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Journal of the Acoustical Society of America

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May 2012

Volume 131, Issue 5, pp. EL355-4232

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A model for the vertical sound speed and absorption profiles in Titan’s atmosphere based on Cassini-Huygens data

Andi Petculescu and Peter Achi

J. Acoust. Soc. Am. Volume 131, Issue 5, pp. 3671-3679 (2012); (9 pages)

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Measurements of thermodynamic quantities in Titan’s atmosphere during the descent of Huygens in 2005 are used to predict the vertical profiles for the speed and intrinsic attenuation (or absorption) of sound. The calculations are done using one author’s previous model modified to accommodate non-ideal equations of state. The vertical temperature profile places the tropopause about 40 km above the surface. In the model, a binary nitrogen-methane composition is assumed for Titan’s atmosphere, quantified by the methane fraction measured by the gas chromatograph/mass spectrometer (GCMS) onboard Huygens. To more accurately constrain the acoustic wave number, the variation of thermophysical properties (specific heats, viscosity, and thermal conductivity) with altitude is included via data extracted from the NIST Chemistry WebBook [URL webbook.nist.gov, National Institute of Standards and Technology Chemistry WebBook (Last accessed 10/20/2011)]. The predicted speed of sound profile fits well inside the spread of the data recorded by Huygens’ active acoustic sensor. In the N2-dominated atmosphere, the sound waves have negligible relaxational dispersion and mostly classical (thermo-viscous) absorption. The cold and dense environment of Titan can sustain acoustic waves over large distances with relatively small transmission losses, as evidenced by the small absorption. A ray-tracing program is used to assess the bounds imposed by the zonal wind—measured by the Doppler Wind Experiment on Huygens—on long-range propagation.
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43.28.Bj Mechanisms affecting sound propagation in air, sound speed in the air
43.20.Hq Velocity and attenuation of acoustic waves

Impulse propagation in the nocturnal boundary layer: Analysis of the geometric component

Philip Blom and Roger Waxler

J. Acoust. Soc. Am. Volume 131, Issue 5, pp. 3680-3690 (2012); (11 pages)

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On clear dry nights over flat land, a temperature inversion and stable nocturnal wind jet lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal propagating in such a duct is received at long ranges from the source as an extended wave train consisting of a series of weakly dispersed distinct arrivals followed by a strongly dispersed low-frequency tail. The leading distinct arrivals have been previously shown to be well modeled by geometric acoustics. In this paper, the geometric acoustics approximation for the leading arrivals is investigated. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the distinct arrivals have been determined. The time delay between and relative amplitudes of the direct-refracted and single ground reflection arrivals have been investigated as parameters for an inversion scheme. A two parameter quadratic approximation to the effective sound speed profile has been fit and found to be in strong agreement with meteorological measurements from the time of propagation.
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43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors
43.28.Js Numerical models for outdoor propagation
43.20.Bi Mathematical theory of wave propagation
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