D.C. Finfer

Clutter suppression and classification using twin inverted pulse sonar (TWIPS)

This paper describes the detection and classification of targets against clutter by distinguishing between linear and nonlinear scatterers and, further, by distinguishing those nonlinear targets that scatter energy at the even-powered harmonics from those that scatter in the odd-powered harmonics. This is done using twin inverted pulse sonar (TWIPS), which can also, in some manifestations, require no range correction (and therefore does not require the a priori knowledge of the environment needed for most remote detection technologies). The method applies, in principle, to a range of sensor technologies, including the use of radar to distinguish between circuitry, metal and soil; Light Detection and Ranging (LIDAR) to detect combustion products; and Magnetic Resonance Imaging (MRI). A sonar application is demonstrated, detecting objects in bubbly water (including in the wake of a ship of 3953 gross register tonnage). A man-made sonar that can operate in bubbly water is relevant: Cold War sonar is not optimized for the shallow coastal waters that typify many current operations. The US Navy use dolphins in such waters. TWIPS arose as a demonstration that echolocation was possible in bubbly water in response to a video showing dolphins generating bubble nets when hunting: if echolocation were impossible in these nets, then during this hunt, the dolphins would have blinded their sonar.

Clutter suppression and classification using twin inverted pulse sonar in ship wakes

Twin inverted pulse sonar (TWIPS) is here deployed in the wake of a moored rigid inflatable boat (RIB) with propeller turning, and then in the wake of a moving tanker of 4580 dry weight tonnage (the Whitchallenger). This is done first to test its ability to distinguish between scatter from the wake and scatter from the seabed, and second to test its ability to improve detectability of the seabed through the wake, compared to conventional sonar processing techniques. TWIPS does this by distinguishing between linear and nonlinear scatterers and has the further property of distinguishing those nonlinear targets which scatter energy at the even-powered harmonics from those which scatter in the odd-powered harmonics. TWIPS can also, in some manifestations, require no range correction (and therefore does not require the a priori environment knowledge necessary for most remote detection technologies).

Hypotheses regarding exploitation of bubble acoustics by cetaceans

Bubbles are the most acoustically active naturally occurring entities in the ocean, and cetaceans are the most intelligent. Having evolved over tens of millions of years to cope with the underwater acoustic environment, cetaceans may have developed extraordinary techniques from which we could learn. This paper outlines some of the possible interactions, ranging from the exploitation of acoustics by humpback whales (Megaptera novaeangliae) in bubble nets to trap prey, to techniques by which coastal dolphins (e.g. of the genus Cephalorhynchus) could successfully echolocate in bubbly water (a hypothesis which has led to the development of a man‐made sonar which can penetrate bubble clouds, and a range of possibilities for homeland security).

Tracking system developed using an optical fiber-based distributed acoustic sensor

This work will present a system for acoustic source tracking and imaging developed using a novel distributed acoustic sensor. This sensor, the intelligent Distributed Acoustic Sensor (iDAS), uses standard off-the-shelf optical fiber as a transducer to capture the audio-frequency acoustic field (both phase and amplitude) simultaneously along the entire length of a fiber with a spatial resolution of about 0.5 m. iDAS was used to track the bearing of acoustic sources in a reverberant environment, with localization being performed in two stages. First, the received signal was preprocessed for the mitigating effects of calibration errors and multipaths. Then tracking was performed via particle filters. These filters can track multiple acoustical sources and are efficient enough to run in nearly real time. The output from this system is presented in the form of an acoustic camera-type video.

Waterfalls in space, and other problems of 'underwater' acoustics on a small planet

Whilst extraterrestrial liquids do occur in the Solar System, todays acoustical oceanographers have fewer sites to which they can apply their experience of Earths oceans than perhaps they would have had in the early Solar System, with its magma oceans. Possible sites are Saturns moons Titan and Enceladus, and Jupiters moon Europa. The ability to transfer our understanding of Earths acoustical oceanography to other moons and planets is particularly valuable, given that current understanding is sufficient to undertake complex inversions to estimate Earths ocean environmental parameters from relatively sparse, or even naturally‐occurring, acoustical signals. However such transference should be done with care, as terms familiar in Earths acoustical oceanography may not be correct on other worlds. For example, in a deep ocean on a small world (such as Europa), the hydrostatic pressure will not equal the product of the density, the depth, and the surface value of the acceleration due to gravity, since th...

Hypotheses on the exploitation of bubble acoustics by cetaceans

Man‐made sonar does not operate well in bubbly water, and yet cetaceans not only function effectively in shallow coastal waters, but also at times generate large bubble fields to assist with catching prey. This paper outlines the challenges faced by cetaceans in using acoustics in such environments, and proposes acoustical techniques which would work. The validity of such proposed acoustical solutions is explored through theory, simulation, and experimentation. The scenarios in question relate to the circular and spiral bubble nets generated to trap prey by humpback whales, and solutions to difficulties associated with echolocation by dolphins and porpoises in bubbly water. Whether the solutions are exploited by cetaceans is uncertain, but their efficacy in test tanks and implications for man‐made sonar are demonstrated.

Aeroacoustics of microphones

Several commercial options exist for prevention of the pseudo‐sound often introduced by a microphone in the presence of a flow field. One such device is a so‐called ‘‘slit‐tube.’’ A slit‐tube is a tube approximately the diameter of the microphone with which it is used. The length of such a tube can vary anywhere from 25 cm to 50 cm, and it is most generally fit with a stream‐lined noise cone. In theory, a perforated slit along the length of the tube encourages net cancellation of turbulent pressure variations, while not interfering with the acoustic field. This poster compares the effectiveness of two similar slit‐tube designs. Each tube was placed inside of a wind‐tunnel and exposed to white noise. The effects of increasing flow velocities on the transfer‐function between the source and receiver signals are analyzed.