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 N(2)-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.

The majority of standard underwater propagation models provide a two-dimensional (range and depth) acoustic field for a single frequency point source. Computational resource demand increases considerably when the three-dimensional acoustic field of a broad-band spatially extended source is of interest. An upgrade of the standard parabolic equation model RAM for use in a high-performance computing (HPC) environment is discussed. A benchmarked upgraded version of RAM is used in the Louisiana Optical Network Initiative HPC-environment to model the three-dimensional acoustic field of a seismic airgun array. Four-dimensional visualization (time and space) of the generated data volume is also addressed. [Research supported by the Louisiana Optical Network Initiative, TeraGrid Fellowship, and the Joint Industry Programme through the International Association of Oil and Gas Producers.]

PACS number(s): 43.28.Bj [ADP] In the original article, 1 the authors used the wrong order of magnitude for viscosity, which resulted in the attenuation coefficient shown in Fig. 5(a) to be erroneously scaled up by a factor of roughly 10 6 m À1 . The correct vertical profiles of the attenuation coefficient a are shown in Fig. 1 below. 1 A. Petculescu and P. Achi, "A model for the vertical sound speed and absorption profiles in Titans atmosphere based on Cassini-Huygens data," J. Acoust.

This research is part of a study investigating the characteristics of thunder on Saturn’s largest moon, Titan. In tandem with electromagnetic signatures, thunder can corroborate and quantify lightning discharges. A physical model for the propagation of thunder on Titan, based on the most recent data collected by the Cassini–Huygens mission, is being developed. The model approximates a tortuous 20 km cloud-to-ground lightning channel by an angle-wise random walk of small discharge segments, each generating a strong cylindrical shock wave, which acquires an N-wave shape after it travels through the relaxation radius, into the acoustic regime. These acoustic waves are then propagated to the far-field detector where they are added linearly to form long-range thunder. The detectability of thunder signatures by a sensor in Titan’s lower atmosphere depends on the moon’s atmospheric structure. In order to constrain the fraction of acoustic energy reaching a detector in Titan’s troposphere, the model accounts for the upward-refracting sound speed profile up to the inversion point at ∼45 km and also for ground effects. The sound speed and attenuation are computed along the length of the lightning channel (20 km) using altitude-dependent pressure, density, and temperature measurements by Cassini–Huygens, as well as thermophysical parameters (specific heats, viscosity, thermal conductivity, and diffusivity) extracted from NIST’s Chemistry WebBook.

The astronauts' ability to communicate easily among themselves or with the ship's computer should be a high priority for the success of the mission. Long-duration space habitats—whether spaceships or surface bases—will likely be larger than present-day Earth-to-orbit/Moon transfer ships. Hence an efficient approach would be to free the crew members from the relative burden of having to wear headsets throughout the spacecraft. This can be achieved by placing microphone arrays in all crew-accessible parts of the habitat. Processing algorithms would first localize the speaker and then perform speech enhancement. The background ``noise'' in a spacecraft is typically fan and duct noise (hum, drone), valve opening/closing (click, hiss), pumps, etc. We simulate such interfering sources by a number of loudspeakers broadcasting various sounds: real ISS sounds, a continuous radio stream, a poem read by one author, etc. To test the concept, we use a linear 30-microphone array driven by a zero-latency professional audio interface. Speaker localization is obtained by time-domain processing. To enhance the speech-to-noise ratio, a frequency-domain minimum-variance approach is used. Time-permitting, we will discuss array weight sensitivity to parameters such as frame length/overlap, windowing, (sub-)array structure, etc. [This work was supported by the Louisiana Space Consortium (LaSPACE).]