Hydrophone Configuration Research Articles

Can we be confident about ship noise measurements?

Increased maritime traffic is leading to more underwater noise. Despite the use of a standardized protocol (ANSI/ASA S12/64-2009), measuring ship noise is complex, as it is difficult to follow the protocol in real-life conditions. The Marine Acoustic Research Station (MARS) project (www.projet-mars.ca/en) designed a recording station and a measurement protocol that follows closely the international standard. The station operated during summer/fall in 2021, 2022 and 2023 in the Laurentian Channel in the St. Lawrence Estuary (eastern Canada). Four dedicated missions were organized, during which the R/V Coriolis II was measured repeatedly, generating a database of 239 measurements. In this study, we assess the measurement repeatability of the MARS station, comparing simultaneous measurements from identical systems, as well as repeated measurements at varying speeds and distances. We quantify the measurement uncertainties associated to the hydrophone configuration and the measurement protocol. Our results illustrate how array redundancy allows to identify and mitigate hardware-related issues and to reduce the duration of the measurement, a costly parameter. We quantify the repeatability of single measurements within the MARS station, we discuss the implications regarding our ability to validate noise reduction actions from ~3 dB and we propose a measurement protocol to further reduce this limit.

Identifying fish sounds of British Columbia with an autonomous audio and video array

Among the ∼400 marine fish species of British Columbia, only 22 have been reported to be soniferous. However, it is likely due to the lack of examination, as more species are suspected to produce sound. Here we describe how an autonomous audio and video array can identify fish sounds in situ. The array is composed of a collapsible PVC frame, an acoustic recorder, six hydrophones, and two custom-made wide-angle autonomous video cameras mounted on the top and side of the array that collect data continuously for up to 10 days. Fish sounds are automatically detected in the acoustic recordings, the time difference of arrivals between pairs of hydrophones is measured by cross correlation, and the 3-D sound-source location and its uncertainty are estimated using linearized inversion. Simulated annealing optimization was used to define the hydrophone configuration that provides the smallest localization uncertainties. The video recordings are used to assign the species of sound-producing fish localized within the array. The array was deployed at several locations around Vancouver Island and used to define the species, sound characteristics, and source levels of several fish sounds. This new information will help making passive acoustics a viable way to monitor fish in the wild.

Passive acoustic localization of fish using a compact hydrophone array

Passive acoustic monitoring of fish in their natural environment is a research field of growing interest and importance. Although many fish species are soniferous, the characterization and biological understanding of their sounds are largely unknown. Many underwater acoustic recordings contain sounds likely produced by fish, but little information can be extracted from them due to the lack of fundamental knowledge about the behaviors they represent. Deploying small hydrophone arrays can help fill some of these knowledge gaps. Passive acoustic localization using fish calls received on multiple hydrophones can be used to estimate swimming speed, calling rate of individual fish, and source level of their calls. This paper focuses on the three-dimensional localization of fish using a compact array of 6 hydrophones using both simulated and measured data. Fish sounds were detected manually on one of the hydrophones. Time difference of arrivals (TDOAs) were then defined by cross correlating the detected signal with signals from the other hydrophones. Linearized Bayesian inversion was employed to localize fish sounds from the measured TDOAs. Localization uncertainties were below 10 cm inside the hydrophone array. Simulated annealing optimization was used to define the hydrophone configuration that could provide the smallest localization uncertainties.

Nearfield and farfield measurements of dolphin echolocation beam patterns: No evidence of focusing.

The potential for bottlenose dolphins to actively focus their biosonar transmissions was examined by measuring emitted clicks in four dolphins using horizontal, planar hydrophone arrays. Two hydrophone configurations were used: a rectangular array with hydrophones 0.2 to 2 m from the dolphins and a polar array with hydrophones 0.5 to 5 m from the dolphins. The biosonar task was a target change detection utilizing physical targets at ranges from 1.3 to 6.3 m with all subjects and "phantom" targets at simulated ranges from 2.5 to 20 m with two subjects. To provide a basis for evaluating the experimental data, sound fields radiated from flat and focused circular pistons were mathematically simulated using transient excitation functions similar to dolphin clicks. The array measurements showed no evidence that the dolphins adaptively focused their click emissions; axial amplitudes and iso-amplitude contours matched the pattern of the simulation results for flat transducers and showed a single region of maximum amplitude, beyond which spherical spreading loss was approximated.

Variability of the coherent arrivals extracted from low-frequency deep-ocean ambient noise correlations

Correlation processing of ocean noise can be used to develop totally passive ocean monitoring methods. Using various hydrophone pair orientations, this study investigates the frequency dependence, seasonal variability, and emergence rate of coherent arrivals from cross-correlations of low frequency ambient noise (f < 40 Hz) recorded on triangular hydrophones arrays. These arrays are located at five existing hydroacoustic stations of the International Monitoring System (IMS), situated in the deep-sound channel, and distributed across the Atlantic, Pacific, and Indian Ocean basins. For the majority of studied sites, persistent and fast-emerging coherent arrivals are reliably obtained if the axis connecting the selected hydrophone pair has a direct line-of-sight with regions of the globe containing stable and diffuse noise sources (e.g., polar-ice or seismic noise). Furthermore, for this favorable orientation, the emergence rate of coherent arrivals extracted between hydrophone pairs separated by long ranges (here ∼130 km) can be approximated based on measurements made between hydrophone pairs separated by short ranges (∼2 km) in the Atlantic Ocean. Hence, results from this study, obtained using existing hydrophone configurations of the IMS hydroacoustic stations, could be used to guide the placement of other hydrophone arrays over the globe for future long-range passive ocean monitoring experiments.

Pressure compensated fiber laser hydrophone: Modeling and experimentation

A pressure compensated metal diaphragm based fiber laser hydrophone configuration that can provide good sensitivity, large bandwidth, and sea state zero noise floor is proposed in this paper. A simplified theoretical model of the proposed sensor configuration is developed in which the acoustic elements of the sensor configuration are modeled using a four-pole acoustic transfer matrix and the structural elements are modeled as second order single degree of freedom elements. This model is then used to optimize the design parameters of the sensor system to achieve the performance objectives. An axisymmetric finite element analysis of the sensor configuration is also carried out to validate the results from the simplified theoretical model. Prototype sensors were fabricated and hydrostatic testing in a pressure vessel validated the static pressure compensation performance of the sensor. Frequency dependent sensitivity of the sensor system was measured through acoustic testing in a water tank. The prototype sensor gave a flat frequency response up to 5 kHz and experimental results compared well with theoretical predictions. The sensor has an acceleration rejection figure on the order of 0 dB ref 1 m/s(2) Pa and the pressure compensation approach worked reasonably well up to a hydrostatic pressures equivalent to a depth of 50 m.

Using hydroacoutic stations as water column seismometers

The Comrehensive Nuclear-Test-Ban Treaty Organization (CTBTO) maintains hydrophones that have been used to study icebergs and T-wave propagation. These stations consist of three hydrophones at about the depth of sound channel in a horizontal triangle array with 2 km sides. We have used data from these stations in the few tenths of a Hertz and below regime to study if we can effectively use these stations as water column seismometers. Among the processing performed was methods to effectively transform the hydrophone configurations to vector sensors. An assortment of signal processing on hydrocoustic data from the December 26th 2004 Great Sumatra Earthquake has been compared to seismograph data of the same event indicating, that the hydrophone stations can indeed be used as surrogate seismometers.

Adaptive Linear Turbo Equalization Over Doubly Selective Channels

Over the last decade, tremendous gains, leading to near-capacity achieving performance, have been shown for a variety of communication systems through the application of the turbo principle, i.e., the exchange of extrinsic information between constituent algorithms for tasks such as channel decoding, equalization, and multiple-input–multiple-output (MIMO) detection. In this paper, we study the practical application of such an iterative detection and decoding (IDD) framework to underwater acoustic communications. We explore complexity and performance tradeoffs of a variety of turbo equalization (TEQ)-based receiver architectures. First, we elaborate on two popular but suboptimal turbo equalization techniques: a channel-estimate-based minimum mean-square error TEQ (CE-based MMSE-TEQ) and a direct-adaptive TEQ (DA-TEQ). We study the behavior of both TEQ approaches in the presence of channel estimation errors and adaptive filter adjustment errors. We confirm that after a sufficient number of iterations, the performance gap between these two TEQ algorithms becomes small. Next, we demonstrate that an underwater receiver architecture built upon the least mean squares (LMS) DA-TEQ technique can leverage and dramatically improve the performance of the conventional implementation based on the decision-feedback equalizer at a feasible complexity. To maintain performance gains over time-varying channels, the slow convergence speed of the LMS algorithm has been improved via two methods: 1) repeating the weight update for the same set of data with decreasing step size and 2) reducing the dimensionality of the equalizer by capturing sparse channel structure. This receiver architecture was used to process collected data from the SPACE 08 experiment (Martha's Vineyard, MA). Receiver performance for different modulation orders, channel codes, and hydrophone configurations is examined at a variety of distance, up to 1 km from the transmitters. Experimental results show great promise for this approach, as data rates in excess of 15 kb/s could readily be achieved without error.

Passive acoustic detection and localization of whales: Effects of shipping noise in Saguenay–St. Lawrence Marine Park

The performance of large-aperture hydrophone arrays to detect and localize blue and fin whales' 15-85 Hz signature vocalizations under ocean noise conditions was assessed through simulations from a normal mode propagation model combined to noise statistics from 15 960 h of recordings in Saguenay-St. Lawrence Marine Park. The probability density functions of 2482 summer noise level estimates in the call bands were used to attach a probability of detection/masking to the simulated call levels as a function of whale depth and range for typical environmental conditions. Results indicate that call detection was modulated by the calling depth relative to the sound channel axis and by modal constructive and destructive interferences with range. Masking of loud infrasounds could reach 40% at 30 km for a receiver at the optimal depth. The 30 dB weaker blue whale D-call were subject to severe masking. Mapping the percentages of detection and localization allowed assessing the performance of a six-hydrophone array under mean- and low-noise conditions. This approach is helpful for optimizing hydrophone configuration in implementing passive acoustic monitoring arrays and building their detection function for whale density assessment, as an alternative to or in combination with the traditional undersampling visual methods.

Pressure-compensated acceleration-insensitive hydrophone

A generally cylindrical hydrophone configuration provides compensation for longitudinal accelerations by placing four identical solid piezoelectric transducer elements along its axis with each transducer element being bonded to a head member, two of which are located generally centrally of the cylindrical housing and fastened thereto and two of which are located near the outside edges of the housing and having slight clearance therewith. Flexible polyurethane boots are clamped to the ends of the housing. The volumes between the centrally disposed and outer transducer head members and between the outer head members and the boots are filled with methyl silicon fluid. Each head member is electrically connected to one side of the electrical output, and the junction between the transducer members is connected to the opposite side, both sides being wired to an electrical contact plate located between the two centrally disposed transducer head members, this volume being filled with electrical potting material. The clearance between the outside transducer heads and the side wall of the housing is controlled to permit long-term pressure equalization without significantly affecting frequency response down to 10 Hz or somewhat lower.

U.S. Navy 0–3 piezocomposite transducers

Interest in 0–3 piezoelectric composite materials for use in hydrophone applications has grown considerably in recent years. Designs which require operation in the hydrostatic mode with a large hydrostatic voltage coefficient gh, are desired to increase free-field voltage sensitivity and meet the ever increasing demands placed on hydrophone performance. U.S. Navy scientists at the Naval Research Laboratory, Underwater Sound Reference Detachment, have investigated the utilization of 0–3 piezocomposite for sonar applications and have developed various hydrophone configurations based on specific program requirements. The requirements of the programs, such as a lightweight, low-profile, and high receive sensitivity, demanded unique designs and the need for utilizing the unique characteristics of 0–3 composite materials. The hydrophones presented in this discussion used a 0–3 composite known at NRL simply as piezorubber or PZR. This material consists of lead titanate particles embedded in a neoprene elastomeric matrix. This discussion will cover the development, including problems and solutions, fabrication, and testing of two unique 0–3 composite hydrophones.

Piezoelectric polymer cylindrical hydrophone

An electroacoustic transducer which uses polyvinylidene fluoride (PVF2) as the piezoelectric element is investigated for its suitability as an acceleration insensitive hydrophone. The fundamental construction concept involves attaching concentric layers of PVF2 to a mechanically compatible end‐capped cylinder. An expression for the electroacoustic sensitivity is developed and compared to measured acoustic data for a variety of hydrophone configurations. The axial acceleration sensitivity is measured and compared to one type of piezoelectric ceramic hydrophone.

Geometric factors affecting hydrophone performance

The application of the finite-element method of analysis to investigate the geometric and material factors which affect the open-circuit voltage sensitivity of a class of underwater acoustic hydrophones is discussed. This class of hydrophones consists of an air-filled, right-circular, piezoelectric ceramic cylinder with flat nonpiezoelectric circular disk end caps. The study examines the factors which constrain the motion of the cylinder in a real hydrophone and cause the open-circuit voltage sensitivity to be less than the value of an ideal hydrophone predicted by the equations developed by R. A. Langevin [J. Acoust. Soc. Am. 26, 421–427 (1953)]. Curves are presented to allow an estimate to be predicted of the reduction in the open-circuit voltage sensitivity which a particular design will produce, and a comparison is made between these predictions and empirical data measured on various hydrophone configurations.

Infrasonic flow-noise measurements using an H-58 omnidirectional cylindrical hydrophone

Infrasonic and low-frequency flow-noise measurements were made during laboratory tests of an H-58 omnidirectional hydrophone. The hydrophone was tested in three configurations: bare, framed, and faired. The incident water speeds varied from 0.25 to 0.45 knot, which correspond to a Reynolds number range from 8800 to 16 000. The faired and framed configurations developed flow-noise levels lower than the bare hydrophone, and the latter configuration developed the highest noise levels at the lowest frequencies of the 1–50−Hz band. Hydrodynamic factors which influence the acoustic measurements made during this test are discussed, and the importance of hydrophone configuration during very low-frequency measurements is demonstrated.

Infrasonic flow-noise measurements using an H-58 hydrophone

Infrasonic and low-frequency flow-noise measurements were made during laboratory tests of an H-58 omnidirectional hydrophone. The hydrophone was tested in three configurations: bare, framed, and faired. Water speeds were varied from 0.25 to 0.45 knots and corresponded to a Reynolds number range from 8800 to 16 000. For different hydrodynamic reasons, the faired and framed configurations developed flow-noise levels lower than the bare hydrophone configuration. The bare hydrophone developed the highest noise levels at the lowest frequencies of the 1–50-Hz band. Hydrodynamic considerations related to the acoustic measurements made during this test are discussed; the importance of hydrophone configuration used in very low-frequency measurements is demonstrated.

Underwater Acoustic Wavefront Variations and Internal Waves

Variations in the measurement of the direction of acoustic wavefronts were obtained in a deep ocean experiment for a fixed source and hydrophone configuration. The nature of the fluctuations are interpreted as resulting from internal ocean waves.