Leo L. Beranek, Fred S. Elwell, John P. Roberts
naca-tn-2079
May 1950
The present work is part of a program, the objective of which is to find practicable ways of reducing the external noise level of light airplanes in order to make them less objectionable to persons on the ground.
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http://naca.central.cranfield.ac.uk/reports/1950/naca-tn-2079.pdf