Noise Control E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 217

Veröffentlichungsjahr: 2014

Beschreibung

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. Visit href="http://www.wiley.com/go/ouebookseries">www.wiley.com/go/ouebookseries

Leseprobe

Noise control

Edited by Shahram Taherzadeh

Published by:John Wiley & Sons LtdThe AtriumSouthern GateChichesterWest SussexPO19 8SQin association with:The Open UniversityWalton HallMilton KeynesMK7 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 [email protected]).

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 basics1.1  Introduction1.2  The nature of sound1.3  Power, pressure and intensity1.4  The decibel and weighting1.5  Adding sound levels1.6  SummarySection 2: Analysing sounds2.1  Introduction2.2  Analysing steady sounds2.3  Point sources and line sources2.4  Directionality2.5  Sound power level2.6  Sound in rooms: reverberation and absorption2.7  Analysing unsteady sounds2.8  Summary Section 3: Outdoor sound3.1  Introduction3.2  Geometric divergence (Adiv)3.3  Atmospheric absorption (Aatm)3.4  Ground absorption (Aground)3.5  Barriers (Abar)3.6  Other factors3.7  Transport noise3.8  Summary Section 4: Noise control at source4.1  Introduction4.2  Choosing which source to control 4.3  Control of noise by design or choice of process 4.4  Isolating structure-borne vibration4.5  Enclosures 4.6  Frequency dependence of noise reduction 4.7  Summary Section 5: Control between source and receiver5.1  Introduction5.2  Active noise control5.3  Indirect sound paths5.4  Absorption and absorbing materials5.5  Barriers 5.6  SummarySection 6: Control at the receiver6.1  Introduction6.2  Sound insulation of dwellings6.3  Ear protection6.4  SummaryGlossaryReferencesAcknowledgements

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 tensionchange in heart rategeneral constriction in the peripheral blood flow (e.g. to the skin)changes in the secretion of saliva and gastric juicereflex 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.

Figure 2  The effects of repeated piston movements (vibrations) on air molecules within a tube

View description

The length of time occupied by a single cycle of compression and rarefaction will influence what we judge to be the pitch of the sound that we hear from the pistonphone. One measure of the length of time occupied by each compression–rarefaction cycle is the number of such cycles that occur in each second. Driving the piston through, say, 100 to-and-fro cycles per second will produce a sound in which 100 complete waves pass a point every second. This measurement is known as the frequency of the sound and is the same as the rate of vibration of the piston in the pistonphone – in this case 100 hertz, usually abbreviated to 100 Hz. The more rapidly the piston vibrates, the higher the frequency will be.

The distance between two successive compressions (or rarefactions) is called the wavelength of the sound. Frequency and wavelength are related by the equation:

(1)

frequency × wavelength = speed of sound

The lowest frequency of sound that someone with normal hearing can hear is about 20 Hz and the highest is about 20 000 Hz, which can also be written as 20 kilohertz or 20 kHz. A pistonphone like the one in Figure 2 is an example of an artificial single-frequency sound source: it produces a pure note with a single frequency – a pure tone – and so can act as a tuning fork. Such sounds are very rare. Even sounds from a musical instrument that appear to have a single pitch, an apparently clear single note, in fact have a complicated mixture of different frequencies. Middle C, for example, is a note with a frequency of 261.626 Hz, but middle C on a piano sounds very different from middle C on a pistonphone.

Usually our impression of a sound’s pitch will correspond to the frequency at which there is most sound power (see Section 1.3) – known as the dominant frequency. Thus ‘middle C’ is the name given to any note with a dominant frequency of 261.626 Hz. Instrumental ensembles tune, usually, to a concert A of 440 Hz. The lowest and highest notes on an 88-key piano have dominant frequencies that are 27.5 Hz and 4186 Hz respectively. This set of frequencies might be called the range of frequencies on the piano.

1.2.2  Speed

The speed at which the sound wave propagates down the tube does not depend on the rate at which the piston moves; it is a fundamental property of air and is related to both the forces between the air molecules and the density of the air. These properties are themselves affected by temperature and pressure, although the corresponding effect of pressure on sound speed is marginal. For most purposes the speed of sound is assumed to be 343 m s−1, which is its value in dry air at 20 ºC.

Sound travels much slower than light, which has a speed of 300 000 km s−1. The difference explains why thunder seems to lag well behind the lightning that causes it. A flash of lightning at a distance of 1 km from an observer will be associated with thunder that is heard 1000/343 seconds (i.e. about 3 seconds) later.

This text is concerned mostly with sound waves in air, but sound can travel through any compressible medium. The denser and less compressible the medium, the higher the speed of sound in the medium. That is why the sound of an approaching train is heard first from the vibration of the rails before one can hear the train itself – the sound travels faster through the rails than it does through the air.

Table 1 gives the speed of sound in various materials. Although values for the speed of sound in steel and lead are provided in this table, it is important to note that these speeds apply only to compression waves of the sort I have described for air. In solid structures, different kinds of waves called shear waves can propagate at different speeds. The possibility of shear waves is an important consideration in noise control in buildings, in particular with regard to paths of noise between rooms that have structural links.

Table 1 Speed of sound in various materials

MediumSpeed of sound (m s−1) Air at standard temperature343Lead~1300Water~1500Steel~5000

1.3  Power, pressure and intensity

In a pistonphone, the greater the displacement of the piston, the greater the pressure changes in the associated sound wave. Consequently, as the power of the piston motion increases, so does the power of the resulting sound. In other words, the sound power of a sound source depends on the power input to the source. Both are expressed in watts (W). Sound power is an intrinsic property of a source, and is unaffected by where the source is placed. However, the speed at which sound propagates does not depend on the sound power; it only depends on the properties of the medium.

The mechanical power rating of mechanical devices gives some idea of their sound power output. Unfortunately a wide range of outputs is possible for a given type of device, depending on whether its basic design is quiet or noisy. For example, a gas turbine can convert between 2 × 10−6 and 5 × 10−5 of its output power to sound – values that are different by a factor of 25. This example also demonstrates that even for the noisiest of machines or processes, only a very small fraction of the mechanical power is converted into sound. Even loudspeakers, which are designed to produce sound, have a maximum conversion factor of only 5–10%. The production of noise cannot therefore be regarded as a waste of energy, although a noisy appliance is often an inefficient one. For example, a failed bearing may produce a lot of noise but it is the increased friction that uses up the additional energy, not the relatively minute amounts required to make the noise.

It is difficult to measure the sound power of a sound source directly. The quantity that can be measured is the air pressure or, more strictly, the difference between the fluctuating value of air pressure and its equilibrium or mean value. Usually the equilibrium value is atmospheric pressure.

Suppose that some means of measuring the pressure fluctuations is placed in the tube of a pistonphone. Figure 3(a) shows how a plot of the variation of pressure difference with time might look. Sometimes the point of measurement will be in a region of compression and at other times in a region of rarefaction. In the rarefaction, the values of pressure difference are negative. This explains the maxima and minima on the curve of pressure difference with time. The maximum value of pressure difference is called the amplitude of the sound wave. The amplitude is indicated in Figure 3(a) together with one complete cycle of pressure-difference values. As the piston vibrates more and more rapidly, the readings on the pressure detector would cycle more and more quickly.

Figure 3  (a) Graph of pressure against time for a single-frequency plane sound wave, showing values of instantaneous pressure and maximum pressure compared with atmospheric pressure and squared and root-mean-square pressure difference; (b) graph of more rapid variation of pressure with time

View description

Another term used in describing an oscillating quantity is phase. It is the relationship in time between the points of an oscillating or repeating quantity (the sound wave in our case) and a fixed reference point, say the start of the wave, or another system. For example, two waves that are exactly aligned are said to be in phase.

Figure 3(b) shows the result of increasing the rapidity of vibrations by a factor of three (in other words, creating a sound wave of the same amplitude but at three times the frequency of that depicted in Figure 3a). As the rapidity of vibration increases, the pressure variation becomes more and more difficult to follow. Even for the slowest rate of vibration that produces audible sound, some kind of average over time is required as a measurable indication of amplitude.

To get this average:

Square every value of the pressure difference over any cycle; this has the effect of making negative pressure differences into positive values. Calculate the arithmetic mean of these squared values.Take the square root of the calculated mean.

The consequences of each of these steps are shown in Figure 3(a). The final result is known as the root mean square (rms) of the sound pressure difference. The term ‘sound pressure’ in this section refers to the rms pressure difference.

Human ears respond to the small variations in sound pressure in a sound wave, like the microphone in a sound level meter. An ear converts sound pressure variations into electrical stimuli in the auditory nerve, which are then interpreted by the brain. Similarly, a microphone converts sound pressure variations into an electrical signal. This signal is amplified and can be recorded or rebroadcast.

Sound intensity represents the power transmitted per unit area at right angles to the direction in which the sound is propagating. The units of sound intensity are watts per square metre (W m−2). As long as the sound wave is not very close to its source and is able to propagate without hindrance from any obstructions, its intensity I is proportional to the square of the sound pressure, with the constant of proportionality being the inverse of the air density multiplied by the speed of sound. This is expressed as:

where

p is the pressure in pascals ρ is the density of the air c is the velocity of sound.

The value of ρc is approximately 410 in air at normal temperature and pressure (i.e. 293 K and 1 atm), giving:

(2)

So, for example, if the rms sound pressure registered at a point is 10−4 Pa, the sound intensity of the unhindered sound wave at that point is given by:

Measured sound intensity is directional – it depends on the direction of a sound source with respect to the measuring position. In the absence of any sound reflections, if a detector is pointed at a source, it will give a much higher reading than if it is pointed at right angles to the source. If pointed away from the source it will give a negative reading. Consequently, measurement of sound intensity is a useful way of locating noise sources.

SAQ 1

The lowest audible sound pressure at 1000 Hz is about 2 × 10−5 Pa. Show that this is equivalent to an intensity of approximately 1 × 10−12 W m−2.

View answer

1.4  The decibel and weighting

Sound intensities involve both very small and very large numbers, as shown in Table 2.

Table 2  Sound pressure and sound pressure level of different sources

Source and locationSound pressure (Pa)Sound pressure level (dB)Peak level of .303 rifle shot at the ear2000160Jet aircraft on take-off at 25 m200140Woodworking machine at 1 m195Heavy diesel lorry at 7 m0.690Ringing alarm clock at 1 m0.280Domestic vacuum cleaner at 3 m0.0670Normal conversation at 1 m0.03565A very quiet room0.000220

For this reason, it is convenient and helpful to use a logarithmic scale of sound pressure and sound intensity, which makes the numbers easier but the maths a little more difficult. The common logarithm of a number is the power to which 10 has to be raised to produce that number. For example, 100 = 102, so the logarithm to base 10 of a hundred is 2. This is written in shorthand form as log10(100) = 2, or more simply as log(100) = 2. Similarly, log10(1000) = 3. So on a logarithmic scale, the difference of a factor of 10 (between 1000 and 100) is reduced to a numerical difference of 1 (between 3 and 2).

One problem with logarithms is that the logarithms of numbers less than 1 are negative. It would be somewhat confusing to represent a sound pressure measurement of a fraction of one pascal, or a sound intensity measurement of less than 1 W m−2, by a negative number. This problem can be overcome by introducing a reference quantity corresponding to a small pressure or small intensity, and always dividing by this before taking logarithms. Thus the sound intensity level (IL) in bels is defined by:

(3)

where

I is the measured intensity I0 is a reference intensity of 10−12 W m−2 (corresponding to the lowest audible intensity at 1000 Hz for someone with normal hearing).

The bel is a ratio between a property of the source in question and a reference value. However, the bel is an inconveniently large unit, so the decibel (dB) is usually used. The decibel is one-tenth of a bel. Thus the sound intensity level (IL) in dB is defined as:

(4)

A similar equation with powers substituted for intensities defines sound power level (SWL or Lw) in dB (see Section 2.5).

If we remember that sound intensity is proportional to the square of sound pressure, i.e.

then:

So, sound pressure level (SPL or Lp) (which is the most common measurement of sound) in dB is defined as:

(5)

where p0 is the reference sound pressure (2 × 10−5 Pa) corresponding to the reference intensity I0. This is the ‘sound pressure level measured in decibels relative to 2 × 10−5 Pa’. Whenever the abbreviations SPL or Lp are used they refer to this mouthful!

1.4.1  Frequency response

The range of frequencies that humans can detect, the audio frequency range, changes with age; the older you get the less sensitive you are, especially to high-frequency sound. For a young adult with normal hearing, the audio frequency range (or hearing range) is usually taken as being from about 20 Hz to about 20 kHz. At the extremes of the audio frequency range, the sound pressures would have to be very high to be heard by someone with normal hearing. In between these extremes, however, there is a range in which the ear is particularly sensitive. Since the ear’s sensitivity varies with frequency, the human hearing system clearly does not behave like the perfect microphone of the advertisement.