Doubly-Resonant Underwater Acoustic Projector for Long-Range Communications and Positioning

The sound source was specially designed for long-range acoustic communications and potentially can be used for navigation of underwater gliders in the Arctic. A doubly resonant organ pipe provides transmission of arbitrary waveforms over a much wider frequency band. As with the single-resonance pipes, the sources can be used at all depths and are efficient and very light if built from composites. The doubly-resonant organ pipes comprise an inner resonator tube with thin walls tuned to a certain frequency surrounded by a larger-diameter tube.

The projector is driven by a monopole acoustic source attached by shock-mounts inside the inner resonator. These resonating tubes are open on both ends and typically made of aluminium or carbon-fiber. The inner tube typically has much thinner walls to allow the sound pressure to spread into the outer tube. The tubes are asymmetrically shifted along the main axis and sound pressure can penetrate from the internal pipe though the area under the shifted external pipe into that external pipe and back. By changing the length of the shifted area the coupling coefficient of two resonators can be regulated to achieve the desired bandwidth. The resulting resonant frequency is proportional to the pipe length. The radiated power from the resonators is proportional to the area of the orifices and the square of the propagated frequencies.

 

To achieve a symmetrical frequency response the radiated power from both resonators should be approximately equal. The system can be expanded to the multi-resonance, multi-frequency case with multiple coaxial pipes coupled through the shifted areas. The preliminary experiments of a doubly-resonant sound source at the Woods Hole Oceanographic Institution show that it exhibits a high electro-acoustical efficiency and a high power output over a large operating band. The WHOI US patent is pending. The frequency response is very sensitive to the resonator parameters and the resonator needs to be tested in water and adjusted according to the test results. The numerical simulation helps to predict the required parameters of the resonator and save some time necessary for fine-tuning in water.

 

The sound source was designed and then simulated with finite element analysis. We used the COMSOL Multiphysics Acoustic-Structural Interaction module and made the simulation for an aluminum variant of the resonator. The prototype of an aluminum variant sound source was built and tested in an acoustic pool. The test results were very close to the simulation. Finally, we built the carbon fiber prototype with the stiffness of pipes close to aluminum variant. The doubly-resonant sound source was tested in water at the Woods Hole Oceanographic Institution and it exhibited a high electro-acoustical efficiency and a high power output over a large operating band. The parameters of the sound source, after a little fine-tuning, were reasonably close to the COMSOL simulations.