485 86th Meeting ß Acoustical Society of America 485

acceptable. Most, however, were clearly acceptable. The maximum ac0eptable level was found to be 85 dBA at 50 ft.

Z:54

VV8. Acoustic Scattering by Turbulence in Typical Atmo- spheres. F. F. HALL, JR., J. E. GAYNOR, AND W. D. NEFF, Wave Propagation Laboratory, NOAA Environmental Research Laboratories, Boulder, Colorado 8030Z.--Acoustic scattering in the atmosphere is determined by velocity structure, C• 2, and temperature structure, CT 2. A knowledge of these structure parameters is important in acoustic echo sounding, and in studies of noise propagation by scattering. Recent acoustic scattering measurements, wind variance studies, and in situ temperature difference measurements have provided a better understanding of C• • and CT •. Under convective conditions, the average value of C• •' over prairie terrain in daytime is nearly constant with height for the first several hundred meters with a value at 100 m of 0.086 m 4• sec -•. Although similarity theory predicts constant values of C• • within the convectively mixed boundary layer, measured values decrease with height from 0.096 at 75 m to 0.068 at 150 m. The standard deviation of C• 2 increases slightly with height, however. Typical C• • values at 100 m under strong convection average to 0.004øC • m-a, with values decreasing with height as z -4• as predicted by similarity theory. When these values are substi- tuted into the acoustic scattering cross-section equation, it becomes apparent that the C•? term is several orders of magni- tude more important in contributing scattered sotind than C• •, especially for scattering in the forward direction.

3:06

VV9. Fluctuations in the Propagation of Sound near the Ground. T. F. W. EMBLETON, N. OLSON, J. E. PIERCY, AND D. F. ROLLIN, Division of Physics, National Research Council, Ottawa, Ontario, Canada.--The fluctuation in the propagation of a pure tone over a flat grassy surface from a source on the ground has been studied over ranges of frequency (300-4000 Hz) of the source as well as height (0-5 m) and distance (15-150 m) of the receiving microphone. Preliminary analysis of the records indicates that the standard deviation of signal level about the mean initially increases nearly linearly with distance as found by previous experimenters, but for longer distances saturates at •6 dB as found previously only in the propagation of light. The dependence on frequency found for the unsaturated region is considerably greater than the 7/12 power predicted for large-scale isotropic turbulence, in keeping with the reduced scale expected near the ground. Signal correlation along a wavefront has also been measured. The results place limitations on the use of coherent acoustic theory for the calculation of the attenuation of noise in practice by barriers and ground interference, as well as the use of modeling techniques for solving outdoor propagation problems.

3:18

VV10. Acoustic Scattering by Reflective Barrier Geometrics. R. G. HOLM AND T. J. TRELLA, Transportation Systems Center, Department of Transportation, Cambridge, Massa- chusetts OZl4Z.--Attenuation of sound by barriers is of much practical interest for abatement of highway and urban rapid transit noise. An effective attenuation prediction scheme cannot be based on the usual diffraction from a rigid semi- infinite plane concept because the ground effect creates localized regions of high attenuation. Assuming a coherent line source radiating above a reflecting plane, theoretical expressions are derived for the scattered sound field of semi- elliptic and wedge-shaped acoustically opaque barriers. Theo-

retical insertion loss predictions, at several measurement locations in the diffracted field, show major dependenceies on the source's position and frequency, but only a minor dependency on barrier shape. Experimental measurements agree favorably with theoretical predictions.

3:30

VVll. Design Analysis of Highway Noise Barriers. HUGH J. SAURENMAN, Cavanaugh Copley Associates, Newtonville, Massa- chusetts 02160.--Use of solid walls and earth berms as highway noise barriers is becoming more widespread. A survey of the methods generally accepted for use in acoustical evaluation and prediction of barrier performance indicates the following: First, most methods are based on the theory of Maekawa (diffraction by a semi-infinite screen). Second, the methods are either too complex to use except for final design evaluation, or they are too simplified for use except in preliminary design stage. This paper describes a method developed to bridge this gap. The method is based on the results of Maekawa and involves the analysis of a single truck pass-by. The vehicle is modeled as three incoherent noise sources representing stack, engine, and tire noise. The attenuation for each octave band is determined; then the octave bands are combined to give an A-weighted sound level. Evaluation of the effectiveness of the sound wall is done by comparing the noise signature with and without a barrier present. The method is simple enough to allow comparison of many barriers in a short period of time, but complete enough to be considered as accurate as the Maekawa results it is based on. Results from use of the

method in the design and analysis of several projects are presented.

3:42

VV12. Model Study of the Propagation of Sound from V/STOL Aircraft into Urban Environs. PAUL R. DONAVAN AND RICHARD H. LYON, Acoustics and Vibration Laboratory, Massachusetts Institute of Technology, Cambridge, Massa- chusetts 02139.•The propagation of sound from elevated noise sources into urban environs was studied with a series of

model experiments. The experiments included the modeling of previous urban propagation field studies, the measurement of sound enhancement and barrier effects in an urban region, and the determination of the significance of building facades in urban propagation. Results of the study were expressed in the difference in sound level observed between a particular urban source-receiver geometry and a corresponding open field geometry. The results expressed in this manner can be applied directly to many urban situations to predict sound levels from known noise sources. In addition to the model

results, a more general method of sound level prediction based solely on urban geometries has been developed. With the model results and elementary psychoacoustic considera- tions, urban geometries which produce the most and least noise intrusion have been rank ordered for the specialized case of V/STOL aircraft. [Work supported by U.S. Depart- ment of Transportation.-]

3:54

VV13. Model Studies of Effects on'Motor Vehicle Noise of Shapes and Arrangements of Buildings along Urban Streets. III. Roadside Barriers. V. C. PLANE AND V. O. KNUDSEN, Department of Physics, University of California, Los Angeles, California 90024.--This paper is Part III of a continuing investigation. Parts I and II were presented at the recent ASA Boston Meeting [J. Acoust. Soc. Am. 54, 341 (A) (1973)-1. The adjustable model investigated since then relates to noise

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486 86th Meeting ß Acoustical Society of America 486

fields resulting from differing geometrical arrangements of roadbed and roadside reflective and absorptive materials. The model has inclined walls 12 ft X6 ft hinged to the 12 ft X 16 ft floor. Walls and floor are •-in. Masonite hardboard (absorption coefficient 0.02 at relevant frequencies); it thus simulates reflective roadbed and roadside boundaries. Noise

reductions resulting from facing of floor and/or walls with 2-in. fiberglas (absorption coefficient 0.85 to 0.90) were measured for five upward inclinations of the walls between 0 ø and 90 ø. The sound source was 425-9600-Hz pink noise, located abov• the floor. Results are presented in curves showing sound pressure levels versus straight-line distance from source across floor, and up and above the inclined walls. A reference curve gives free-field levels and thus the curves reveal the increase in levels from reflective boundaries and the decrease from

absorptive ones, which is very large at near grazing incidence. Applications to highway designing are discussed, including estimates of possible noise reductions.

VV14. Abstract withdrawn.

FRIDAY, 2 NOVEMBER 1973 GARDEN Roon EAST, 1:30 P.M.

Session WW. Architectural Acoustics IV: Sound Absorption and Sound Transmission Loss

PAUL B. OSTERGAARD, Chairman

Osterœaard Associates, Caldwell, New Jersey 07006

Contributed Papers (15 minutes)

WWl. Abstract withdrawn.

1:30 sound power was electroacoustical--an array of four loud- speakers having their audio input voltage waveforms ampli- tude-modulated at frequencies F=0.! to 0.4 Hz. The sound pressure I p•'l was measured with an array of four microphones. A trihedral vane rotated at the center of the chamber. The

phase • was obtained by means of an analog computer (phase-meter) based on a least-squares principle of design. The results for the absorption cross section A are compared with cross sections obtained by the ASTM standard method of test based on the time decay of sound energy in a reverbera- tion chamber. The recording phase meter displayed • as a running function of time. From the recordings we obtained also (a) the influence of the vanes on diffusion, (b) the inter- action between the loudspeaker source and the sound energy in the chamber, and (c) the relative effects of bands of noise and frequency modulation at various audio frequencies.

1:45

WW2. Results of Measurement of Absorption and Sound Power with Modulated Reverberation. RICHA}•D K. COOK, National Bureau of Standards, Washington, D.C. 20234.-- Measurements of absorption A and sound power W0 have been made at various audio frequencies in the 425-m 3 rever- beration chamber of the Bureau. The modulated source of

2:00

WW3. A Unique Method to Measure the Sound Absorption Coefficient for Oblique Incidence. I. L. Via, Bolt Beranek and Newman Incorporated, Cambridge, Massachusetts 02138. m The method requires subsequent accurate recording of the absolute value of the transfer function between omnidirec-

tional sound source and microphone in an anechoic environ- ment for three configurations: (1) without the sample or barrier, (2) without the sample but with a barrier blocking the line of sight between source and receiver, and (3)with both the barrier and the sample in place. The first two record- ings "calibrate" the barrier, while the second and third provide an interference pattern between the sound refracted around the edge of the barrier and that reflected from the sample. The reflection factor given as

J. Acoust. Soc. Am., Vol. 55, No. 2, February 1974

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