Predicting Outdoor Sound

Predicting Outdoor Sound

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Predicting Outdoor Sound Keith Attenborough, Kai Ming Li and Kirill Horoshenkov

LONDON AND NEW YORK

First published 2007 by Taylor & Francis 2 Park Square, Milton Park, Abingdon, Oxon OX 14 4RN Simultaneously published in the USA and Canada by Taylor & Francis 270 Madison Ave, New York, NY 10016 Taylor & Francis is an imprint of the Taylor & Francis Group, an informa business This edition published in the Taylor & Francis e-Library, 2007. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” © 2007 Keith Attenborough, Kai Ming Li and Kirill Horoshenkov All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any efforts or omissions that may be made. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data Attenborough, K. (Keith). Predicting outdoor sound/Keith Attenborough, Kai Ming Li and Kirill Horoshenkov. p. cm. Includes bibliographical references and index. I. Outdoor sounds—Measurement. 2. Sound—Recording and reproducing. I. Li, Kai Ming. II. Horoshenkov, Kirill. III. Title. QC246.A88 2006 534–dc22 2006013566 ISBN 0-203-08873-5 Master e-book ISBN

ISBN 10: 0-419-23510-8 (Print Edition) ISBN 13: 978-0-19-23510-1 (hbk) ISBN 10: 0-203-08873-5 (Print Edition) ISBN 13: 978-0-203-08873-9 (ebk)

Contents Preface

ix

List of symbols

x

1 Introduction

1

1.1 Early observations

1

1.2 A brief survey of outdoor sound attenuation mechanisms

1

1.3 Data including combined effects of ground and meteorology

2

1.4 Classification of meteorological conditions for outdoor sound prediction

9

1.5 Typical sound speed profiles

14

1.6 Air absorption

21

2 The propagation of sound near ground surfaces in a homogeneous medium 25 2.1 Introduction

25

2.2 Mathematical formulation for a point source near to the ground

25

2.3 The sound field above a locally reacting ground

34

2.4 The sound field above a layered extended-reaction ground

42

2.5 The propagation of surface waves above a porous ground

50

2.6 Experimental data and numerical predictions

54

2.7 The sound field due to a line source near the ground

60

3 Predicting the acoustical properties of outdoor ground surfaces

66

3.1 Introduction

66

3.2 Models for ground impedance

67

3.3 Effects of surface roughness

84

3.4 Effects of ground elasticity

97

4 Measurements of the acoustical properties of ground surfaces and comparisons with models 4.1 Impedance measurement methods

110 110

4.2 Comparisons of impedance data with model predictions

118

4.3 Measured and predicted roughness effects

120

4.4 Measured and predicted effects of ground elasticity

130

4.5 Comparisons between ‘template’ fits and direct impedance fits for ground impedance

135

4.6 Measured flow resistivities and porosities

142

4.7 Effective flow resistivities and other fitted parameters

143

5 Predicting effects of source characteristics on outdoor sound

148

5.1 Introduction

148

5.2 The sound field due to a dipole source

148

5.3 The sound field due to an arbitrarily orientated quadrupole

168

5.4 Source directivity and railway noise prediction

173

6 Predictions, approximations and empirical results for ground effect excluding meteorological effects 6.1 Approximations for frequency and range dependency

177 177

6.2 Approximations and data for A-weighted levels over continuous ground 180 6.3 Predictions of the variation of A-weighted noise over discontinuous surfaces 7 Influence of source motion on ground effect and diffraction

185 191

7.1 Introduction

191

7.2 A monopole source moving at constant speed and height above a ground surface

192

7.3 The sound field of a source moving with arbitrary velocity

200

7.4 Comparison with heuristic calculations

206

7.5 Diffraction of sound due to a point source moving at constant speed and 210 height parallel to a rigid wedge 7.6 Source moving parallel to a ground discontinuity

215

7.7 Source moving along a rigid barrier above the ground

222

8 Predicting effects of mixed impedance ground

229

8.1 Introduction

229

8.2 Single discontinuity

229

8.3 Propagation over impedance strips

239

8.4 Effects of refraction above mixed impedance ground

244

8.5 Predicted effects of porous sleepers and slab-track on railway noise

250

9 Predicting the performance of outdoor noise barriers

258

9.1 Introduction

258

9.2 Analytical solutions for the diffraction of sound by a barrier

258

9.3 Empirical formulations for studying the shielding effect of barriers

274

9.4 The sound attenuation by a thin barrier on an impedance ground

279

9.5 Noise reduction by a finite length barrier

285

9.6 Adverse effect of gaps in barriers

287

9.7 The acoustic performance of an absorptive screen

293

9.8 Other factors in barrier performance

296

9.9 Predicted effects of spectral variations in train noise during pass-by

303

10 Predicting effects of vegetation, trees and turbulence

311

10.1 Effects of vegetation and crops on excess attenuation spectra

311

10.2 Propagation through trees and tall vegetation

315

10.3 Meteorological effects on sound transmission through trees

321

10.4 Combined effects of vegetation, barriers and meteorology

323

10.5 Turbulence and its effects

325

11 Analytical approximations including ground effect, refraction and turbulence 11.1 Ray tracing

342 342

11.2 Linear sound speed gradients and weak refraction

352

11.3 Approximations for A-weighted levels and ground effect optimization in the presence of weak refraction and turbulence

355

11.4 A semi-empirical model for A-weighted sound levels at long range

375

12 Prediction schemes

380

12.1 Introduction

380

12.2 ISO 9613–2

380

12.3 CONCAWE

389

12.4 Calculation of road traffic noise (CRTN)

392

12.5 Calculation of railway noise (CRN)

402

12.6 NORD2000

404

12.7 HARMONOISE

404

12.8 Performance of railway noise prediction schemes in high-rise cities

406

13 Predicting sound in an urban environment

414

13.1 Introduction

414

13.2 Improved corrections for the reflection of road traffic noise from a building façade

415

13.3 Improved correction for the multiple reflections between parallel building façades

417

13.4 Traffic noise attenuation along a city street

420

13.5 Noise in tunnels

422

13.6 Prediction of the acoustic effect of a single building façade with balconies

426

13.7 Sound propagation between two parallel high-rise building façades

430

13.8 Modelling of 3-D sound propagation between two high-rise building façades

443

13.9 Sound propagation in city streets

453

Index

466

Preface The subject of outdoor sound propagation is of wide-ranging interest not only for noise prediction but also in studies of animal bio-acoustics and in military contexts. The purpose of this book is to provide a comprehensive reference about aspects of outdoor sound and its prediction that should be useful to practitioners, and yet is respectable from the academic point of view. It is based on a joint experience of more than 50 years of research and consultancy. Many current prediction schemes for outdoor sound are empirical. To some extent this is understandable in view of the complicated source characteristics and complex propagation paths that are often of interest. Yet there has been significant progress in theories and computational methods for the various phenomena that are involved. These theories have been validated extensively by comparisons with data and help with our understanding of the important effects. No current text is devoted to bringing the leading theories and data together. Neither is the practitioner provided with the basis for deciding which model or scheme to use in a given situation. This text is a step towards a remedy for both of these deficiencies. The book covers recent advances in theory, new and old empirical schemes, available data and comparisons between theory and data. Where possible, examples of results of the application of prediction schemes have been included. Enough of the background theoretical detail is available to make the reader/user aware of the inherent approximations, restrictions and/or difficulties of any of the prediction methods being discussed. The book has had a long gestation since it was started in 1999. In 2001, Computational Atmospheric Acoustics by Erik Salomons was published. There have been three consequences of this publication for our endeavours in Predicting Outdoor Sound. The first is that we have not attempted to duplicate the fine, thorough and approachable treatments of the computational methods to be found in Salomons’ book. Second, given that Salomons’ book does not include any data, we have emphasized data where it is available. Finally, we have concentrated on those aspects of our research that complement Salomons’ work. Probably, students, researchers and consultants interested in outdoor sound prediction would do best if they are in possession of both texts. Keith Attenborough Kai Ming Li Kirill Horoshenkov November 2005

Symbols Ag c(z) c0 C(ω) C d D(z) f F(w) g0 gβ G0 Gβ k k h H I L ∆L M n1 N Nc NPR P q2 Q r R1 R2 R′ Rp Rs Reff

A-weighted ground attenuation (ISO9613–2) Sound speed as a function of height Ambient (adiabatic) sound speed Complex compressibility Elastic constant for Biot model Layer thickness, distance from carriageway edge Doppler factor Frequency; hourly traffic flow Boundary loss factor Viscosity correction function 3-D Green’s function for sound propagation above rigid boundary 3-D Green’s function for sound propagation above impedance boundary 2-D Green’s function for sound propagation above rigid boundary 2-D Green’s function for sound propagation above impedance boundary Von Karman constant Propagation constant (=ω/c) Mean propagation path height Percentage molar concentration of water vapour; Elastic constant for Biot model Hankel function of the first kind and order 0 Root mean square roughness height Acoustic intensity; integral term Bessel function of the first kind and order 0 Obukhov length; number of layers; outer (inertial) scale of turbulence; sound pressure level Change in sound pressure level Elastic constant for Biot model; Mach number Refractive index Fresnel number Fractional cloud cover Prandtl number Pasquill class Tortuosity Spherical wave reflection coefficient Horizontal range Source-receiver distance Length of specularly-reflected path Shortest source-edge-receiver path Plane wave reflection coefficient Flow resistivity Effective flow resisitivity

rh s0 S0 sA, sB te T T* T0 Tav T(z) u* u10 u(z) Vj w W Z zM zH z0 α αe β βR γ γg Γ λ Λ, Λ′ φ µ ν θ0 θ θs ρ, ρ(ω) σ σe σs

τe, τv ψ

Relative humidity Steady flow shape factor Source strength Dynamic pore shape parameters Emission time Tortuosity Scaling temperature °K Temperature °C at zero height Average temperature °C Temperature as a function of height Friction velocity (m/s) Wind speed at reference height of 10 m Wind speed as a function of height Plane wave reflection coefficient of layer j Numerical distance Ratio of minimum to mean roughness element spacing Specific normalized impedance Momentum roughness length Heat roughness length Roughness length Air absorption coefficient; air absorption parameter Rate of porosity change Specific normalized admittance Energy reflection coefficient Specific heat ratio Ground parameter (Makarewicz) Adiabatic correction factor Velocity potential, log2 base pore dimension Dimensionless parameter for complex density; wavelength Viscous and thermal characteristic lengths Velocity potential Dynamic viscosity; polar angle Kinematic viscosity Mean square refractive index Angle of incidence Polar angle Scattering angle; angle of view Density, complex density; transverse correlation Standard deviation of log-normal pore size distribution Effective flow resistivity Scattering cross section Variance of wind velocity fluctuations Variance of temperature fluctuations Thermal and viscous relaxation times Azimuthal angle

ψM ψH χM χH ω Ω

Diabatic momentum profile correction (mixing) function Diabatic heat profile correction (mixing) function Inverse diabatic influence or function for momentum Inverse diabatic influence function for momentum Angular frequency Porosity

Chapter 1 Introduction 1.1 Early observations The way in which sound travels outdoors has been of interest for several centuries. Initial experiments were concerned with the speed of sound [1]. The Francisan (Minimite) friar, Marin Mersenne (1588–1648), suggested timing the interval between seeing the flash and hearing the report of guns fired at a known distance. William Derham (1657–1735), the rector of a small church near London, was first to observe the influence of wind and temperature on sound speed and the difference in the sound of church bells at the same distance over newly fallen snow and over a hard frozen surface. Many records of the strange effects of the atmosphere on the propagation of sound waves have been associated with war [2, 3]. In June 1666, Samuel Pepys wrote that the sounds of a naval engagement between the British and Dutch fleets were heard clearly at some spots but not at others a similar distance away or closer. Pepys spoke to the captain of a yacht that had been positioned between the battle and the English coast. The captain said that he had seen the fleets and run from them, ‘…but from that hour to this hath not heard one gun…’. The effects of the atmosphere on battle sounds were not studied in a scientific way until after the First World War. During that war, acoustic shadow zones, similar to those observed by Pepys, were observed during the battle of Antwerp. Observers also noted that battle sounds from France only reached England during the summer months and were best heard in Germany during the winter. After the war there was great interest in these observations among the scientific community. Large amounts of ammunition were detonated throughout England and the public was asked to listen for sounds of explosions. Although there was considerable interest in atmospheric acoustics after the First World War, the advent of the submarine encouraged the greatest efforts in underwater acoustics research during and after the Second World War. The theoretical and numerical methods widely deployed in predicting sound propagation in the oceans have been adapted subsequently for use in atmospheric acoustics. A meeting organized by the University of Mississippi and held on the Mississippi Gulf Coast in 1981 was the first in which researchers in underwater acoustics met with scientists interested in atmospheric acoustics and stimulated the adoption and adaptation of the numerical methods, used for underwater acoustics, in the atmosphere [4].

1.2 A brief survey of outdoor sound attenuation mechanisms Outdoor sound is attenuated by distance, by topography (including natural or artificial barriers), by interaction with the ground and with ground cover and by atmospheric effects including upward refraction and absorption. When the source is downwind of the

Predicting outdoor sound

2

receiver, the sound has to propagate upwind. As height increases, the wind speed increases and the amount being subtracted from the speed of sound increases, leading to a negative sound speed gradient. In terms of rays, a negative sound gradient means that rays bend upwards. This is called upward refraction. Consequently, there is a ray that leaves the source, grazes the ground at some point and does not reach a receiver positioned beyond this point. Ray tracing ceases to be valid beyond this ground-grazing ray. The ray tracing model for outdoor sound is considered in more detail in Chapter 11. Upward refraction leads to the creation of a sound shadow at a distance from the source that depends on the gradient. The shadow zone is penetrated by sound scattered by turbulence and this sets a limit to the noise reduction within the sound shadow. Turbulence effects are considered further in Chapter 10. A negative sound speed gradient also results when the temperature decreases with height. This is called a temperature lapse condition and is the normal condition on a dry sunny day with little wind. A combination of slightly negative temperature gradient, strong upwind propagation and air absorption has been observed, in carefully monitored experiments, to reduce sound levels, 640 m from a 6 m high source over relatively hard ground, by up to 20 dB more than expected from spherical spreading [5]. Atmospheric absorption acts as a low pass filter at long range. It results from heat conduction losses, shear viscosity losses and molecular relaxation losses. The total attenuation of a sound outdoors can be expressed as the sum of the reduction due to geometric spreading, atmospheric absorption and extra attenuation including, for example, ground effects, vegetation effects, refraction in the atmosphere and diffraction by barriers. Ground effects (for elevated source and receiver) are the result of interference between sound travelling directly from source to receiver and sound reflected from the ground. Since it involves interference, there can be enhancement as well as attenuation. Enhancement tends to occur at low frequencies. The presence of vegetation tends to make the surface layer of ground including the root zone more porous. The layer of partially decayed matter on the floor of a forest is highly porous. In addition, propagation through trees involves reverberant scattering by tree trunks and viscous scattering by foliage. These ground and scattering effects are explored in detail in Chapters 3 and 10 respectively.

1.3 Data including combined effects of ground and meteorology Pioneering studies of the combined influences of the ground surface and meteorological conditions were carried out by Parkin and Scholes [6–10] using a fixed Rolls Royce Avon jet engine as a source at two airfields (Hatfield and Radlett). In his 1970 Rayleigh Medal Lecture, one of the investigators, the late Peter Parkin, remarked [6] These horizontal propagation trials showed up the ground effect, which at first we did not believe, thinking there was something wrong with the measurements. But by listening to the jet noise at a distance, one could clearly hear the gap in the spectrum.

Introduction

3

Parkin was among the first people to note and quantify the change in ground effect with the type of surface. Parkin and Scholes data showed a noticeable difference between the ground effects due to two grass covers. The ground attenuation at Hatfield, although still a major propagation factor, was less than at Radlett and its maximum value occurred at a higher frequency. The weather at the time of their measurements also enabled them to observe the different effect of snow. measurements [were] made at Site 2 [Radlett] with 6 to 9 in. of snow on the ground. The snow had fallen within the previous 24 hours and had not been disturbed. The attenuations with snow on the ground were very different from those measured under comparable wind and temperature conditions without snow…. The maximum of the ground attenuation appears to have moved down the frequency scale by approximately 2 octaves…. Examples of the Parkin and Scholes data are shown in Figure 1.1. They quoted their data as the difference in sound pressure levels at 19 m (reference location) and more distant locations corrected for the decrease expected from spherical spreading and air absorption. Clearly ground effect is sensitive to the acoustical properties of the surface. These depend on the substance of which the surface is composed. Different ground surfaces have different porosities. Soils have volume porosities of between 10 and 40%. Snow which has a porosity of around 60%, and many fibrous materials which have porosities of about 97%, have fairly low flow resistivities whereas a wet compacted soil surface will have a rather high flow resistivity. The thickness of the surface porous layer also is important and whether or not it has an acoustically hard substrate. The Parkin and Scholes data revealed the large effect at low frequencies (63 and 125 Hz octave bands) in the presence of thick snow. It should be noted moreover that even without snow there are significant differences between summer and winter excess attenuation in the Parkin and Scholes’ Radlett data. The classical experiments by Parkin and Scholes involved relatively little meteorological monitoring. In particular, the fine-scale fluctuations in wind speed and hence the turbulence were not monitored; perhaps since the important role of turbulence was not appreciated at the time. Recently there has been a similar experiment to that carried out by Parkin and Scholes. Simultaneous acoustic and meteorological measurements have been made using a jet engine source at a disused airfield operated as test facility by Rolls Royce at Hucknall [11]. In addition to wind and temperature gradient measurements, the fluctuation in wind velocity measurements was recorded and used as a measure of turbulence. Some of the data obtained under low wind and low turbulence conditions over continuous grassland are shown in Figure 1.2. Also shown is the third octave power spectrum of the Avon engine source between 100 and 4000 Hz deduced from the measured spectrum at 152.4 m corrected for spherical spreading and ground effect. The data obtained at the longest range is noise-limited above 3 kHz. The significant dips in the received spectra between 100 and 500 Hz are clear evidence of ground effect. The ground effect at Hucknall is different from that at either at Radlett or Hatfield.

Predicting outdoor sound

4

Figure 1.1 Parkin and Scholes’ data for the level difference between 1.5 m high microphones at 19 and 347 m from a fixed jet engine source (nozzle-centre height 1.82 m) corrected for wavefront spreading and air absorption. The symbols □ and ◊ represent data over airfields (grass-covered) at Radlett and Hatfield respectively with a positive vector wind between source and receiver of 1.27 m s−1 (5 ft s−1). Crosses (×) represent data over approximately 0.15 m thick (6–9 in.) snow at Hatfield with a positive vector wind of 1.52 m s−1 (6 ft s−1).

The influence of small changes in the wind speed and turbulence strength on the measured spectra at the longest range is demonstrated in Figure 1.3. The associated meteorological conditions are detailed in Table 1.1. The ground effect between 100 and 400 Hz is fairly stable and is significantly greater for the low microphones and shifted in frequency compared with that for the high microphones. The data for both microphone heights show considerable variability between 400 and 2 kHz as a result of changes in wind velocity and turbulence.

Introduction

5

Figure 1.2 Data recorded at 1.2 m high receivers at horizontal ranges of 152.4 m (solid line), 457 m (dotted line), 762 m (dashed line) and 1 158 m (dash-dot line) from a fixed Rolls Royce jet engine source with the nozzle centre 2.16 m above an airfield at Hucknall, Notts. These data represent simultaneous recordings averaged over 26 s during zero wind and low turbulence conditions (block 20 of run 454, see Figure 1.3). Also shown (connected circles) is the deduced third octave power spectrum of the Avon jet engine source after subtracting 50 dB. Figures 1.4 and 1.5 show A-weighted levels deduced from consecutive 26 s average spectra measured at 1.2 m height and ranges of 152.4 m, 457.6 m, 762.2 m and 1158.4 m over grasslands at Hucknall. Figure 1.4 shows data for low wind speed (less than 2 m s−1 from source to receiver) and low turbulence conditions. Figure 1.5 shows data for moderate downwind conditions (approximately 6 m s−1 from source to receiver) and for higher turbulence intensities. The details of the meteorological conditions are listed in Tables 1.2 and 1.3.

Predicting outdoor sound

6

Figure 1.3 Simultaneously measured narrow band (25 Hz interval) spectra at low (1.2 m) and high (6.4 m) microphones between 50 and 10 kHz at 1158.2 m from a fixed Avon jet engine source averaged over 26 s intervals during low wind, low turbulence conditions at Hucknall (Notts, UK). The conditions are specified in Table 1.1 and the key.

Introduction

7

Table 1.1 Meteorological conditions corresponding to data in Figure 1.3 Direction Temperature Temperature Turbulence Run Wind speed Wind 454 at ground speed at relative to at ground (°C) at 6.4 m (°C) variable Block (0.025 m) 6.4 m line of mics. (°) No. (m s−1) height (m s−1) 2 3 4 5 6 7 19 20

1.57 1.34 1.27 0.00 0.00 0.00 0.00 0.00

1.86 1.61 1.96 1.57 1.46 1.81 0.00 0.00

23.3 26.9 349.0 343.2 346.0 342.8 301.6 236.9

10.4 10.4 10.5 10.5 10.5 10.7 10.2 10.2

9.9 9.9 9.8 9.8 9.8 9.9 9.8 9.8

Figure 1.4 Comparison of A-weighted sound levels (26 s averages) deduced from low wind, low turbulence octave band measurements at 1.2 m height over grassland at Hucknall (Run 454 blocks 11, 12, 13(×); 14, 15, 16(+); 17, 18, 19, 20(○)).

0.0486 0.0962 0.0672 0.0873 0.1251 0.2371 0.0000 0.0000

Predicting outdoor sound

8

Note that there is a considerable spread in the measured levels at the longer ranges in Figure 1.4 as a result of the variation in wind speed and direction (up to approximately 2 m s−1 downwind at 6.4 m height) and turbulence levels. The data for stronger downwind conditions (up to approximately 6.5 m s−1 at 6.4 m height) in Figure 1.5 exhibit consistently higher levels than the relatively low wind speed data and a smaller spread. Although only four averages are shown in Figure 1.5, their spread is smaller than for any four averages exhibited in Figure 1.4. This is consistent with the assertion in ISO 9613–2 [12] that the variation in sound levels is less under ‘moderate’ downwind conditions. The average downwind level measured at Hucknall is about 10 dB higher than the levels for the lowest wind speed and turbulence conditions at 1.1 km from the source.

Figure 1.5 Comparison of A-weighted sound levels deduced from consecutive 26 s average downwind octave band measurements at 1.2 m height above grassland at Hucknall (Run 453 blocks 3(×); 4(+); 5(□) and 6(◊)).

Introduction

9

Table 1.2 Meteorological data corresponding to sound level data shown in Figure 1.4 Run 454 Block No.

Direction Temperature Temperature Turbulence Wind speed Wind at ground speed at relative to at ground (°C) at 6.4 m (°C) variable 6.4 m line of mics. (0.025 m) (°) (m s−1) height (m s−1)

11 0.00 12 0.00 13 0.00 14 0.00 15 1.02 16 0.00 17 0.00 18 0.00 19a 0.00 20a 0.00 Note a Also listed in Table 1.1.

1.97 1.97 1.09 0.01 1.53 1.58 0.92 1.16 0.00 0.00

348.1 324.8 356.9 357.7 50.6 38.5 20.9 14.0 301.6 236.9

10.5 10.6 10.6 10.4 10.4 10.4 10.2 10.2 10.2 10.2

9.8 9.8 9.8 9.9 9.9 10.0 9.9 9.9 9.8 9.8

0.0805 0.0607 0.0678 10.1489 0.0764 0.0928 0.1792 0.0424 0.0000 0.0000

Table 1.3 Meteorological data corresponding to sound level data in Figure 1.5 Run 453 Block No. 3 4 5 6

Direction Temperature at Temperature Turbulence Wind Wind ground (°C) at 6.4 m (°C) variable speed at speed at relative to ground (m 6.4 m line of mics. s−1) (°) height (m s−1) 4.09 4.09 4.33 3.89

6.44 6.11 5.93 6.07

10.0 20.5 17.8 10.8

15.0 14.9 14.9 14.9

15.0 15.0 15.0 15.0

0.1202 0.1606 0.1729 0.1028

1.4 Classification of meteorological conditions for outdoor sound prediction The atmosphere is constantly in motion as a consequence of wind shear and uneven heating of the earth’s surface (see Figure 1.6). Any turbulent flow of a fluid across a rough solid surface generates a boundary layer. Most interest from the point of view of outdoor noise prediction focuses on the lower part of the meteorological boundary layer called the surface layer. In the surface layer, turbulent fluxes vary by less than 10% of their magnitude but the wind speed and temperature gradients are largest. In typical daytime conditions the surface layer extends over 50–100 m. Usually it is thinner at night.

Predicting outdoor sound

10

In most common daytime conditions, the net radiative energy at the surface is converted into sensible heat. This warms up the atmosphere thereby producing negative temperature gradients as indicated in Figure 1.6. If the radiation is strong (high sun, little cloud cover), the ground is dry, and the surface wind speed is low, the temperature gradient is large. The atmosphere exhibits strong thermal stratification. If the ground is wet, most of the radiative energy is converted into latent heat of evaporation and the temperature gradients are correspondingly lower. In unstable daytime conditions, the wind speed is affected by the temperature gradient and exhibits slightly less variation with height than for the isothermal case. On the other hand, ‘stable’ conditions prevail at night. The radiative losses from the surface cause positive temperature gradients. There is a considerable body of knowledge about meteorological influences on air quality in general and the dispersion of plumes from stacks in particular. Plume behaviour depends on vertical temperature gradients and hence on the degree of mixing in the atmosphere. Vertical temperature gradients decrease with increasing wind. The stability of the atmosphere in respect of plume dispersion is described in terms of Pasquill classes. This classification is based on incoming solar radiation, time of day and wind speed. There are six Pasquill classes (A−F) defined in Table 1.4. Data are recorded in this form by meteorological stations and so, at first sight, it is a convenient classification system for noise prediction.

Figure 1.6 Schematic of the daytime atmospheric boundary layer and eddy structures. The sketch graph on the left shows the mean wind speed (U) and the potential where temperature profiles (θ=T+γdz, γd=0.098°C km−1 is the dry adiabatic lapse rate, T is the temperature and z is the height).

Introduction

11

Class A represents a very unstable atmosphere with strong vertical air transport, that is, mixing. Class F represents a very stable atmosphere with weak vertical transport. Class D represents a meteorologically neutral atmosphere. Such an atmosphere has a logarithmic wind speed profile and a temperature gradient corresponding to the normal decrease with height (adiabatic lapse rate). A meteorologically neutral atmosphere occurs for high wind speeds and large values of cloud cover. This means that a meteorologically neutral atmosphere may be far from acoustically neutral. Typically, the atmosphere is unstable by day and stable by night. This means that classes A−D might be appropriate classes by day and D−F by night. With practice, it is possible to estimate Pasquill Stability Categories in the field, for a particular time and season, from a visual estimate of the degree of cloud cover. The Pasquill classification of meteorological conditions has been adopted widely as the basis of a meteorological classification system for noise prediction schemes [e.g. 13]. However, it is clear from Table 1.2, that the ‘meteorologically neutral’ category (C), while being fairly common in a temperate climate, includes a wide range of wind speeds and is therefore not very suitable as a category for noise prediction. In the CONCAWE scheme [13], this problem is addressed by defining six noise prediction categories based on Pasquill categories (representing the temperature gradient) and wind speed. There are 18 sub-categories depending on wind speed. These are defined in Table 1.5. CONCAWE category 4 is specified as one in which there is zero meteorological influence. So CONCAWE category 4 is equivalent to acoustically neutral conditions. Table 1.4 Pasquill (meteorological) stability categories Wind speeda Daytime incoming solar radiation mW cm−2 (m s−1) >60 30–60 6.0 D D D D D D D D Notes a Measured to the nearest 0.5 m s−1 at 11 m height. b Category G is an additional category restricted to the night-time with less than 1 octa of cloud and a wind speed of less than 0.5 m s−1.

Table 1.5 CONCAWE meteorological classes for noise prediction Meteorological category

Pasquill stability category and wind speed (m s−1) (positive is towards receiver) A, B C, D, E F, G

1 2 3 4a

v