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Apr 1982

Volume 71, Issue S1, pp. S1-S113

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back to top Session Q. Noise III: Outdoor Noise Barriers
Invited Papers
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Analytical models of sound diffraction by barriers: A review (A)

Allan D. Pierce

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S34-S35 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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During the past few years, considerable success has been achieved in the further development and numerical implementation of analytic solutions for barrier diffraction. The older Fresnel‐Kirchhoff theory of diffraction has proven to be inadequate for many cases of practical application, but is an unnecessary oversimplification because simple formulas (including the Fresnel number approximation) result in the uniform asymptotic expansion limit for the exact solution of diffraction by a rigid wedge. Although the MacDonald solution in its original form is cumbersome to apply to general source‐listener configurations, numerical calculations become simple when contour deformation techniques are employed. Also significant is Medwin's application of FFT algorithms to the transient solution of Biot and Tolstoy, which we now know is equivalent to MacDonald's solution. Williams' and Maliuzhinetz's solution for impedance wedges has led to approximate models for nonrigid barriers. Three‐sided barriers can be handled using Keller's geometrical theory of diffraction (GTD), which also allows the theory for plane wave diffraction to be used when the source is localized and when the barrier rests on the ground. Hayek and others have used creeping wave expansions for curved barriers; Fock's theory allows a smooth transition into the shadow zone.
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Barriers and ground effects: practice, model experiments, and theories (A)

T. F. W. Embleton, J. E. Piercy, and G. Daigle

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S35-S35 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The sound field behind a barrier is usually determined by both diffraction and interference. This interference is dependent on the acoustic impedance of the ground surface, and is significant when predicting insertion loss (as opposed to attenuation) of barriers in many typical locations. Field measurements behind a highway barrier show small but systematic deviations from expected results that suggest scattering above about 4 kHz and the need for more precise diffraction theory at all frequencies. Controlled experiments outdoors using a pure‐tone sound source, and simultaneously measuring the temperature and velocity structure constants CT2 and CV2 of the turbulent atmosphere, show reasonable agreement between predicted and measured effects of scattering by turbulence into the shadow region behind a barrier. Scale model experiments indoors (no turbulence) show, with greater precision, similar discrepancies between measured results and those predicted by Kirchhoff‐Fresnel and related diffraction theories−from 2 to 10 dB depending on geometrical configuration. These discrepancies can be eliminated using predictions based on either layer‐potential theory [e.g., P. Filippi, J. Sound Vib. 54, 473–500 (1977)] or direct and accurate evaluation of the diffraction integral [W. J. Hadden and A.D. Pierce, J. Acoust. Soc. Am. 69, 1266–1276 (1981)].
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Impulse studies of scattering by finite noise barriers (A)

Herman Medwin

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S35-S35 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Laboratory studies of shadowing by noise barriers most often use impulse sources, primarily because this permits the separation of reflections from diffractions and it avoids interference from extraneous boundaries of the experiment. A computer implementation of an exact, closed‐form impulse theory of diffraction and reflection by plates and wedges has similar advantages: the delta function source permits direct calculation of reflection and diffraction components due to each face and edge of a finite barrier, as well as reflections from the ground, so that the times and amplitudes of all contributions are clearly presented. When the total impulse response is fast Fourier transformed, excellent agreement is found with shadowing losses measured in the frequency domain. Computer optimization of finite barrier design is simple, and double diffraction at a thick barrier becomes physically meaningful by use of a discrete impulse interpretation of Huygens' principle. [Work supported by the Office of Naval Research.]
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Highway traffic noise prediction with and without barriers—Some results and observations (A)

J. M. Lawther

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S35-S35 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Although the past decade has been a time of rapid increase in sophistication of models for predicting highway traffic noise levels including the effects of barriers, several important questions remain unanswered. This paper describes some barrier and near‐ground propagation modeling work recently completed and nearing completion at Penn State, and discusses questions arising therefrom. A point of view looking towards the needs of the utlimate users of traffic noise prediction models is attempted. Experiences in both indoor and outdoor barrier testing and with both controlled sources and free‐flowing traffic sources will be drawn upon in the presentation.
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Cost‐effective barrier design along roadways (A)

Grant S. Anderson

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S35-S36 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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The Federal Highway Administration (FHWA) method for prediction of highway traffic noise and of noise‐barrier performance is incorporated in their computer program STAMINA. For the FHWA, we have recently developed and documented an adjunct computer program OPTIMA, which combines the acoustical output of STAMINA with information on barrier costs and receiver noise criteria, to guide the user towards a balanced barrier design. A balanced design is one that provides the best barrier performance for a given cost, or equivalently, one that costs the least for a given performance. The measure of barrier costs is dollars, with unit costs supplied by the user. The measure of barrier performance is the reduction in acoustic energy density, summed over all receivers, after normalization to each receiver's noise criterion. This paper introduces the mathematical concepts within OPTIMA. The use of OPTIMA, together with STAMINA, is now being taught by the FHWA to state and regional highway engineers. [Work supported by FHWA.]
Contributed Papers
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Sound propagation over an impedance plane (A)

Matthew A. Nobile and Sabih I. Hayek

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S36-S36 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A comprehensive model developed by ARL/PSU [J. Acoust. Soc. Am. Suppl. I 68, S54 (1980)] is currently being used to predict sound levels behind a vertical highway noise barrier. An integral part of the theoretical problem involves the propagation of sound above an impedance plane (as is modeled the ground on either side of the noise barrier), to which a modified version of Lawhead and Rudnick's solution [J. Acoust. Soc. Am. 23, 546–549 (1951)] has been adapted. This solution, however, relies heavily on an approximation that greatly simplifies and reduces the field integral to the complementary error function form. As an alternative, a transformation of variables on a similar integral allows a direct evaluation in terms of two asymptotic series in inverse powers of kr, one of which is multiplied by the complementary error function. The first term in the overall solution is identical to Thomasson's solution [J. Acoust. Soc. Am. 61, 659–674 (1977)], and further terms should afford an improvement in accuracy. The importance of the higher‐order terms relative to kr, ground impedance, and angle of incidence is explored.
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Outdoor noise propagation above a porous half‐space (A)

Louis R. Quartararo and Sabih I. Hayek

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S36-S36 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Assuming an extended reaction in the lower half‐space, Attenborough et al. [J. Acoust. Soc. Am. 68, 1493–1501 (1980)] have obtained an exact solution for the propagation of noise due to a point source above a homogeneous and isotropic porous ground. The acoustic pressure calculated by this extended reaction model depends upon the complex values of the density, ρ, and the sound speed, c, of the ground. In contrast, the local reaction models of Thomasson et al. and Lawhead and Rudnick depend upon the complex normal impedance of the ground, given by the product ρc. For independently varied ρ and c, noise levels are predicted using the extended reaction model, and these levels are compared to the levels predicted by the local reaction models. Measurements of noise propagating over grass, by Embleton and Piercy, and by The Applied Research Laboratory, are compared to both models. Values of ρ and c for the measured cases were obtained from the measured complex impedance of the grass, and its frequency dependent flow resistance. [Work supported by FHWA.]
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