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The design criteria for a deep-submergence source utilizing the acoustic principle of the Helmholtz resonator is discussed. For frequencies below 500 Hz, the size of any acoustic source is necessarily large if any great amount of power is to be radiated. The advantage of the Helmholtz resonator becomes apparent when compared to a typical Tonpiltz resonator. Disadvantages are highly reactive electrical loads, which require high driving voltage and reactive power. Trade-offs between physical size and bandwidth can be made for maximum efficiency.
Helicoidal (vortex) acoustic waves have received much interest lately and can be generated by a variety of means, such as applying a prescribed phase shift to the elements of conventional 2-D planar arrays or by employing a leaky wave antenna in a circular arrangement. In the context of array signal processing, the beampattern associated with vortex wave generation has a strong central null that can be exploited to achieve far field super-resolution in a variety of applications. Although recent physical acoustics research into vortex waves has focused on active applications such as particle manipulation or propagating sub-diffraction-limit features into the far field, vortex waves can also be utilized in passive applications such as source localization. This work investigates the signal processing implications of using vortex waves in both active and passive regimes. Key trade-offs between conventional, adaptive, and vortex-wave-based array signal processing methods are examined by simulating a variety of array geometries and source/target configurations. Results indicate the potential for vortex wave methods to provide performance improvements over conventional and adaptive methods such as MVDR (minimum variance distortionless response) and MUSIC (multiple signal classification) in snapshot-deficient and high-bearing-rate target scenarios.
Presently, the most common method of measuring the surface vibration of a fluid-loaded structure is through the use of accelerometers mounted on the surface. When the surface of the structure consists of a low-density, compliant material it is necessary to use small, low-mass accelerometers to avoid mass loading of the surface. There are a variety of commercially available, low-mass accelerometers all of which have low sensitivities. For high-frequency measurements the low sensitivities are not a major problem and the accelerometers are generally acceptable. However, at low frequency, where the acceleration is small, the signal-to-noise ratio obtained with the low-sensitivity accelerometers is generally unacceptable. To address the problem of making low-frequency measurements of the vibration of a submerged compliant surface the Underwater Sound Reference Detachment of the Naval Research Laboratory has developed a class of neutrally buoyant capacitive displacement sensors. The dynamic mass of these sensors is equal to the mass of the displaced fluid, thus the sensor does not add any additional loading to the surface. Since the sensors respond to the displacement of the surface these sensors are ideally suited for low-frequency measurements. Both the theory of operation as well as experimental results will be presented. [Work supported by ONR Code 452.]
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