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The sources and characteristics of sounds at altitudes of 60 000 ft were investigated with a balloon-borne acoustic probe and telemetering system. The system is described and samples of analyzed data presented. Wideband background levels of 0.2 d/cm2 were measured consistently. Spectral energy per cycle was found to decrease with frequency by about 6 db per octave, a result in good agreement with current theories of noise radiated by turbulence. Cross-correlated data from a two-probe array indicated that most of this energy propagates from lower altitudes. It follows that a major source of high-altitude acoustic noise is lower altitude atmospheric turbulence. In addition to background noise, some specific types of acoustic signals have been detected and identified. Analyzed samples are presented. Some observed Doppler effects are discussed and their possible use for calculating absorption coefficients of low frequency sound in air is explained. (The acoustic probe and financial support for this work were furnished by the U. S. Army Signal Missile Support Agency under contract.)

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