Overground Excess Sound Attenuation (ESA). Volume 2. Analysis of Data for Flat Grassy Terrain Conditions.
Final rept. Aug 82-Feb 84,
BOLT BERANEK AND NEWMAN INC CANOGA PARK CA
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Analysis of the ESA data acquired under the Air Force measurements at Wright-Patterson AF Base show the expected large variations with weather and ground surface conditions for flat grassy terrain. Multiple linear regression analyses show low to moderately high correlations of excess attenuation data with the wind component in the direction of sound propagation, the temperature gradient and the ground surface grass or snow. A characteristic of the data is the maximum excess attenuation typically occurring between 160 and 250 Hz which is related to the ground impedance. This maximum is evident in all data except under large negative wind component and negative temperature gradient conditions. Theoretical calculations show a reasonable fit of the ESA data for surface flow resistivity values of 100 to 200 cgs units for grass and 20 to 50 cgs units for snow. The extent of changes in ESA values with changes in wind component or temperature gradient varies considerably with frequency and distance. At low frequencies, typically 25 to 100 Hz, attenuation values generally show well-ordered increases with decreasing wind component and temperature gradient values. At frequencies above about 300 Hz, ESA sensitivity to wind component and temperature gradient increases, with generally a greater range of change occurring for negative values of wind component and temperature gradient than for positive values. Comparison of the ESA values for near zero wind component and neutral temperature conditions with theoretical models shows general good fit if one allows for a moderate increase in the flow resistivity with distance.
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