Reducing vessel noise: An example of a solar-electric passenger ferry.

Concern over the impacts of anthropogenic noise on aquatic fauna is increasing, as is the number of vessels in the world's oceans, lakes, and rivers. Sound signatures of different vessel types are increasingly characterized, yet few reports are available on solar-electric powered vessels. Such data are important to model the sound levels experienced by marine fauna and their potential impacts. Sounds from two vessel types were recorded in the shallow waters of the Swan River, Western Australia, using bottom-mounted OceanInstruments SoundTraps. Multiple passes from two 10-m solar-electric powered passenger ferries and, for comparison, two 25-m conventionally powered (inboard diesel engine) passenger ferries were selected. Analysis was conducted on 58 and 16 passes by the electric ferries (in 2016 and 2017-2018, respectively) and 10 and 14 passes by the conventional ferry (2016 and 2017-2018, respectively) at 5-m range. At 55-m range, analysis was conducted on 17 and 1 passes by the electric ferry (2016 and 2017-2018, respectively) and 9 and 3 passes of the conventional ferry (2016 and 2017-2018, respectively). Measured received levels and modeled sound propagation were then used to estimate monopole source levels (MSL) and radiated noise levels (RNL). At 55-m range, the conventionally powered ferry type produced 156 and 157 dB re 1 µPa2m2 MSL and RNL, respectively, while the same metrics for the electric ferry were 12 dB lower. At frequencies below 500 Hz, spectral levels of the electric ferry at a range of <5 m were 10-25 dB lower than those of the conventional ferry, implying a potential benefit for animals that use low-frequency communication, if electric motors replaced petrol or diesel engines.

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.

Non-negative intensity for coupled fluid-structure interaction problems using the fast multipole method.

The non-negative intensity (NNI) method is applied to large-scale coupled fluid-structure interaction (FSI) problems using the fast multipole boundary element method (FMBEM). The NNI provides a field on the radiating structure surface that consists of positive-only contributions to the radiated sound power, thus avoiding the near-field cancellation effects that otherwise occur with the sound intensity field. Thus far the NNI has been implemented with the boundary element method (BEM) for relatively small problem sizes to allow for the full BEM coefficient and inverse matrices to be explicitly constructed and stored. In this work, the FMBEM is adapted to the NNI by calculating the eigenvalue solution of the symmetric acoustic impedance matrix using the FMBEM via a two-stage solution method. The FMBEM implementation of the NNI is demonstrated for a large-scale model of a submerged cylindrical shell. The coupled FSI problem is first solved using a finite element-FMBEM model and the resulting surface fields are then used in the FMBEM calculation of the NNI. An equivalent reactive NNI field representing the evanescent near-field radiation is demonstrated and the effect of the chosen number eigenvectors on the NNI field is investigated.

A modelling comparison between received sound levels produced by a marine Vibroseis array and those from an airgun array for some typical seismic survey scenarios.

Marine Vibroseis (MV) may provide a marine seismic sound source that has less environmental impact than conventional airguns. Modelled sound levels from a realistic MV array and airgun array with similar downward energy at frequencies <100Hz were compared under three scenarios: shallow, deep, and slope. Changing the layout of the MV array's higher frequency sources reduced sound exposure levels (SELs) by 4dB. At 100m range this MV was 20dB lower in peak-to-peak sound pressure level vs. the airgun array, decreasing to 12dB lower at 5km, the maximum modelled range for peak levels. SELs were less clear-cut, but for both shallow and deep water, MV produced 8dB lower SELs than the airguns at 100km range because of MV's reduced bandwidth. Overall, MV produced lower broadband SELs, especially at long range, and lower peak pressure, especially at short range, than airguns.

Fluctuations of the peak pressure level of man-made impulsive sound signals propagating in the ocean.

The theory of wave propagation and fluctuations in random media has been broadly studied; however the works studying the influence of a changing underwater acoustic environment on the spatial decay and fluctuations of the peak pressure in broadband and impulsive signals are limited. Using a method based on the formulation developed by Dyer and Makris to estimate intensity fluctuations of sound signals in the ocean in conditions of saturated multipath propagation, this paper presents an approach to model peak pressure fluctuations of transient signals propagating underwater. In contrast to the formulation of Dyer and Makris, the approach presented in this work applies extreme value theory using the properties of the peak pressure as a maximum value taken from a Rayleigh distributed amplitude. The location and scale parameters obtained from the best fit to a Gumbel distribution are used to estimate the probability of the peak pressure level staying below a certain threshold. The theory was applied to measurements of signals from an airgun array and offshore impact pile driving, resulting in good agreement in both cases.

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 1 m below the sea surface. Broadband source levels (10-48 000 Hz) increased from 134 to 171 dB re 1 µPa @ 1 m with speed from 3 to 16 m/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.

Regional Variations and Trends in Ambient Noise: Examples from Australian Waters.

Studies of ambient noise south of Australia show higher levels at low frequencies in the deep water off the continental shelf compared with locations on the shelf. The difference arises because of differences in transmission loss. Marine animals would experience significantly different noise levels and directionality in the two regions and while crossing the boundary, provide positional information. Opportunities for long-range, low-frequency communication by animals would be significantly limited by the higher background noise in the open ocean. Measures of long-term sea noise trends highlight the influence of biological sources and the importance of local sound transmission regimens.

Acoustic coupled fluid-structure interactions using a unified fast multipole boundary element method.

This paper presents a numerical model for the acoustic coupled fluid-structure interaction (FSI) of a submerged finite elastic body using the fast multipole boundary element method (FMBEM). The Helmholtz and elastodynamic boundary integral equations (BIEs) are, respectively, employed to model the exterior fluid and interior solid domains, and the pressure and displacement unknowns are coupled between conforming meshes at the shared boundary interface to achieve the acoustic FSI. The low frequency FMBEM is applied to both BIEs to reduce the algorithmic complexity of the iterative solution from O(N(2)) to O(N(1.5)) operations per matrix-vector product for N boundary unknowns. Numerical examples are presented to demonstrate the algorithmic and memory complexity of the method, which are shown to be in good agreement with the theoretical estimates, while the solution accuracy is comparable to that achieved by a conventional finite element-boundary element FSI model.

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.

In situ source levels of mulloway (Argyrosomus japonicus) calls.

Mulloway (Argyrosomus japonicus) in Mosman Bay, Western Australia produce three call categories associated with spawning behavior. The determination of call source levels and their contribution to overall recorded sound pressure levels is a significant step towards estimating numbers of calling fish within the detection range of a hydrophone. The source levels and ambient noise also provide significant information on the impacts anthropogenic activity may have on the detection of A. japonicus calls. An array of four hydrophones was deployed to record and locate individual fish from call arrival-time differences. Successive A. japonicus calls produced samples at various ranges between 1 and 100 m from one of the array hydrophones. The three-dimensional localization of calls, together with removal of ambient noise, allowed the determination of source levels for each call category using observed trends in propagation losses and interference. Mean source levels (at 1 m from the hydrophone) of the three call categories were calculated as 163 ± 16 dB re 1 µPa for Category 1 calls (short call of 2-5 pulses); 172 ± 4 dB re 1 µPa for Category 2 calls (long calls of 11-32 pulses); and 157 ± 5 dB re 1 µPa for Category 3 calls (series of successive calls of 1-4 pulses, increasing in call rate).