Investigation of sound pressure waveforms helps the selection of appropriate metrics to evaluate their effects on marine life in relation to noise thresholds. As marine animals move farther away from a sound source, the temporal characteristics of sound pressure may be influenced by interactions with the sediment and the sea surface. Sound pressure kurtosis and root-mean-square (rms) sound pressure are quantitative characteristics that depend on the shape of a sound pulse, with kurtosis related to the qualitative characteristic “impulsiveness.” After verifying the propagation modeling approach using selected test cases from the JAM Workshop held in Cambridge, UK, in 2022, the time dispersion values of pressure signals produced by an individual airgun shot across various sediment types are analyzed. The results reveal that there is significant pulse dispersion when the seabed consists of predominantly sand-type sediments: i.e., the airgun signal duration increases considerably over long distances, thus decreasing the kurtosis of a sequence of pulses, whereas the pulse dispersion is more limited for clay and silt-type sediments. The range variations of frequency weighted kurtosis and rms sound pressure differ from those of the unweighted kurtosis, depending on the corresponding lower and upper roll-off frequencies corresponding to different marine animal groups.

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Effective measures for protecting and preserving the marine environment require an understanding of the potential impact of anthropogenic sound on marine life. A crucial component is a proper assessment of the anthropogenic soundscape: which sounds are present where, when and how strong? We provide an extensive case study modelling the spatial, temporal and spectral distribution of sound radiated by several anthropogenic sources (ships, seismic airguns, explosives) and a naturally occurring one (wind) in the Dutch North Sea. We present the results as a series of sound maps covering the whole of the Dutch North Sea, showing the spatial and temporal distribution of the energy from these sources. Averaged over a two year period, shipping is responsible for the largest amount of acoustic energy (∼1800 J), followed by seismic surveys (∼300 J), explosions (∼20 J) and wind (∼20 J) in the frequency band between 100 Hz and 100 kHz. Our study shows that anthropogenic sources are responsible for 100 times more acoustic energy (averaged over 2 years) in the Dutch North Sea than naturally occurring sound from wind. The potential impact of these sounds on aquatic animals depends not only on these temporally averaged and spatially integrated broadband energies, but also on the source-specific spatial, spectral and temporal variation. Shipping is dominant in the southern part and along the coast in the north, throughout the years and across the spectrum. Seismic surveys are relatively local and spatially and temporally dependent on exploration activities in any particular year, and spectrally shifted to low frequencies relative to the other sources. Explosions in the southern part contribute wide-extent high energy bursts across the spectrum. Relating modelled sound fields to the temporal and spatial distribution of animal species may provide a powerful tool for understanding the potential impact of anthropogenic sound on marine life. Detailed sound maps of the Dutch North Sea, which can serve as a basis for further impact assessment studies and legislation, are calculated and analyzed.

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Computer models can be used to predict the sound field near an airgun or airgun array. The accuracy of such predictions can be assessed either by a process of verification (comparison with alternative theoretical solution to the same computational problem) or by validation (comparison with measurement). A set of verification test problems is described, as used originally by participants in the International Airgun Modeling Workshop held in Dublin, Ireland on July 16, 2016, and now by other authors in this special issue. The main inputs specified are the characteristics of the source (array geometry, airgun type and volume, and chamber pressure) and of the propagation medium (mainly water depth and bottom type). Also specified are source waveforms for individual airguns for authors wishing to focus only on the propagation aspects of the problem. The outputs are specified in terms of metrics derived from the sound pressure and sound particle acceleration.@en

An analytical formulation for the calculation of the range and depth-dependence of propagation loss in isospeed water is introduced. The range-dependent bathymetry is handled with Weston's ray invariant approach, requiring a gradually varying water depth. The depth dependence of propagation loss is formulated using a transformation from an incoherent mode sum to an integral over angle, in the adiabatic approximation, and the results obtained in this way are tested by comparison with a full adiabatic normal mode sum. The validity of the adiabatic approximation itself is then investigated by means of a comparison with parabolic equation and coupled normal mode results for selected test cases from the Weston Memorial Workshop, held at the University of Cambridge in April 2010. Propagation loss calculations based on different methods are compared. The proposed analytical approach provides practical, fast and accurate results.@en

Comparisons are made of sound pressure and particle acceleration predicted by two methods in the vicinity of two arrays of marine-seismic airguns. Data describing the array properties and the environmental conditions are taken from test cases designed to facilitate intermodel comparison. The two propagation approaches, one based on method of images and the other on wave number integration, are shown to be capable of giving line-on-line agreement when the latter method implements the full form of the Hankel transform; when the more approximate Fourier transform is used, predictions are shown to differ at ranges of a few meters from the source.@en

The European Union requires member states to achieve or maintain good environmental status for their marine territorial waters and explicitly mentions potentially adverse effects of underwater sound. In this study, we focused on producing maps of underwater sound from various natural and anthropogenic origins in the Dutch North Sea. The source properties and sound propagation are simulated by mathematical methods. These maps could be used to assess and predict large-scale effects on behavior and distribution of underwater marine life and therefore become a valuable tool in assessing and managing the impact of underwater sound on marine life.@en

Sound mapping over large areas can be computationally expensive because of the large number of sources and large source-receiver separations involved. In order to facilitate computation, a simplifying assumption sometimes made is to neglect the sound speed gradient in shallow water. The accuracy of this assumption is investigated for ship generated sound in the Dutch North Sea, for realistic ship and wind distributions. Sound maps are generated for zero, negative and positive gradients for selected frequency bands (56 Hz to 3.6 kHz). The effect of sound speed profile for the decidecade centred at 125 Hz is less than 1.7 dB.@en

The production of underwater sound is more and more considered to be an environmental risk. This has already been the case for military sonar for more than a decade, as sonar was identified as a possible cause of marine mammal strandings. The approach we adapted for military sonar is the following. The risk is characterized by computing the exposure (sound produced by the sonar) in an area around the source and by coupling that information to the effects it causes on a certain animal species. The risk is then quantified by taking into account the probability of the presence of that species in the area. If too large, the risk can be mitigated. We observe a trend of shifting the focus from individual disturbance to more general population consequences. A similar approach is advised to characterize the risks involved in the use of airguns in seismic acquisition.

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Concern about possible impact of human activity on marine life has led to regulation in the USA and the EU. The EU's Marine Strategy Framework Directive requires member states to achieve Good Environmental Status (GES) by 2020. Descriptor 11 of GES is "Introduction of energy, including underwater noise, is at levels that do not adversely affect the marine environment", for which two indicators are specified. The effects of underwater sound on aquatic animals is a research topic in need of multidisciplinary effort between acousticians and biologists, in order to provide a quantitative description of the sound field and to understand the impact of the sound on aquatic life. Sound exposure level (SEL), sound pressure level (SPL) and peak pressure are widely used as metrics for assessing environmental impact . This paper described the calculation of SEL and SPL for impulsive sounds in shallow water (relevant to Indicator 11.2.1 of GES Descriptor 11), with particular attention to the difficulties associated with the effect of time dispersion, which can lead to large uncertainties and errors in SPL.@en