Abstract
A balloon-borne acoustic interferometer for determining the local virtual air temperature of the upper atmosphere (up to 45 km) through phase-velocity measurements of sound is described. The spacing between a plane-wave emitter and a receiver-reflector is held fixed while the frequency of the emitter input is periodically varied between 24 and 30 kc/sec. A standing wave is generated between the emitter and receiver; and the receiver output, consisting of a frequency-amplitude modulated waveform, is telemetered to a ground station. The phase velocity of sound is determined by an amplitude-frequency measurement of the received signal, and the virtual air temperature is then calculated. Under certain conditions, measurements of sound absorption and specific acoustic impedance are possible by amplitude and frequency bandwidth measurements of the received signal. These measurements can then be used to determine the air density and coefficient of air viscosity at high altitudes. The effects of wind, atmospheric pressure, and other end effects of the sound transducers are described. The results of contemplated balloon flights are summarized. [Supported in part by U. S. Army Signal Corps Grant No. DA-SIG-36-039-62-G17 and Air Force Cambridge Research Laboratories.]
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