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- Development of a high-power multilayer PVDF acoustic projector for 40 to 80 kHz bandPublication . Silva, António; Hughes, Ashley; Pozzatti, Daniel; Zabel, Friedrich; Viegas, Rúben; Martins, MarcosA piston type projector using the PVDF piezoelectric polymer was developed for operating in underwater environment, below 100 kHz. For those frequencies PZT piezoelectric ceramic is usually a preferable choice and PVDF is only considered for frequencies above a few hundreds of kHz. This paper will show that efficient underwater acoustic projectors for frequencies below 100 kHz can be implemented regarding an appropriate impedance adapter is being used. The developed project presents a calibrated transmitting voltage response (TVR) of approximately 166, 160 and 175 dB at 40, 50 and 75 kHz, respectively. The PVDF TVR values are compatible with the PZT projectors available on market with the advantage of having a larger bandwidth than most PZT projectors. To the authors knowledge this is the first time that a PVDF projector attain such characteristics. Although theoretically the PVDF projector bandwidth is larger than 40 to 80 kHz, in practice it was observed that only between those frequencies the project presents a stable operation for the transmission of long-term signals.
- Underwater acoustic spiral source: pool tests and calibrationPublication . Viegas, Sergio; Zabel, Friedrich; Silva, AntónioUnderwater acoustic spiral sources are able to generate spiral acoustic fields where the phase depends on the bearing angle. It allows to estimate the bearing angle relatively to a receiver by subtracting the phases of a spiral and a circular wavefront, and can be used to estimate bearing angles with a single hydrophone/projector pair, e.g., for unmanned underwater vehicles localization. The developed spiral source comprises four monopoles/quadrants and it will be shown that the spiral source behavior is linear, which means that the generated acoustic signals are the sum of the four acoustic signals from each separate monopole, for any amplitude value. This work presents two calibration methods for spiral acoustic sources and the linearity evaluation for the used spiral source. Unlike the calibrations performed on signal reception, the two proposed calibration methods adjust the phase of the emitted signals to form the acoustic spiral field. The first calibration method rectifies the phase of one quadrant based on the contribution of the four quadrants. This method was tested and presented a performance lower than the performance of the calibration on the receiving side. After evaluating the linearity, a new calibration approach is suggested. This new approach uses the signals from separate quadrants instead of the contribution of the four quadrants. This method needs to be tested experimentally to check its validity.
- In-Lab demonstration of an underwater acoustic spiral sourcePublication . Viegas, Rúben; Zabel, Friedrich; Silva, AntónioUnderwater acoustic spiral sources can generate spiral acoustic fields where the phase depends on the bearing angle. This allows estimating the bearing angle of a single hydrophone relative to a single source and implementing localization equipment, e.g., for target detection or unmanned underwater vehicle navigation, without requiring an array of hydrophones and/or projectors. A spiral acoustic source prototype made out of a single standard piezoceramic cylinder, which is able to generate both spiral and circular fields, is presented. This paper reports the prototyping process and the multi-frequency acoustic tests performed in a water tank where the spiral source was characterized in terms of the transmitting voltage response, phase, and horizontal and vertical directivity patterns. A receiving calibration method for the spiral source is proposed and showed a maximum angle error of 3° when the calibration and the operation were carried out in the same conditions and a mean angle error of up to 6° for frequencies above 25 kHz when the same conditions were not fulfilled.
- Integrated approach for modeling acoustic propagation and projectors/hydrophones electronicsPublication . Viegas, Rúben; Pozzatti, Daniel; Zabel, Friedrich; Silva, AntónioUnderwater acoustic propagation models (APM) are useful tools to predict acoustic propagation, making it possible to implement and test equalization algorithms for Underwater Acoustic Communication (UWAC) systems. To our knowledge, none of the APMs developed so far consider the distortion induced by the associated electronic circuits, impedance adaptors and acoustic transducers on signal propagation, which are important mainly in broadband applications. This paper describes the functioning of a new model capable of predicting the aforementioned distortions on the projector and hydrophone. The electro-Acoustic Propagation Model (eAPM) calculates the frequency response of the circuits with frequency-dependent characteristic components (transducers and impedance adaptors) using SPICE simulations and simulates the acoustic propagation using an Time-Variable APM (TV-APM), all embedded in a single model. SPICE simulations require the insertion of electrical impedance measurements from the transducers and impedance adaptors. eAPM also uses the projector's Transmitting Voltage Response (TVR) and the hydrophone's Open Circuit Voltage Response (OCVR), that can be obtained through equipment calibration. The model output signals have a good agreement with the signal experimentally recorded, showing that the eAPM allows for in-lab prediction of the distortion induced by the transducers and electronics and its impact on an application in a realistic acoustic propagation environment. The developed model can be used to predict distortions on broadband UWAC systems and also to support the development of new transducers, especially those with a wide bandwidth response.
- Comparing noise vessel azimuth tracking with a planar hydrophone array and a single vector sensorPublication . SOARES, CRISTIANO; Zabel, Friedrich; Jorge Maia dos Santos, Paulo; Silva, AntónioVector sensors are appealing for monitoring underwater noise due to its inherent directivity. While acoustic pressure sensors are ambiguous in all directions, vector sensors permit the aquisition of directional information through the measurement of particle velocity, which enables the possibility of azimuth tracking of underwater noise sources. The MARREAL marine observatory is a marine observatory equipped with a number of sensors and subsystems, including an acoustic acquisition system made of four hydrophones and a vector sensor. The observatory was deployed in September 2022 in Sagres, Portugal, near the Baleeira Port which is accessed by fisher boats and recreational boats. This paper shows preliminary results on azimuth estimation of boats passing in the deployment area, obtained independently with a 4- hydrophone planar array and a vector sensor. The results indicate that a single vector sensor can provide fair results on azimuth tracking of boats passing in the area. At high signal-to-noise ratio (SNR) the vector sensor is able to yield results similar to those obtained with the planar array. When the SNR is low the planar array outperforms the vector sensor with actual processing methods used.