Noise Control

By Shahram Taherzadeh

Designed to accompany the new Open University course in Environmental Monitoring and Protection, this is one of four new titles which will equip the reader with the tools to undertake Environmental Impact Assessments (EIAs). Used in planning, decision-making and management, EIAs review both the theoretical principles and environmental considerations of engineering and environmental projects to help steer fundamental legislation in the right direction. This book will cover the basic principles and concepts of sound and sound propagation, covering units, criteria and indices.  It considers noise propagation and attenuation, before leading on to assessment methods for both industrial and transport noise. It includes models for predicting sound levels both indoors and outdoors, and details methods for noise control and abatement.

Discover our e-book series on Environmental Monitoring and Protection, published in partnership with The Open University!
Find out more about the series editors, the titles in the series and their focus on water, noise, air and waste, and The Open University courses in Environmental Management.
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• Language: English
• Publisher: Wiley
• Release date: Mar 20, 2014
• ISBN: 9781118863855

Book preview

Noise control

Edited by Shahram Taherzadeh

Published by:

John Wiley & Sons Ltd

The Atrium

Southern Gate

Chichester

West Sussex

PO19 8SQ

in association with:

The Open University

Walton Hall

Milton Keynes

MK7 6AA

First published 2014.

Copyright © 2014 The Open University

Cover image © Artur Marciniec/Alamy

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, transmitted or utilised in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without written permission from the publisher or a licence from the Copyright Licensing Agency Ltd. Details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS (website www.cla.co.uk).

Edited and designed by The Open University.

This publication forms part of the Open University module T868 Environmental monitoring and protection. Details of this and other Open University modules can be obtained from the Student Registration and Enquiry Service, The Open University, PO Box 197, Milton Keynes MK7 6BJ, United Kingdom (tel. [phone]; email [e-mail]).

www.open.ac.uk

British Library Cataloguing Publication Data:

A catalogue record for this book is available from the British Library.

Library of Congress Cataloging-in-Publication Data:

A catalog record for this book has been requested.

ISBN 978 1 1188 6385 5

1.1

Contents

Section 1: Noise basics

1.1  Introduction

1.2  The nature of sound

1.3  Power, pressure and intensity

1.4  The decibel and weighting

1.5  Adding sound levels

1.6  Summary

Section 2: Analysing sounds

2.1  Introduction

2.2  Analysing steady sounds

2.3  Point sources and line sources

2.4  Directionality

2.5  Sound power level

2.6  Sound in rooms: reverberation and absorption

2.7  Analysing unsteady sounds

2.8  Summary

Section 3: Outdoor sound

3.1  Introduction

3.2  Geometric divergence (Adiv)

3.3  Atmospheric absorption (Aatm)

3.4  Ground absorption (Aground)

3.5  Barriers (Abar)

3.6  Other factors

3.7  Transport noise

3.8  Summary

Section 4: Noise control at source

4.1  Introduction

4.2  Choosing which source to control

4.3  Control of noise by design or choice of process

4.4  Isolating structure-borne vibration

4.5  Enclosures

4.6  Frequency dependence of noise reduction

4.7  Summary

Section 5: Control between source and receiver

5.1  Introduction

5.2  Active noise control

5.3  Indirect sound paths

5.4  Absorption and absorbing materials

5.5  Barriers

5.6  Summary

Section 6: Control at the receiver

6.1  Introduction

6.2  Sound insulation of dwellings

6.3  Ear protection

6.4  Summary

Glossary

References

Acknowledgements

Section 1: Noise basics

1.1  Introduction

Any work done by a moving device is inevitably accompanied by energy conversion and degradation. A small fraction of the work done is not converted directly into useful work or heat, but is radiated as sound. Eventually this in turn becomes heat, which is the ultimate fate of all work.

‘Noise’ is unwanted sound. Sound accounts for a very small fraction of the mechanical energy transformed during any process, so noise control cannot be justified solely on the grounds of minimising waste. On the other hand, a noisy process is often wasteful or inefficient.

The ability to make and detect sound allows humans to communicate with each other and receive useful information from the environment. Sound can give warnings (e.g. fire alarm), useful information (e.g. radio news broadcast) and enjoyment (e.g. music). Unwanted or extraneous sound can interfere with all these.

High noise levels in industry are unwanted because they are hazardous to hearing, hinder communication and cause unnecessary stress. Noises associated with environmental sources such as transport systems and industrial plant are unwanted because they impose a burden of annoyance, distraction, intrusion and interference upon people who receive no immediate or direct benefit from the noise-producing system. This can be true of any sound in any context, but there is a greater variety of artificial sound sources now than ever before.

The primary effect of prolonged exposure to high levels of noise in the workplace is the development of industrial or occupational deafness. This results in damage to the inner ear. It is often called noise-induced hearing loss (NIHL) to distinguish it from hearing loss due to increasing age or resulting from particular diseases of the hearing system. NIHL results from long-term exposure to noise, of the order of a working lifetime, although sufficiently intense impulsive noise could produce instantaneous damage known as ‘blast deafness’. Short periods of exposure to high noise levels produce temporary hearing loss, followed by a gradual recovery.

Vasoconstriction – a narrowing of the blood vessels, which reduces the flow of blood to various parts of the body – is a well-documented circulatory response to noise (Levak et al., 2008; van Kamp and Davies, 2008; Sørensen et al., 2011). It is a ‘startle reaction’.

There are other physiological effects of sudden or transient sounds, which may be related to natural responses and reflexes to audible warning signals. These include:

muscle tension

change in heart rate

general constriction in the peripheral blood flow (e.g. to the skin)

changes in the secretion of saliva and gastric juice

reflex movements of the gastrointestinal tract.

There is some evidence to suggest that prolonged exposure to intense noise may affect digestion. It is now considered conceivable that some disorders (especially cardiovascular disorders) and increase of susceptibility to disease are caused or accelerated by exposure to high levels of noise (van Kamp and Davies, 2008).

Interference with rest or sleep and the consequent irritability, reduced efficiency or lack of concentration are obvious and annoying effects of noise. The effect of such sleep disturbance is not readily either identifiable or quantifiable, but is most likely to be significant among the older age groups. Note that it is not necessary for people to be woken up to suffer loss of the correct type of sleep.

One of the most common and undesirable effects of noise is interference with communication. In industry, this can result in inefficiency and accidents. In people’s homes and during music-based leisure activities, loud noise might make speech unintelligible and warning sounds inaudible. At the least, speech might be less easily understood and warnings unheard. Speech interference is a particularly important consideration in educational establishments.

Even at levels where the risk to hearing is not large, noise in the working environment can affect concentration, efficiency and output. Most annoyance with noise in the home seems to be associated with its impact on conversation or enjoyment of radio and television. One person’s music is another person’s noise – according to George Bernard Shaw (1856–1950), ‘Hell is full of musical amateurs’.

Advances in music reproduction and amplification have led to widespread nuisance from amplified music. Complaints to local authorities about noise nuisance, rowdy parties and noisy neighbours have been increasing dramatically in some areas of the UK (DOE, 2012).

Exposure to unaccustomed high levels of noise tends to change our emotional responses. We tend to become more agitated or less reasonable than usual. There is some evidence, but not conclusive, that noise is associated with mental illness (Stansfeld and Matheson, 2003). It may lead to general psychological distress, and this in itself will mean an increased susceptibility to noise, which may precipitate some sort of crisis.

A series of studies carried out by researchers in London has shown a strong correlation between traffic noise and reduction in the concentration levels and learning ability of primary school children (Shield and Dockrell, 2002; Shield et al., 2010). Furthermore, a report commissioned by the World Health Organization and the European Union concludes:

There is sufficient evidence from large-scale epidemiological studies linking the population’s exposure to environmental noise with adverse health effects. Therefore, environmental noise should be considered not only as a cause of nuisance but also a concern for public health and environmental health.

(WHO, 2011, p. xvii)

We should not, however, aim for the complete elimination of noise, even if it were possible. Complete silence for prolonged periods can be very disturbing since it is a form of sensory deprivation. Moreover, the noise of industrial machinery or of vehicles can often be used to judge their general working order.

Society’s recognition of the problems caused by noise has resulted in legislation to control noise in the workplace, and to protect people living alongside roads and near airports exposed to high levels of noise. It has also led to widespread local authority powers to control noise nuisance.

This section of Noise control will cover the basics of sound, introducing some of the mathematical formulae used to calculate different aspects of sound levels and their effects. Subsequent sections will focus on noise pollution and the monitoring of noise, its health and environmental effects, and methods of control, with reference to relevant legislation.

The self-assessment questions (SAQs) located throughout the text will help you to review and remember what you have read.

1.2  The nature of sound

Since noise is unwanted sound, to understand the ways in which noise can be measured and how the measurement depends on where we measure it, it is necessary to know something about the nature of sound.

To grasp a number of concepts necessary for understanding the nature of sound, it is worth considering ripples on the surface of water and thinking about the similarities to and differences from sound waves.

If you throw a stone into a pond, you will see ripples (water waves) spread out across the water (Figure 1). The ripples seem to travel outwards, in ever-expanding circles, from the point where the stone hit the surface of the pond. The speed at which the waves spread does not depend on the size of the stone or the splash. Rather, the waves move at a constant speed (depending on the depth of the pond). Another important point to note is that water molecules do not travel outwards with the ripples at all, but move up and down (oscillate).

Copyright © Robert Harding Picture Library/Alamy

Figure 1  Water waves (ripples) spreading out as circles from a point

View description

The water waves discussed above are on the surface of the pond, i.e. they are two-dimensional. When a source creates sound waves, the waves spread out from the source in three dimensions. The situation is similar to that of the stone in the pond, but the waves spread out in spheres rather than circles. The speed with which the waves spread on the water does not depend on the size of the stone; similarly, the speed of sound waves does not depend on how loud the sound is, but is a property of the medium through which the waves are travelling.

Sound is a form of energy. The sensation that we call sound is caused by the interaction between small pressure variations in the air around us and our hearing mechanism. The small pressure variations are associated with the oscillations of the molecules of which air is composed. These oscillations originate with the disturbance of air molecules adjacent to any vibrating surface in air – which we call a sound source – and are transmitted subsequently through the air from molecule to molecule, as a consequence of the forces between these molecules. As with the water waves, the molecules are not carried along with the disturbance – they simply oscillate to and fro about their equilibrium (or average undisturbed) positions as the sound wave disturbance passes.

Note that the pressure variations involved in sound are usually very small. Pressure is measured in newtons per square metre (N m−2), usually called pascals (Pa). Typical pressure variations in a sound wave are a small fraction of a pascal, whereas atmospheric pressure has a typical value of about 100 000 Pa or 100 kPa.

1.2.1  Frequency and wavelength

Consider the simple example of a pistonphone as a sound source. A pistonphone is a device used to calibrate microphones and sound measurement or recording systems. It consists of a piston that can be driven backwards and forwards in a regular manner at one end of a circular cylindrical tube. As the piston is driven forward, the air molecules next to its surface will bunch together, producing a high-pressure region – a compression. As the piston moves backwards, the air molecules next to its surface will spread out, producing a low-pressure region – a rarefaction.

If the forward-and-backward movement of the piston is made continuous and regular, a series of regularly spaced compressions and rarefactions will form and travel along the tube (Figure 2). In other words, there will be a pressure wave in the tube (as shown at the bottom of the figure). If you were to put your ear to the opposite end of the tube to the piston, and the piston were to be moving rapidly enough, you would hear this pressure wave as sound.