This paper presents a comprehensive review of hydrophones, their design considerations, physical aspects, and structures by reviewing many publications that have been published over the last two decades. The investigation of the fundamental aspects and essential considerations of hydrophones in the literature are distributed in different papers and have not been brought together in a conclusive manner. This paper has collected and classified all the information about hydrophones including the parameters that affect their performance as well as their features. We have categorized the hydrophones according to their mechanism of operation, structure design, frequency response, and application. The literature has been summarized in a way that enables the readers' easy referral to appropriate designs for their desired application scenarios.

Hydrophones, fundamental features, design considerations, and various structures: A review

A voluntary commercial vessel Slowdown trial was conducted through 16 nm of shipping lanes overlapping critical habitat of at-risk southern resident killer whales (SRKW) in the Salish Sea. From August 7 to October 6, 2017, the trial requested piloted vessels to slow to 11 knots speed-through-water. Analysis of AIS vessel tracking data showed that 421 of 951 (44%) piloted transits achieved speeds within one knot of the target (i.e., ≤12 knots) and 55% achieved speeds ≤13 knots. Slowdown results were compared to ‘Baseline’ noise of the same region, matched across lunar months. A local hydrophone listening station in Lime Kiln State Park, 2.3 km from the shipping lane, recorded 1.2 dB reductions in median broadband noise (10-100,000 Hz, rms) compared to the Baseline period, despite longer transit durations during the trial. Filtering for commercial vessels within 6 km radius of Lime Kiln listening station (aiming to remove confounding effects of variability in vessel numbers), and excluding small boat noise, high current and wind speed periods, median noise was reduced by 2.5 dB. The reductions were highest in the 1st decade band (-3.1 dB, 10-100 Hz) and lowest in the 4th decade band (-0.3 dB reduction, 10-100 kHz). A regional vessel noise model predicted noise for a range of traffic volume and vessel speed scenarios for a 1133 km2 ‘Slowdown region’ containing the 16 nm of the trial’s shipping lanes. A temporally and spatially-explicit simulation model was used to evaluate the changes in traffic volume and speed on SRKW in their foraging habitat within this Slowdown region. The simulation model tracked the number and magnitude of noise-exposure events that impacted each of 78 SRKW across different traffic volume-speed scenarios. These metrics were simplified to a cumulative effect termed ‘potential lost foraging time’. The model predicted that the voluntary Slowdown trial achieved 22% reduction in ‘potential lost foraging time’ for SRKW, with 40% reductions under 100% 11 knot participation. Slower vessel speeds reduced underwater noise in the Slowdown area despite longer passage times and therefore suggest this is an effective way to benefit SRKW habitat function in the vicinity of shipping lanes.

http://summit.sfu.ca/system/files/iritems1/18841/fmars-06-00344.pdf

Potential Benefits of Vessel Slowdowns on Endangered Southern Resident Killer Whales

A pattern recognition system is described for recognizing shipbuilding parts using artificial neural networks and Fourier descriptors. The system uses shape contour information that is invariant of size, translation, and rotation. Fourier descriptors provide information, and the neural networks make decisions about the shapes. A brief review of the current state of the art is included, and results from testing show that the system distinguished between various shapes and proved to be a valid approach.

/pdf/recognizing-shipbuilding-parts-using-artificial-neural-kjvffhhbm4.pdf

Recognizing shipbuilding parts using artificial neural networks and Fourier descriptors

One calendar year of Automatic Identification System (AIS) ship-traffic data was paired with hydrophone recordings to assess ambient noise in northern Admiralty Inlet, Puget Sound, WA (USA) and to quantify the contribution of vessel traffic. The study region included inland waters of the Salish Sea within a 20 km radius of the hydrophone deployment site. Spectra and hourly, daily, and monthly ambient noise statistics for unweighted broadband (0.02-30 kHz) and marine mammal, or M-weighted, sound pressure levels showed variability driven largely by vessel traffic. Over the calendar year, 1363 unique AIS transmitting vessels were recorded, with at least one AIS transmitting vessel present in the study area 90% of the time. A vessel noise budget was calculated for all vessels equipped with AIS transponders. Cargo ships were the largest contributor to the vessel noise budget, followed by tugs and passenger vessels. A simple model to predict received levels at the site based on an incoherent summation of noise from different vessels resulted in a cumulative probability density function of broadband sound pressure levels that shows good agreement with 85% of the temporal data.

http://staff.washington.edu/jmt3rd/Publications/Bassett_etal_2012_VesselNoise.pdf

A vessel noise budget for Admiralty Inlet, Puget Sound, Washington (USA)

The most vital step in the development of novel and existing surface acoustic wave (SAW)-based sensors and transducers is their design and optimization. Demand for SAW devices has been steadily increasing due to their low cost, portability, and versatility in electronics, telecommunications, and biosensor applications. However, a full characterization of surface acoustic wave biosensors in a three-dimensional (3D) finite element model has not yet been developed. In this study, a novel approach is developed for analyzing shear horizontal Love wave resonator devices. The developed modeling methodology was verified using fabricated devices. A thorough analysis of the 3D model and the experimental device was performed in this study including scattering parameters (S-parameters), reflection coefficient parameters, transmission parameters, and phase velocity. The simulated results will be used as a design guideline for future device design and optimization, which has thus far resulted in close matching between prediction and experimental results. This manuscript is the first to demonstrate a 3D finite element model to correlate the sensitivity of the SAW device with the magnitude of the phase shift, the real and imaginary part of the response, insertion loss, and the frequency shift. The results show that the imaginary part of the response shift has a higher sensitivity compared to other parameters.

https://www.mdpi.com/1424-8220/19/8/1749/pdf

Finite Element Analysis for Surface Acoustic Wave Device Characteristic Properties and Sensitivity.

Time-variant sea surface scattering and Doppler spreading due to source/receiver motion cause the signal fading stochastically in amplitude and phase fluctuation. Consequently, the performance of underwater acoustic communication systems are degraded, and high-speed digital communication is disrupted. In this study, a channel simulator for flat fading over time-variant sea surface scattering and Doppler spread in source/receiver motion is presented. Rayleigh and Rice fading models are adopted and their distributions are compared with the distribution measured in a water tank. The bit error rate of binary phase shift keying (BPSK) and binary frequency shift keying (BFSK) for flat fading is examined to evaluate the simulator performance.

http://iopscience.iop.org/article/10.1143/JJAP.49.07HG10/pdf

Underwater Acoustic Communication Channel Simulator for Flat Fading

The performance of an underwater acoustic communication system in shallow water is strongly affected by the water surface and the seabed acoustical properties. Time-variant water surface scattering and grazing-angle-dependent bottom reflection limit the received signal coherency. Consequently, the performance of underwater acoustic communication systems is degraded, and high-speed digital communication is disrupted. In this study, the authors consider two different multipath channels, such as a water tank and a river, and conduct experiments to quantify these effects on the image transmission using binary frequency shift keying (BFSK) modulation. The quality of the received image in a water tank experiment is shown to be affected by water surface scattering. In the 9-m-deep Nakdong river experiment, it is shown to be affected by grazing-angle-dependent bottom reflection. The analysis is based on the coherence bandwidth of the multipath channel response.

Coherence Bandwidth Effects on Underwater Image Transmission in Multipath Channel

The results of an experiment conducted to understand the frequency dependence of acoustic propagation in very shallow water using a ship as an acoustic source are presented and the possible causes of the features appearing in the results are discussed. The result shows large propagation losses at a frequency of approximately 140 Hz. The results of numerical simulation including the shear property and layered structure of the bottom imply that the features might be caused by the shear property of the lower sediment that lies below the thin upper fluid sediment.

Measurement and Analysis of Broadband Acoustic Propagation in Very Shallow Water

In underwater acoustic communication, the transmitted signal is affected by multipath fading and background noise. Therefore, the intersymbol interference (ISI) of underwater acoustic communication systems depends on the effects of these parameters. To cope with ISI due to multipath fading, various equalizers and orthogonal frequency division multiplexing systems have been developed. Forward error correction coding is also adopted to reduce the error by background noise. Here, the performances of convolution code (CC) and Reed–Solomon (RS) code are evaluated in a multipath fading channel. The CC shows better performance in frequency nonselective fading but the RS code shows better performance in frequency selective fading.

https://iopscience.iop.org/article/10.7567/JJAP.53.07KG02/pdf

Performance comparison of convolution and Reed–Solomon codes in underwater multipath fading channel

The forward error correction (FEC) is achieved by increasing redundancy of information. Convolutional coding with Viterbi decoding is a typical FEC technique in channel corrupted by additive white gaussian noise. But the FEC effectiveness of convolutional code is questioned in multipath frequency selective fading channel. In this paper, how convolutional code works in multipath channel in underwater, is examined. Bit error rates (BER) with and without 1/2 convolutional code are analyzed based on channel bandwidth which is frequency selectivity parameter. It is found that convolution code performance is well matched in non selective channel and also effective in selective channel.

https://iopscience.iop.org/article/10.7567/JJAP.52.07HG01/pdf

Effectiveness of Convolutional Code in Multipath Underwater Acoustic Channel

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