Issues associated with sound exposure experiments in tanks

For practical reasons it is often necessary to carry out sound exposure experiments on marine animals in tanks or pools that may have dimensions ranging from less than one meter to a few tens of meters. The boundaries of such tanks are almost invariably highly reflective to underwater sound, resulting in a sound field that can vary spatially in unexpected ways, and in which the relationship between pressure and particle velocity is quite different from that in an animal's natural environment. In this paper a numerical simulation based on the finite difference method is used to illustrate these effects. The results show that, at frequencies below the tank's lowest resonant frequency, the particle velocity and pressure fields vary smoothly in space and with changes in frequency, but that both the ratio of the particle velocity to the pressure and the way in which their amplitudes vary with distance from the source are different than in a freefield situation. At frequencies above the lowest resonant frequency the particle velocity and pressure fields, and their ratio, vary rapidly both spatially and with changes in frequency. Experimental measurements of pressure and particle velocity in a tank agree qualitatively with these results. © 2016 Acoustical Society of America

Development of a simple underwater acoustic channel simulator for analysis and prediction of horizontal data telemetry

ABSTRACT This study seeks to identify the various mechanisms affecting the performance of horizontal underwater acoustic communication. In order to better understand the impact of oceanographic parameters on acoustic transmission, a simple channel simulator was developed. This was based on the Bellhop propagation model, utilising Gaussian beam tracing. Using a simulated underwater environment and given parameters regarding transmitter and receiver locations, the impulse response was obtained and a subsequent prediction of a received signal was achieved. Using this model, FSK methods of underwater data telemetry were investigated for a variety of scenarios including both deep and shallow water. These predictions were also compared to signals obtained in the field for two different deployments. The results indicated the simulator works effectively to determine areas of more difficult reception for a given environment, despite predicting lower success rates than those found during the trials. It was concluded that additions to environmental data to provide a more realistic simulation may rectify this. In addition, further work in replicating the error correction of the modems will also provide more valuable information about the results observed by the end user. Overall, the simulations worked effectively in providing a simple but effective method of predicting the distortions observed by an acoustic modem receiver

Characteristics of sound propagation in shallow water over an elastic seabed with a thin cap-rock layer

Measurements of low-frequency sound propagation over the areas of the Australian continental shelf, where the bottom sediments consist primarily of calcarenite, have revealed that acoustic transmission losses are generally much higher than those observed over other continental shelves and remain relatively low only in a few narrow frequency bands. This paper considers this phenomenon and provides a physical interpretation in terms of normal modes in shallow water over a layered elastic seabed with a shear wave speed comparable to but lower than the water-column sound speed. A theoretical analysis and numerical modeling show that, in such environments, low attenuation of underwater sound is expected only in narrow frequency bands just above the modal critical frequencies which in turn are governed primarily by the water depth and compressional wave speed in the seabed. In addition, the effect of a thin layer of harder cap-rock overlaying less consolidated sediments is considered. Low-frequency transmission loss data collected from an offshore seismic survey in Bass Strait on the southern Australian continental shelf are analyzed and shown to be in broad agreement with the numerical predictions based on the theoretical analysis and modeling using an elastic parabolic equation solution for range-dependent bathymetry

Underwater sound of rigid-hulled inflatable boats

Underwater sound of rigid-hulled inflatable boats was recorded 142 times in total, over 3 sites: 2 in southern British Columbia, Canada, and 1 off Western Australia. Underwater sound peaked between 70 and 400 Hz, exhibiting strong tones in this frequency range related to engine and propeller rotation. Sound propagation models were applied to compute monopole source levels, with the source assumed 1m below the sea surface. Broadband source levels (10–48 000Hz) increased from 134 to 171 dB re 1μPa @ 1m with speed from 3 to 16m/s (10–56 km/h). Source power spectral density percentile levels and 1/3 octave band levels are given for use in predictive modeling of underwater sound of these boats as part of environmental impact assessments

t|ket> : A retargetable compiler for NISQ devices

We present t|ket>, a quantum software development platform produced by Cambridge Quantum Computing Ltd. The heart of t|ket> is a language-agnostic optimising compiler designed to generate code for a variety of NISQ devices, which has several features designed to minimise the influence of device error. The compiler has been extensively benchmarked and outperforms most competitors in terms of circuit optimisation and qubit routing

Song variation of the South Eastern Indian Ocean pygmy blue whale population in the Perth Canyon, Western Australia

Sea noise collected over 2003 to 2017 from the Perth Canyon, Western Australia was analysed for variation in the South Eastern Indian Ocean pygmy blue whale song structure. The primary song-types were: P3, a three unit phrase (I, II and III) repeated with an inter-song interval (ISI) of 170–194 s; P2, a phrase consisting of only units II & III repeated every 84–96 s; and P1 with a phrase consisting of only unit II repeated every 45–49 s. The different ISI values were approximate multiples of each other within a season. When comparing data from each season, across seasons, the ISI value for each song increased significantly through time (all fits had p < 0.001), at 0.30 s/Year (95%CI 0.217–0.383), 0.8 s/Year (95% CI 0.655–1.025) and 1.73 s/Year (95%CI 1.264–2.196) for the P1, P2 and P3 songs respectively. The proportions of each song-type averaged at 21.5, 24.2 and 56% for P1, P2 and P3 occurrence respectively and these ratios could vary by up to ± 8% (95% CI) amongst years. On some occasions animals changed the P3 ISI to be significantly shorter (120–160 s) or longer (220–280 s). Hybrid song patterns occurred where animals combined multiple phrase types into a repeated song. In recent years whales introduced further complexity by splitting song units. This variability of song-type and proportions implies abundance measure for this whale sub population based on song detection needs to factor in trends in song variability to make data comparable between seasons. Further, such variability in song production by a sub population of pygmy blue whales raises questions as to the stability of the song types that are used to delineate populations. The high level of song variability may be driven by an increasing number of background whale callers creating ‘noise’ and so forcing animals to alter song in order to ‘stand out’ amongst the crowd

Low volumetric flow rate injection system

Current research is being studied in polymer drag reduction within a turbulent boundary layer. This research requires precise control of the volumetric flux of solution injected into a developing boundary layer. Because of significant uncertainty in the current system, a critical developing boundary layer. Because of significant uncertainty in the current system, a critical need exists for an improved injection system that is both mobile and has a wide range of operation. This project includes the design and construction of a mobile syringe pump system that has a mobile platform, digital control of the injection rate, ability to operate over a wide range of volumetric injection fluxes, and can be used with water and polymer solutions. The system design consisted of fluid flow analysis, sizing a motor, design of the system components, and digital controls. The system was tested to verify the flow rates that could be achieved. This calibration found the maximum achievable flow rate to be 8Qs. Additional calibration will be done to the system onve the system construction is finalized

Single-carrier iterative frequency-domain equalization with soft decision feedback in shallow underwater acoustic communication

This paper investigates a single-carrier iterative frequency-domain equalization (SC-IFDE) scheme for high-rate underwater acoustic (UA) communication systems. This scheme is based on the minimum mean-squared error (MMSE) criterion, and soft decision feedback is applied to improve the reliability of the equalizer decision. The proposed algorithm is applied to the data received during the UA communication experiment conducted in December 2012 in the Indian Ocean off Rottnest Island, Western Australia. It is demonstrated that using one transmitting transducer and one receiving hydrophone, the proposed SC-IFDE algorithm achieves an average of 3% uncoded bit-error-rate (BER) with quaternary phase shift keying (QPSK) modulated signals over a range of 1 km

Channel estimation based on compressed sensing in high-speed underwater acoustic communication

The underwater acoustic (UA) channel is dispersive in both time and frequency with severe frequency-dependent signal attenuation. Efficient channel estimation and tracking are crucial to coherent high-rate UA communication. In this paper, we propose a new compressed sensing (CS) based channel estimation method with block-by-block channel tracking for UA communication. Compared with conventional channel estimation algorithms, the proposed method efficiently exploits the sparsity of the UA channel, and improves the channel tracking capability of UA communication system. The proposed algorithm was tested during our UA communication experiment conducted in December 2012 in the Indian Ocean off Rottnest Island, Western Australia. At a data rate of 8 kbps (QPSK constellations), average uncoded bit-error-rates (BERs) of 3% and 14% have been achieved over 1 km and 6 km ranges, respectively, using MMSE equalization based on the proposed channel estimation and tracking method