1,720,979 research outputs found
The opportunities and challenges in the use of extra-terrestrial acoustics in the exploration of the oceans of icy planetary bodies
No full textAcoustic radiation is the signal of choice for exploring Earth’s oceans. Its potential application for the oceans of icy moons requires investigation. However acoustic technology needs to be treated with care for extra-terrestrial purposes. Instruments, calibrations, and predictive codes that have served well on Earth may require fundamental redesign for use on other worlds. However when such an assessment is achieved, acoustic signals open up the possibility of exploring volumes exceeding one million cubic kilometres in a few minutes. This paper begins at tutorial level for novice acousticians, illustrating the principles by which acoustics can be used to monitor the environment at great distances from the source, both by projecting out signals and by using natural signals of opportunity. It then progresses to calculations for a generic icy moon (which resembles, but does not model Europa), proceeding from tutorial calculations of ‘flat world’ models to calculate the propagation times for pulses to circumpropagate around the entire moon. Given that a single emitted pulse can produce multiple arrivals from different propagation paths, the paper discusses how the structure of the received time history can be used to monitor changes in the temperature profile in the ocean, position of the water/ice layer and the asphericity of the moon during orbit.<br/
Cavitation and cetacean
No full textBubbles 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 techniques from which we
could learn. This paper outlines some of the possible interactions, ranging from the exploitation
of acoustics in bubble nets to trap prey, to techniques for echolocating in bubbly water, to the
possibility that man-made sonar signals could be responsible for bubble generation and death
within cetaceans
Bubble acoustics in shallow water: Possible applications in nature
No full textGas bubbles are the most potent naturally-occurring entities that influence the acoustic environment in liquids. Upon entrainment under breaking waves, waterfalls, or rainfall over water, each bubble undergoes small amplitude decaying pulsations with a natural frequency that varies approximately inversely with the bubble radius, giving rise to the 'plink' of a dripping tap or the roar of a cataract. When they occur in their millions per cubic metre in the top few metres of the ocean, bubbles can dominate the underwater sound field. Similarly, when driven by an incident sound field, bubbles exhibit a strong pulsation resonance. This paper discusses three examples of how bubble acoustics may find applications in Nature. The first of these is the determination of bubble size distributions through inversion of the sound fields that bubbles generate on entrainment. This can be used not only in testing models of bubble cloud evolution under breaking waves, but also in extraterrestrial environmental assessment. The second application lies in the possible enhancement by humpback whales of the efficiency of the bubble nets they use in fishing. The third speculates on the apparent conundrum, that unless dolphins employ better signal processing than humans currently do, then when they use bubble nets to hunt they are, in this visually confusing environment, nullifying their own most spectacular sensory apparatus
Ocean acoustic circumpropagation in the ice seas of Europa
No full textIn recent years increased attention has been paid to the potential uses of acoustics forextraterrestrial exploration. A number of important papers have discussed propagationon Europa, primarily with respect to sound in the ice sheet which is believed to covera salt water ocean. The models used to date assume a flat ice surface and agravitational acceleration which does not vary with depth. Models of long rangeacoustic propagation through Europa’s ice seas require models which do not makethese two assumptions. This report applies such a model to simple Europangeometries to show how observables can be affected by the values of physicalparameters: the report considers the specific case of the effect on the travel time of acircumpropagating pulse of the ice and water thicknesses, and water temperature(assumed to be uniform), on a simple spherical planet. Such effects could be the basisof acoustic inversion experiments, although it is recognised that the complexities anduncertainties associated with the actual ice seas of Europa would make the task verymuch more challenging than the calculations undertaken in this simple study
Issues relating to the use of a 61.5 db conversion factor when comparing airborne and underwater anthropogenic noise levels. (In special issue on: the detection of buried marine targets)
No full textAlthough a considerable amount of the current underwater acoustics literature deals with the proper documentation and analysis of underwater anthropogenic noise levels, mistakes and misconceptions can occur when attempts are made (often by non-experts) to make these data accessible for legislators, journalists and the public. This is because it is difficult for humans to assess qualitatively underwater sound level and quality. It can even be difficult for researchers to judge whether a given underwater sound should be classified as “loud” or “soft”. Many practitioners have suggested that the difference between airborne and underwater sound can be accounted for by applying a 61.5 dB comparison factor (in an attempt to compensate for the different acoustic impedances, and dB reference level conventions, which characterize acoustics in air and water). Whilst use of such a factor is preferable to use of none (which has led to misleading comparisons between levels in-air and water) nevertheless its existence could confer a false sense of security that the comparison is sound, whereas in fact, depending on the details of the comparison, a range of other issues would have to be rigorously taken in to account. Those issues include the perception of sound and annoyance underwater, and the problematic issue of making comparisons across species. This paper does not offer solutions to those issues, but rather outlines the thinking behind the 61.5 dB comparison factor, and shows the intriguing results of it blind application in some interesting example scenarios
Sonar which penetrates bubble clouds
No full textMan-made active sonar does not operate well in bubbly water. However
dolphins and porpoises not only function effectively in shallow coastal waters, but also at
times generate large bubble fields to assist with catching prey. Possible physics solutions
to target detection in bubbly water are proposed, and the validities of such proposed
acoustical solutions are explored through theory, simulation and experimentation.
Whether the solutions are exploited by cetaceans is uncertain. However the efficacy of the
new methodology in test tanks, and the implications for man-made sonar, are
demonstrated
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