Improving AEDT Noise Modeling of Mixed Ground Surfaces (2017) / Chapter Skim
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Pages 5-18
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3-1 CHAPTER 3. THEORY The calculation of propagation of sound through the atmosphere consists of solving the wave equation, which for a uniform medium is shown in equation (1)
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3-2 Assuming that the acoustic waves are planar, the acoustic field, p, at the receiver can be described by: ൌ ݁భభ ݎଵ ܴ ݁భమ ݎଶ (2) where p0 is the pressure a unit distance from the source, k1 is the wave number in air, r1 is the length of the direct path, r2 is the length of the reflected path (where r2 = ra + rb)
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3-3 where the "ground wave" (also called the Boundary Loss Factor) function F(w)
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3-4 t = difference in transit time between the reflected ray path and the direct ray path = the phase angle of Q = 2f/(2f)
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3-5 porosity. Typical surfaces for which the parameters for both the one and two parameter model of the ground can be found in Table 1 and Table 2, respectively (ANSI/ASA S1.18, 2010)
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3-6 ߚଶ ݇ଵ ൌ 10.3 ൬ 1000݂ ߪ ൰ ି.ହଽ (21) Table 1 from the ANSI standard is based on work by Embleton et al.
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3-7 FIGURE 2. Basic ray geometry with reflection and single impedance discontinuity.
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3-8 being the Fresnel integral. The plus sign between the Fresnel integrals in Equation (22)
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3-9 path, and ܨఒ is an empirical constant, here set to 1/3 after Boulanger (1997)
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3-10 FIGURE 4. Example source-receiver geometry over strip of different impedance.
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3-11 modeling due to the computation resources involved. However, it is noted that the method does agree well with other models, but is not a practical solution to the problem at hand.
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3-12 one-third octave band noise measurements from 50 to 10000 Hz. This confirms the applicability of both ray theory and the one-parameter ground model for propagation over mowed grassland.
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3-13 impedances are likely to change more or less randomly both in their geometry as well as their impedance values. It should be noted that for the periodic case, Boulanger et al.
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3-14 Isolating the efficacy of a theory in how it compares to measurements is difficult at large distances because propagation over the intervening distance between source and receiver does not account for effects not addressed by the theory under study. As noted above, the objectives of this project will be met if the effects of ground impedance can be isolated and quantified.
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