The interaction of the wind with the ocean surface has long been recognized as the major source of ocean ambient noise. High-frequency noise data 1200 to 2 kHz) has consistently been found to have strong wind-dependent characteristics associated with spray, splashes, bubbles, and rain. Recently, wave-wave interaction has been shown to be a source of infrasonic (0.2 to 2 Hz) noise and ocean bottom microseisms. Generally, the low-frequency noise (2 to 20 Hz) is associated with noise from distant ships. However, narrow-hand (as opposed to 1/3-octave) measurements show in addition to the noise from ships a wind-dependent characteristic. Furthermore, mid-ocean basin vertical directionality measurements show noise intensity near the horizontal with a broad frequency characteristic in diverse geographic locations. These results suggest a wind-generated noise due to a mechanism such as wave-wave interaction, wind turbulence, or the interaction of surface waves with turbulence is coupled into the mid-basin sound channel by either a shallowing sound channel such as found at high latitudes or a down-slope conversion process due to the basin boundaries and sea mounts. Theoretical expressions are derived from first principles following the approaches of Yen and Pertone [Naval Underwater Systems Center TR5833 (1979)] and Huon Li [Naval Ocean R&D Activity, TN89 (1981)] yielding the frequency-dependent radiation characteristics for wave-wave interaction, wind turbulence, and wave-turbulence interaction. These results show that wave-turbulence interaction is a possible source of wind-driven noise in the 10- to 200-Hz regions. Other possible mechanisms such as nonlinear capillary wave interactions are discussed and compared to this wave-turbulence mechanism.
Observed human-gait features in Doppler sonar grams are explained by using the Boulic-Thalmann (BT) model to predict joint angle time histories and the temporal displacements of the body center of mass. Body segments are represented as ellipsoids. Temporally dependent velocities at the proximal and distal end of key body segments are determined from BT. Doppler sonar grams are computed by mapping velocity-time dependent spectral acoustic-cross sections for the body segments onto time-velocity space, mimicking the Short Time Fourier Transform used in the Doppler sonar processing. Comparisons to measured data indicate that dominant returns come from trunk, thigh and lower leg.
The focus of this paper is to experimentally extract the Doppler signatures of a walking human's individual body segments using an ultrasonic Doppler system (UDS) operating at 40 kHz. In a human's walk, the major contribution to Doppler velocities and acoustic scattering is from the foot, lower leg, thigh (upper leg) and torso. The Doppler signature of these human body segments are extracted experimentally. The measurements were made by illuminating one of these body segments at a time and blocking the remaining body segments using acoustic screens. The results obtained in our experiment were verified with the results published by Bradley using a physics-based model for Doppler sonar spectrograms.
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