《ESDU-79011-C-2003.pdf》由会员分享,可在线阅读,更多相关《ESDU-79011-C-2003.pdf(38页珍藏版)》请在三一文库上搜索。
1、ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 79011 Endorsed by The Royal Aeronautical Society ESDU An IHS GROUP Company Engineering Sciences Data Unit TM Issued September 1979 With Amendment C June 2003 Estimation of noise shielding by barriers Associated softwar
2、e: ESDUpac A7911 ESDUpac B7911 ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 79011 ESDU Engineering Sciences Data Unit TM ESDU DATA ITEMS Data Items provide validated information in engineering design and analysis for use by, or under the supervision of, professio
3、nally qualified engineers. The data are founded on an evaluation of all the relevant information, both published and unpublished, and are invariably supported by original work of ESDU staff engineers or consultants. The whole process is subject to independent review for which crucial support is prov
4、ided by industrial companies, government research laboratories, universities and others from around the world through the participation of some of their leading experts on ESDU Technical Committees. This process ensures that the results of much valuable work (theoretical, experimental and operationa
5、l), which may not be widely available or in a readily usable form, can be communicated concisely and accurately to the engineering community. We are constantly striving to develop new work and review data already issued. Any comments arising out of your use of our data, or any suggestions for new to
6、pics or information that might lead to improvements, will help us to provide a better service. THE PREPARATION OF THIS DATA ITEM The work on this particular Data Item was monitored and guided by the Aircraft Noise Committee. The Aircraft Noise Committee first met in 1971 and now has the following me
7、mbership: The person with overall responsibility in this subject area is Chairman Mr R.A. Pinker QinetiQ, Pyestock Vice-Chairman Mr L.W. Illston BAE SYSTEMS, Military Aircraft Division, Farnborough Members Mr W.D. Bryce* * Member Emeritus Bryce Research, Farnham Dr M.J. Fisher Institute of Sound and
8、 Vibration Research, University of Southampton Mr P.J. Hopkins Rolls-Royce plc, Derby Mr P. Lempereur Airbus France, Toulouse Mr W.J. Readman Civil Aviation Authority, Airworthiness Division Mr D.C. Riordan Bombardier Aerospace Short Brothers plc, Belfast Mr P. Robinson Dowty Aerospace Propellers, G
9、loucester Mr D.P. Garber Corresponding Member. NASA Langley Research Center, USA. Dr C.B. Chinoy Head of the Aircraft Noise and Structural Dynamics Group. -,-,- ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 1 79011 ESDU Engineering Sciences Data Unit TM ESTIMATION
10、 OF NOISE SHIELDING BY BARRIERS 1.NOTATION Units SIBritish speed of soundm/sft/s Fresnel integrals shortest distance from vertical plane through barrier edgemft frequencyHzHz auxiliary Fresnel functions height above ground planemft parameter in ss number of routes being considered constant in m/(s K
11、)ft/(s K) total sound attenuation dBdB sound attenuation due to diffraction round an edgedBdB difference in source sound pressure levels due to angular characteristics of source dBdB inverse square law correctiondBdB correction to at low values of shieldingdBdB shortest path via barrier edge between
12、 source and observermft angular function of , see Section 2.3.2 angular function of , see Section 2.3.2 shortest distance through barrier between source and observermft greater of and mft a C X() S X(), d Ff X1()f X2()+ f f X() g X(), Gg X1()g X2()+ H J2Jf = j kakT= LLL1L2L3L4+= L1 L2 L3 L4L1 l M1os
13、+ M2os R R1rsro Issued September 1979 With Amendment C - 34 pages ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 2 79011 ESDU Engineering Sciences Data Unit TM lesser of and mft cylindrical polar coordinates * distance, see Sketch 2.2mft sound pressure leveldBdB fr
14、ee field sound pressure level radiating in direction of observer dBdB free field sound pressure level radiating in direction of barriers edge dBdB temperatureKK argument in auxiliary Fresnel functions for argument in auxiliary Fresnel functions for Cartesian co-ordinates non-dimensional lateral sepa
15、ration of source and observer wedge angle, see Sketch 2.4degdeg , , distance parameters (, see Equations (2.6) and (2.7) parameter given by ss function of , see Section 3.2 angle between barrier face on observers side and vertical plane through barrier edge, see Sketch 2.4 degdeg angle dependent on
16、and given by degdeg , functions of and , see Section 2.3.2degdeg angle between barrier and ray to observer or source, see Sketch 2.2 degdeg wavelength mft angle of incidence of ray onto barrier, see Sketch 2.2degdeg angle into shadow, see Sketch 2.4degdeg For footnotes refer to end of Notation Secti
17、on R2rsro r z, r S SD SE T X1os+ X2os x y z, Z Zzozs/ rors+()=() 1212= 33/f = s= 12 a/f=() -,-,- ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 3 79011 ESDU Engineering Sciences Data Unit TM 2.NOTES 2.1General When a barrier is placed between a sound source and an
18、observer, sound reaches the observer by diffraction over the top of the barrier, or round the barrier edges, and by direct transmission through the barrier. In many practical cases the barrier is designed such that the transmission path is so highly attenuated that it may be neglected. Hence this It
19、em is only concerned with the sound reaching the observer via the edge of the barrier, i.e. propagating along any of the paths SBO shown in Sketch 2.1. A method is provided for estimating the shielding effect of a barrier in terms of the difference in sound level received at a given location on the
20、side of the barrier remote from the noise source and the sound level received at the same location without the barrier. The sound attenuation due to a barrier is therefore dependent on the redistribution of sound energy by diffraction over the top, and round the edges, of the barrier and the increas
21、ed propagation losses which result from the change in the length and direction of the sound propagation path due to the presence of the barrier. In the procedure used in this Item for estimating the loss due to diffraction it is assumed that the noise is emitted by a point source and that both the s
22、ource and observer are at large distances from the diffracting edge relative to the wavelengths of the sound considered. Also it is assumed that barriers are rigid, wedge-shaped (as described below) and have hard surfaces and sharp edges. The theory breaks the attenuation due to diffraction into two
23、 parts, and . The expressions defining and are given in Appendix A, Section A1. For attenuations greater than 10 dB is negligible. The wedge-shaped barrier can vary between an infinitesimally thin wall, where the wedge has become a surface. When the barrier would represent a right-angled wedge which
24、 for example could be the corner of a building. The increased sound propagation path length due to the presence of the barrier introduces additional attenuations due to increases in spherical spreading loss and atmospheric attenuation. Often these effects are very small and may be neglected in calcu
25、lating shielding effects. However, in the case when both source and observer are close to the barrier the sound spreading loss, , may be significant and should be calculated using the inverse square law as described in Section 2.4.2. The effects of the atmosphere are discussed in Section 2.4.3 and,
26、in the calculation of attenuation due to diffraction, a steady uniform atmosphere is assumed. The attenuation due to diffraction is measured relative to the source level, , propagating in the direction of the diffracting edge. For a directional source there will be an additional effect, , due to the
27、 different intensities of radiation along the directions of the direct path and the path via the edge of the barrier (see Section 2.4.1). This Item provides a means of estimating the total attenuation at an observer position due to the presence Suffixes barrier route identifier observer source * The
28、 reference pressure for all decibels is Pa (0.0002 dyn/cm2 or lbf/ft2). b i o s 204.17710 7 L1L4L1L4 L4 270= L3 SE L2 -,-,- ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 4 79011 ESDU Engineering Sciences Data Unit TM of a barrier, interposed between the source and
29、 observer, relative to the free-field level when no barrier is present. The total attenuation L is given by . During any preliminary investigations into the attenuation by barriers, when the source characteristics are not known, it is usual to assume that . However, in some cases source directivity
30、patterns should be investigated before a barrier design is finalised. The attenuation due to shielding is frequency dependent; as it increases slowly with frequency, the centre frequency of a frequency band should be used when estimating the attenuation for the band. When considering frequency weigh
31、ted units, e.g. dBA, PNdB etc., it is difficult to achieve levels of shielding greater than 20 dB although at high frequencies the attenuation at a frequency could be greater than 30 dB. Appendix A gives a numerical method which enables the values of attenuation due to shielding to be calculated. A
32、computer program, known as ESDUpac A7911, is provided. This program offers the user the additional facility of adjusting spectra at one-third octave band centre frequencies for barrier attenuation. These spectra must be entered via the common input data file, see Appendices B and C. Implementation o
33、f the program is described in Appendix B. 2.2Geometric Relationships The geometric arrangement of source, observer and barrier is shown in Sketches 2.1, 2.2 and 2.3. For convenience the barrier is represented as a thin wall (i.e. ). Sketch 2.1 shows a practical example of a finite barrier with the s
34、ource and observer on the opposite sides of the barrier and indicates the various propagation paths round and over the barrier which should be considered when estimating noise shielding. Sketch 2.1 LL1L2L3L4+= L20= 360= S Source O Observer B B B This page Amendment C -,-,- ESDU product issue: 2004-0
35、1. For current status, contact ESDU. Observe Copyright. 5 79011 ESDU Engineering Sciences Data Unit TM The total attenuation of the barrier shown in Sketch 2.1 is given by the combination of the attenuations due to the propagation paths over the top and round the edges of the wall. The attenuation o
36、f sound by each propagation path is defined in terms of the minimum length of ray path via the barrier edge under consideration. In the method for estimating attenuation it i0s assumed that the barrier extends to infinity in the line of the diffracting edge being considered. In practice it is often
37、only necessary to consider the edge giving the least sound attenuation as attenuations which are 10 dB more than the lowest can be neglected. A more complete discussion of finite barriers is given in Section 2.5. Considering any one of the sound paths SBO, shown in Sketch 2.1, the problem is simplif
38、ied by representing the barrier as an infinitely long barrier with the sound being diffracted by the top edge, as illustrated in Sketch 2.2. The diffracting edge of the barrier is located in the xz-plane with the edge at x = 0, stretching from to . Sketch 2.2 General arrangement Sketch 2.3 (View on
39、plane SBO) z=z += ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? B R S O xs xo rs* ro* Barrier -,-,- ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 6 79011 ESDU Engineering Sciences Data Unit TM The basic geometric relationships relat
40、ing the position of the source, barrier and the observer are as follows. The length, R, of the direct path, SO, is given by ,(2.1) and the length of the path via the edge, SBO, is given by ,(2.2) where the conversion from Cartesian to cylindrical polar co-ordinates is made using the following relati
41、onships (2.3) (2.4) and.(2.5) 2.3Shielding Parameters The effectiveness of a barrier depends on the geometric relationships relating the positions of the noise source, observer and the barrier. Shielding is evaluated using the four parameters defined in the following subsections. 2.3.1Distance param
42、eters The distance parameter takes account of the relative distances of the source and observer from the barriers edge and is given by ,(2.6) where is the greater of the source or observer distances from the barriers edge. The parameter is the acoustic scaling function and is given by , where is the
43、 lesser of the source or observer distances from the barriers edge. It is convenient to express as ,(2.7) Rxoxs()2yoys()2zozs()2+= lrors+()2zozs()2+= xrcos= yrsin= rx2y2+= 1 1 2R1 l - - = R1 2 2 R2 - - = R2 2 2Jf = -,-,- ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyrigh
44、t. 7 79011 ESDU Engineering Sciences Data Unit TM where.(2.8) For air k = 20.0468 m/(s K) or 65.7703 ft/(s K). 2.3.2Angle dependent parameters For clarity Sketches 2.1 to 2.3 are drawn for a thin flat barrier. The arrangement for a typical wedge-shaped barrier is illustrated in Sketch 2.4. The view
45、in Sketch 2.4 is in the vertical plane normal to the barrier. Sketch 2.4 General barrier shape (View normal to barrier) The limiting cases for are an infinitesimally thin vertical wall, where , and a flat surface, where . The two angle dependent parameters and are functions of and of and respectivel
46、y. Expressions for these quantities are given in Appendix A, Section A1.1. The values of and are found from Figure 2, using the following angular values for and . For use where Since is a function of and , the value of is dependent on the wedge angle and the angular symmetry of the source and observ
47、er with respect to the sides of the barrier. Also as =when and=when, and for use where =when and=when. J R2 kT - - = s o x S O 360= 180= M1M2os+()os() M1M212 M11 1os+os+ 12os+()os1os+458.1=360= M11.30= 2M2 os= 114344.1= 230.1= 2,s f X( ) 10.926X+() 21.792X3.104X2+() - -= g X( )24.142X3.492X26.670X3+() 1 = 210 3 Rxoxs()2yoys()2zozs()2+= -,-,- ESDU product issue: 2004-01. For current status, contact ESDU. Observe Copyright. 25 79011 ESDU Engineering Sciences Data Unit TM (iii)Calculate l from . (iv)Calculate using . (v)
链接地址:https://www.31doc.com/p-3756015.html