying practical and cost effective noise control techniques 10. The following options to minimise noise within a machine shop, for example, could be considered bearing in mind other factors such as access and fire risk:

1. Damping materials could be applied to existing machine guards and panels to reduce the resonant response to vibration and its radiation as airborne noise.

2. Gaps in guards and panels that allow noise to enter the workplace should be minimised by introducing or repairing flexible seals where possible.

3. Absorptive materials used to line the inside of panels can be effective at reducing the build up of reverberant noise.

4. Anti-vibration mounts fitted to the support of motors, pumps and gearboxes can be used to isolate it from the supporting structure and minimise the
transmission of energy which can also be radiated into the workplace.

The effectiveness of measures should be verified by repeating the assessment of exposure to noise in the machine shop after the measures have been applied. If this has not been effective then other approaches are suggested as follows:

• Identification of particularly noisy plant for replacement with quieter alternatives

• Changes in the working patterns of operators to limit time within the machine shop for example by job rotation

• Investigate the use of screens and refuges and whether changes in the layout of the area or the relocation of plant could be effective and feasible

Personal Protective Equipment (PPE)

The use of hearing protection to limit exposure should only be considered where additional protection is needed once all practical measures to minimise levels of noise in the workplace are effective. It is preferable to develop a methodical approach to noise control – reducing noise at source, for example.

The two main types of hearing protection are ear muffs and ear plugs. 

Careful selection of hearing protection is important to ensure that the performance of the product is compatible with the noise climate in which it is used. Selection should be based on an assessment that is informed by an accurate record of the level and duration of the employee’s exposure to noise during tasks. The minimum specification of any hearing protection can then be determined in order to reduce exposure to within safe limits.

It should be noted that the benefit of any muffs or plugs may be reduced if it is incorrectly specified, poorly fitted, worn, damaged or removed intermittently.This emphasises the need to ensure employees are adequately trained in fitting and using any specified PPE and that it is appropriately maintained and replaced.

The use of hearing protection that is too effective in reducing noise levels should be avoided so that the wearer does not become isolated from their surroundings. This is called ‘over-protection’ and is expected to occur where the level at the ear falls below 70dB(A). This can endanger the wearer by masking audible warnings in the workplace such as alarms, approaching vehicles or catastrophic equipment failure.

Hearing protection is available that has been designed to significantly reduce exposure at most frequencies but provides audibility at frequencies that aid the identification and location of warning signals.

Practical issues such as compatibility with other PPE (for example hard hats) and user comfort should be considered to enable employees to make effective use of PPE.

Summary

Noise is an everyday phenomenon which is generally tolerated. Hearing damage is something that can occur due to a sudden, extreme noise event but the
majority of problems arise as a result of prolonged, regular exposure to high noise levels. As a result, it is sometimes difficult to recognise the associated
hazards. The exposure of workers to noisy environments has the potential to cause hearing damage unless adequate precautions are not taken.

The consequences of noise-induced hearing loss can lead to psychological effects that may severely impair health and quality of life. Hearing damage cannot be repaired but if the symptoms of progressive deterioration are spotted early enough then measures to limit further damage can be introduced.

Employers are legally obliged to ensure that the exposure of their employees is within safe limits and have a duty to minimise noise levels. Organisations are also liable to compensation claims from employees who have suffered hearing damage if the legal obligations have not been met.

This report has outlined an approach for assessing workplace noise. It is based on investigating the levels of exposure and time taken in undertaking daily tasks. This is used to ascertain whether limits are exceeded and prioritise any action to be taken. Where there is a need to reduce exposure then practical measures should always be considered in preference to the blanket provision of hearing protection. 

Author details:

Andrew Monk-Steel is a member of the Institute of Acoustics and a Chartered Engineer with 12 years’ experience of noise and vibration in the rail, automotive, environmental and wind farm industries.

Andrew Monk-Steel is a Senior Acoustic Engineer with Mott MacDonald in Southampton, England, and holds the Institute of Acoustics Certificate of Competence in Workplace Noise Assessment. With contributions from Max Forni, Derek Mackay, Maureen Marsden, Tim Mason and Dan Doherty at Mott MacDonald (www.mottmac.com).

References
1 World Health Organization ‘Prevention of noise-induced hearing loss: Report of an informal consultation’ (1997)

2 F Alton Everest and KC Pohlmann, ‘Master Handbook of Acoustics’ (2009)

3 C Morfey, ‘Dictionary of Acoustics’ Academic Press ISBN 0 12 506940 5b (2000)

4 H Fletcher and W.A. Munson, ‘Loudness, it’s definition, measurement and calculation’, Journal of the Acoustical Society of America 5, 82-108 (1933)

5 The Environment Agency, Horizontal Guidance for Noise Part 2 Noise Assessment and Control (2002)

6 Health and Safety Executive, ‘Controlling Noise at Work: The Control of Noise at Work Regulations 2005’. ISBN 0 7176 6164 4 (2005).

7 W Burns and DW Robinson, ‘Hearing and Noise in Industry’ HMSO Second impression SBN 760022 9 (1970)

8 World Health Organization, ‘Occupational exposure to noise: evaluation, prevention and control’ Edited by B Goelzer, CH Hansen and GA Sehrndt.

9 Directive 2003/10/EC of the European Parliament and of the Council of 6 February 2003 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise) (2003) 10 www.hse.gov.uk/pubns/top10noise.pdf

Andrew’s contact details are:
andrew.monk-steel@mottmac.com

www.osedirectory.com/health-and-safety.php

Published: 01st Mar 2011 in OSA Magazine

Author


Andrew Monk-Steel


Andrew Monk-Steel is a member of the Institute of Acoustics and a Chartered Engineer with 12 years’ experience of noise and vibration in the rail, automotive, environmental and wind farm industries.

Andrew Monk-Steel is a Senior Acoustic Engineer with Mott MacDonald in Southampton, England, and holds the Institute of Acoustics Certificate of Competence in Workplace Noise Assessment.
 


Andrew Monk-Steel

Website:
http://

Email:
andrew.monk-steel@mottmac.com

andrew.monk-steel@mottmac.com
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