High-Altitude Acoustic Research

Abstract

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.)

Infrasound in the middle stratosphere measured with a free‐flying acoustic array

“…Each balloon payload had one to two pressure sensors that sent data to a ground‐based mobile tracking station via a radio link. The sensors detected signals from propeller aircraft and other more enigmatic sources superimposed on an acoustic background originating from turbulence in the troposphere [ Wescott , , , ; Meecham and Wescott , ]. This, along with an Air Force initiative in the 1950s named Project Mogul [ Weaver and McAndrew , ], appears to be the totality of in situ stratospheric infrasound measurements reported in scientific literature prior to the one described here.…”

The AtmoSOFAR Channel: First Direct Observations of an Elevated Acoustic Duct

“…The use of balloons for infrasound monitoring was first attempted over 50 years ago (Weaver & McAndrew, 1995; Wescott, 1964). However, until recently their potential was largely unexplored.…”

High-altitude atmospheric turbulence and infrasound measurements using a balloon-launched small uncrewed aircraft system

“…Many experiments have conducted stratospheric turbulence measurement using balloon-borne instruments (e.g. Wescott et al, 1964;Ehrenberger, 1992;Haack et al, 2014) with previously published studies of stratospheric turbulence dating back to the 1960s (Enlich and Mancuso, 1968). Among the most relevant conclusions from these studies is that stratospheric turbulence tends to form in relatively thin atmospheric layers due to intrinsic static stability at these altitudes.…”