Tunable and broadband resonator pipe sound sources for ocean acoustic tomography, communications and long-range navigation

Abstract

High-efficient tunable sound sources in the ocean and bottom tomography have 15 years of operating history. The sound source is efficient, powerful, and has unlimited operational depth, as well as a minimum level of high frequency harmonic content. The projector uses a narrow-band, highly efficient sound resonator, which is tuned to match the frequency and phase of a reference frequency-modulated signal. The high-Q resonator tunes to match the frequency and phase of a reference frequency-modulated signal. The projector transmits a digitally synthesized frequency swept signal and mechnicallytunes the organ pipe to match the frequency and phase of a reference signal. The computer timing system uses a high precision Cesium atomic clock. The resonator tube projector consists of a volume source in the form of a pressure balanced symmetric Tonpilz driver and aluminum free flooded pipe. The actuator smoothly tunes the frequency of the resonator tube over a large frequency band. The transmission duration can vary from one second to a few minutes, and the frequency ranges from 140 Hz to 1200Hz. The first tunable organ-pipe was successfully tested on 11.09.2001. Since 2001, this type of sound source has been used in many experiments: Pacific Ocean, Pioneer Seamount (2001); MOVE Experiment (2004–2005); Pacific Ocean, Hoke Seamount (2002–2004); NPAL04, SPICE04, Pacific Ocean (2004–2005); Fram Strait 2008–2012; Philippine Sea (2009, 2010–2011); Newfoundland, Canada (2014–2015). In 2013 TWR specially designed a sound source for a sea floor deployment. The bottom-deployed swept frequency array can be used for high-resolution seismic imaging of deep geological formations. The Teledyne underwater tunable resonant sound source demonstrated exceptional performance. However, the tunable transducers have limitations when used for arbitrary waveforms. They can only transmit frequency-modulated signals. When the single-resonance organ-pipes do not provide sufficient bandwidth, 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 (Morozov 2014, US patent 8670293). The projector is driven by a manual acoustic source attached with shock-mounts inside the inner resonator. These resonating tubes are open on both ends and typically made of aluminum or carbonfiber. The inner tube 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 the external pipe and back. By changing the length of the shifter 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 doubly-resonant sound source was tested in the water at Woods Hole Oceanographic Institution and exhibited a high electro-acoustical efficiency and a high power output over a large operating band. Different variants of dual-resonant broadband organ-pipes were considered. The new tunable and broadband variants of organ pipes are analyzed by the COMSOL multi-physics simulation. The finite analysis, computer simulation gives a picture of the tunable resonant acoustics. For a clear interpretation of sound pressure levels (SPL) the analysis was done for the standard spherical piezo-ceramic driver. The finite element simulation shows the structural acoustics of the tunable resonator, and helps to improve the acoustics of the sound source. The analysis gave the correct solution for a tuning mechanism within the octave frequency range. Application of COMSOL finite element analysis predicted optimal parameters of the resonator and avoided a long series of water tests with parameter adjustments. The results are compared with the experimental test. The experimental parameters of the sound source are close to the COMSOL simulations. Teledyne Marine Systems continues innovating promising solutions for ocean acoustic tomography, navigation and seismo-acoustic applications.