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Army Research Office - Study of Sound Propagation in Air

The absorption Of sound by the atmosphere must be known to predict 
sound levels at receiving sites vell removed from the aoise source and te 
correct recorded noise spectra to standard conditions When making 
cæparisons Of the noise generated by different sources. 
factors which affect the intensity of the sound arriving at the receiving 
site can be classified as 
Spreading Losses 
Uniform spherical spreading 
Non—unif spreading 
Reflection by finite boundaries 
Refraction by non—uniform atmosphere 
Dif fraction (scattering by non—stationary atmosphere) 
Absorption Losses 
Absorption by ground and ground cover 
Absorption by the atmosphere 
Classical absorption 
Molecular relaxation absorptior. 
Of these sound propagation effects, different ones nay be the ing 
factor for different atmospheric conditions and for varying source—receiver 
placement; for any type of condition or any type of scund propa— 
gation path, the absorption due to classical and molecular effects are 
fixed for a hmogeneous atmosphere and are functions only of the propagation 
path distance. the humidity content, gas impurities, and the tenperature. 
The purpose Of the research reported here was to establish the correct 
values Of those fixed absorpcim losses. 
Prior vork In collaboration with Wyle Laboratories, with U. S. Army 
support. Led to the developrnt of a tiw.retical model of sound absorption 
in air. L This requires as input parmeters. the rate at vhich vihra— 
tional and rotational energy is transferred during binary molecular 
collisions. There are twenty four energy transfer rates Important for an