885095951

10:10

8NS6. Recent progress in outdoor sound propagation prcdiction by the parabolic equation method.

Kenneth E. Gilbert, Xiao Di, and Chulsoo You (Natl. Ctr. for Physical Acoust., University. MS 38677)

During the past 5 years significant progress has been madę in predicting long-range sound propagation in the almosphere using numerical methods based on the parabolic approximation to the Helmholtz equa-tion. The theory for the parabolic approximation is discussed starting from the one-way wave approximation to the two-way (Helmholtz) wave equation. The application of the theory for practical numerical algo-rithms is briefly reviewed, and some results are presented for sound propagation in a turbulcnt atmosphere. Finally, a new Grccn’s function approach to the parabolic cquation is presented. The new method is approximately 100 times faster than existing parabolic equation methods for outdoor sound propagation. Hence the new algorithm has the potential for useful calculations in the field using a desktop Computer. (Work supported by NASA.)

10:35

8NS7. Norma! modę solution for a downward refracting atmosphere above a complex impedance piane.

Richard Raspet and Gordon Baird (Physical Acoust. Res. Group. Univ. of Mississippi, University, MS 38677)

The development of the fast field and parabolic equation Solutions to the wave equation has madę it possible to solve for the combined effects of refraction in a layered atmosphere and the intcraction of sound with a complex impedance ground surface. In many respects the numerical methods have advanced beyond an understanding of the basie phenomena. In an earlier study [J. Acoust. Soc. Am. 89, 107-114 (1991)], the residue series solution for upward refraction was investigated and provided insight into the naturę of the interaction of refraction and ground reflection. In this paper, results of a similar normal modę solution are presented for downward refraction by a bilinear sound velocity profile above a complex impedance ground surface. This model is used to investigate when the surface wave is excited for downward refraction conditions and to develop criteria for the maximum rangę of cylindrical decay as a function of ground impedance phase and magnitude and the magnitude of the sound velocity gradient. (Work supported by NASA Langley Research Center.]

Contributed Papers

11:00

8NS8. A simple exact solution for a point source above a reacting surface. Xiao Di and Kenneth E. Gilbert (Natl. Ctr. for Physical Acoust., University, MS 38677)

An exact analysis is developed for a spherical source in air above a reacting surface. The solution, which is valid for both local and ex-tended reaction, proceeds along the standard route of an expansion of the spherical wave as a superposition of piane waves. Use of a Laplace transform on the plane-wave reflection coefficient leads to a finał form that requires a simple numerical integration of an image integral. The resulting solution is exact for all angles of propagation. For a locally reacting ground surface, the image integral is rapidly convergent. A number of limiting cases are investigated and compared with previous Solutions. (Work supported by NASA.]

11:15

8NS9. Correlation between excess attenuation and geometrie or meteorological parameters. Charles T. Moritz and John S. Lamancusa (Ctr. for Acoust. and Vib., Penn State Univ., University Park. PA 16802)

Extcnsive cxpcrimcntal measurements of sound propagation and vertical sound-speed profiles over level agricuitural terrain are presented. A high-power speaker generating pure tones at approximately onc-third ociavc band center frcqucncics from 160—4000 Hz waa uscd aa the noise source. Sound lcvels were measured and recorded at ranges of 2, 125, 250, and 500 m from the source during various time periods of interest from 3-7 July 1990. Simultaneously, meteorological conditions were measured using a vanety of tower mounted anemometers and temperaturę sensors. A total of 50 sets of 10-min averagcd data at each

1991 J. Acoust. Soc. Am., Vol. 89. No. 4, Pt. 2, April 1991 of 12 frequencies and 4 ranges was obtained. Temperaturę and ve!ocity profiles were reconstructed using Monin-Obukhov scaling theory. A statistical regression analysis to determinc the correlation between cx-cess attenuation and mcasurable geometrie and meteorological parameters was completed. Sample measurements are presented and compar-isons are madę to the Daigle model (J. Acoust. Soc. Am. 65, 45-^49 (1979)].

11:30

8NS10. The use of a one-parameter model to characterize the acoustical properties of asphalt. K. Van Wyk, J. S. Bolton, and P. J. Sherman (Ray W. Herrick Labs., School of Mech. Eng., Purdue University, West Lafayette, IN 47907)

Following a suggestion by Delany and Bazley (J. Sound Vib. 16. 315-322 (1971)], a single parameter (flow resistivity) fibrous materiał model has frequently been used to model the acoustical properties of outdoor surfaces. Embleton et al.[J Acoust. Soc. Am. 74. 1239-1244 (1983)] have shown that an effective flow resistivity for an outdoor surface may be inferred by matching measured and predictcd excess attenuations. In this paper, our experience in using such an approach to characterize the acoustical properties of asphalt surfaces will be de-scribed. A point source placed directly on the asphalt surface has been used to generate a broadband noise signal. A microphone placed directly on the surface was used to measure l/3-octave band sound levels as a function of frcqucncy (1-20 kHz) and distancc from the source (0.1-12.8 m). The flow resistivity of the surface was estimated by com-paring theoretical and measured level differences between 0.1 m and the other measurement locations. It was found ihat it is possible to obtain consistent estimates of flow resistivity if the data subset (i.e., the rangę of frequencies and distances to which the theory is compared) is chosen

121st Meeting: Acoustical Society of America 1991