Abstract
Experimental values of sound absorption were obtained from noise signals recorded at five ground positions during a series of 20 flyovers by two aircraft during a single day of field measurements. Differences in 13-oct band noise levels observed at the ground positions for the same angle of radiation from the aircraft were used to determine absorption values which are compared to calculated values of sound absorption based upon both surface and altitude measurements of temperature and humidity. The analysis did not assume uniformity of the sound source from flight to flight nor uniformity of sound power output or directivity characteristics for flights made at different altitudes. Experimental atomspheric absorption values based upon combined data for all flyovers showed good agreement (±0.8 dB per 1000 ft) with calculated absorption values for frequencies from 1250 to 6300 Hz. For the frequency range from 400 to 1000 Hz, experimental values (typically 2 dB per thousand feet) were significantly greater than calculated values. [Work supported by NASA.]
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