Acoustic multipath arrivals in the horizontal plane due to approaching nonlinear internal waves

Author Posting. © Acoustical Society of America, 2011. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 129 (2011): EL141-EL147, doi:10.1121/1.3553374.Simultaneous measurements of acoustic wave transmissions and a nonlinear internal wave packet approaching an along-shelf acoustic path during the Shallow Water 2006 experiment are reported. The incoming internal wave packet acts as a moving frontal layer reflecting (or refracting) sound in the horizontal plane. Received acoustic signals are filtered into acoustic normal mode arrivals. It is shown that a horizontal multipath interference is produced. This has previously been called a horizontal Lloyd’s mirror. The interference between the direct path and the refracted path depends on the mode number and frequency of the acoustic signal. A mechanism for the multipath interference is shown. Preliminary modeling results of this dynamic interaction using vertical modes and horizontal parabolic equation models are in good agreement with the observed data

Observation of sound focusing and defocusing due to propagating nonlinear internal waves

Fluctuations of the low frequency sound field in the presence of an internal solitary wave packet during the Shallow Water ’06 experiment are analyzed. Acoustic, environmental, and on-board ship radar image data were collected simultaneously before, during, and after a strong internal solitary wave packet passed through the acoustic track. Preliminary analysis of the acoustic wave temporal intensity fluctuations agrees with previously observed phenomena and the existing theory of the horizontal refraction mechanism, which causes focusing and defocusing when the acoustic track is nearly parallel to the front of the internal waves

The Modeling of Anisotropic Fuselage Lining Material

In this paper a theoretical model that can account for the effect of lining anisotropy on sound transmission through fuselage structures is developed. The model allows for anisotropic flow resistivity, tortuosity and elastic moduli. Implicit to the theory is a characteristic dispersion relation of sixth order that yields the allowed wave numbers for wave propagation in anisotropic elastic porous media. In addition, explicit expressions for field variables such as displacements and stresses appropriate for anisotropic foams are derived. Predictions of random incidence sound transmission loss for double panels with anisotropic linings have been performed. To verify the prediction, the theoretical results have been compared with random incidence transmission loss measurements

Highfrequency acoustic propagation in the presence of ocean variability

Broadband mid-to-high frequency (0.6-18 kHz) acoustic wave propagation in shallow coastal waters (< 20 m) is influenced by a variety of oceanographic conditions. Physical parameters such as temperature and salinity as well as hydrodynamic parameters such as surface waves, tide and current can influence amplitude and travel time of signal transmissions. In this paper a unique set of simultaneous ocean and acoustic observations that reveal interesting temporal behavior of the acoustic signal and its correlation with environmental variability are presented. The temporal variations in salinity, including those induced by the semi-diurnal tides and a northerly wind event, are accurately predicted by using the measured acoustic signals and temperature profile

3D sound intensity variability in shallow water in presence of internal waves in SWARM'95 experiments

Broadband shot signal data are analyzed from the SWARM'95 experiment to investigate acoustic variability in presence of internal solitons. A 10 to 15 minute temporal variations in the intensity of the received signals were observed. These temporal variations are azimuthally dependent on variability of water column in the presence of internal solitary waves. These fluctuations should be explained by significant horizontal refraction (3D-effects) taking place when orientation of acoustic track is close to direction of the wave front of internal solitons. Analysis on the base of both equation of vertical modes/horizontal rays and PE in horizontal plane is carried out, good agreement between theoretical calculations and experimental data is obtained