Techniques for distinguishing between impulsive and non-impulsive sound in the context of regulating sound exposure for marine mammals

Recommendations on bioacoustical metrics relevant for regulating exposure to anthropogenic underwater sounda)

Metrics to be used in noise impact assessment must integrate the physical acoustic characteristics of the sound field with relevant biology of animals. Several metrics have been established to determine and regulate underwater noise exposure to aquatic fauna. However, recent advances in understanding cause-effect relationships indicate that additional metrics are needed to fully describe and quantify the impact of sound fields on aquatic fauna. Existing regulations have primarily focused on marine mammals and are based on the dichotomy of sound types as being either impulsive or non-impulsive. This classification of sound types, however, is overly simplistic and insufficient for adequate impact assessments of sound on animals. It is recommended that the definition of impulsiveness be refined by incorporating kurtosis as an additional parameter and applying an appropriate conversion factor. Auditory frequency weighting functions, which scale the importance of particular sound frequencies to account for an animal's sensitivity to those frequencies, should be applied. Minimum phase filters are recommended for calculating weighted sound pressure. Temporal observation windows should be reported as signal duration influences its detectability by animals. Acknowledging that auditory integration time differs across species and is frequency dependent, standardized temporal integration windows are proposed for various signal types.

Multidimensional comparison of underwater soundscapes using the soundscape codea)

The soundscape of a given habitat is a product of its physical environment, human activity, and presence of soniferous marine life, which can be used to understand ecosystem processes, habitat quality, and biodiversity. Shallow coral habitats are hotspots of biodiversity and marine life. Deep-sea coral environments, in comparison, are generally poorly understood. Four soundscapes along the U.S. Outer Continental Shelf (OCS) and one soundscape from the Great Barrier Reef were quantified to explore how differences in habitat, depth, and substrate manifest acoustically. Comparisons were made between (1) deep, cold-water and shallow, warm-water coral reefs and (2) deep-sea coral and sandy bottom habitats. Application of the soundscape code to recordings in each location seeded cluster analyses of soundscape metrics and an assessment of daily trends to quantitatively compare the soundscapes. The shallow, tropical reef soundscape differed from the deep-sea soundscapes in amplitude and impulsiveness. Differences in soundscape properties among the deep-sea soundscapes suggested cold-water coral sites produce different soundscapes than the deep sites without live hard bottom. This initial assessment of deep-sea soundscapes along the U.S. OCS provides baseline acoustic properties in a region likely to experience changes due to climate and human use.

Evaluation of kurtosis-corrected sound exposure level as a metric for predicting onset of hearing threshold shifts in harbor porpoises (Phocoena phocoena)

Application of a kurtosis correction to frequency-weighted sound exposure level (SEL) improved predictions of risk of hearing damage in humans and terrestrial mammals for sound exposures with different degrees of impulsiveness. To assess whether kurtosis corrections may lead to improved predictions for marine mammals, corrections were applied to temporary threshold shift (TTS) growth measurements for harbor porpoises (Phocoena phocoena) exposed to different sounds. Kurtosis-corrected frequency-weighted SEL predicted accurately the growth of low levels of TTS (TTS_1-4 < 10 dB) for intermittent sounds with short (1-13 s) silence intervals but was not consistent with frequency-weighted SEL data for continuous sound exposures.

Kurtosis analysis of sounds from down-the-hole pile installation and the implications for marine mammal auditory impairment

Sounds from down-the-hole pile installation contain both impulsive and non-impulsive components. Kurtosis values (β) were determined for two datasets to investigate the impulsiveness of piling sounds. When the hammer struck the pile(s), β was 21-30 at 10 m and approximately 10 at 200 m. When the hammer was used for drilling without contacting the pile, β was 4-6 at all distances. These findings suggest that a simple dichotomy of classifying sounds as impulsive or non-impulsive may be overly simplistic for assessing marine mammal auditory impacts and studies investigating the impacts from complex sound fields are needed.

A brief overview of current approaches for underwater sound analysis and reporting

Soundscapes have substantially changed since the industrial revolution and in response to biodiversity loss and climate change. Human activities such as shipping, resource exploration and offshore construction alter natural ecosystems through sound, which can impact marine species in complex ways. The study of underwater sound is multi-disciplinary, spanning the fields of acoustics, physics, animal physiology and behaviour to marine ecology and conservation. These different backgrounds have led to the use of various disparate terms, metrics, and summary statistics, which can hamper comparisons between studies. Different types of equipment, analytical pathways, and reporting can lead to different results for the same sound source, with implications for impact assessments. For meaningful comparisons and derivation of appropriate thresholds, mitigation, and management approaches, it is necessary to develop common standards. This paper presents a brief overview of acoustic metrics, analysis approaches and reporting standards used in the context of long-term monitoring of soundscapes.

Acoustic characteristics from an in-water down-the-hole pile drilling activity

Sound generated by pile installation using a down-the-hole (DTH) hammer is not well documented and differs in character from sound generated by conventional impact and vibratory pile driving. This paper describes underwater acoustic characteristics from DTH pile drilling during the installation of 0.84-m shafts within 1.22-m steel piles in Ketchikan, Alaska. The median single-strike sound exposure levels were 138 and 142 dB re 1 μPa²s at 10 m for each of the two piles, with cumulative sound exposure levels of 185 and 193 dB re 1 μPa²s at 10 m, respectively. The sound levels measured at Ketchikan were significantly lower than previous studies, and the sound was determined to be non-impulsive in this study as compared to impulsive in previous studies. These differences likely result from the DTH hammer not making direct contact with the pile, as had been the case in previous studies. Therefore, we suggest using the term DTH pile drilling to distinguish from DTH pile driving when the hammer strikes the pile. Further research is needed to investigate DTH piling techniques and associated sound-generating mechanisms and to differentiate the various types of sound emitted, which has important implications for the underwater sound regulatory community.

International harmonization of procedures for measuring and analyzing of vessel underwater radiated noise

The habitat of the endangered southern resident killer whale (SRKW) overlaps major international shipping lanes near the Port of Vancouver, British Columbia. Shipping is a dominant source of underwater noise, which can hinder SRKW key life functions. To reduce environmental pressure on the SRKWs, Vancouver Fraser Port Authority offers incentives for quieter ships. However, the absence of a widely accepted underwater radiated noise (URN) measurement procedure hinders the determination of relative quietness. We review URN measurement procedures, summarizing results to date from two Canadian-led projects aimed at improving harmonization of shallow-water URN measurement procedures: One supports the International Organization for Standardization (ISO) in the development of a URN measurement standard; the other supports the alignment of URN measurement procedures developed by ship classification societies. Weaknesses in conventional shallow-water URN metrics are identified, and two alternative metrics proposed. Optimal shallow-water measurement geometry is identified.

Deep embedded clustering of coral reef bioacoustics

Deep clustering was applied to unlabeled, automatically detected signals in a coral reef soundscape to distinguish fish pulse calls from segments of whale song. Deep embedded clustering (DEC) learned latent features and formed classification clusters using fixed-length power spectrograms of the signals. Handpicked spectral and temporal features were also extracted and clustered with Gaussian mixture models (GMM) and conventional clustering. DEC, GMM, and conventional clustering were tested on simulated datasets of fish pulse calls (fish) and whale song units (whale) with randomized bandwidth, duration, and SNR. Both GMM and DEC achieved high accuracy and identified clusters with fish, whale, and overlapping fish and whale signals. Conventional clustering methods had low accuracy in scenarios with unequal-sized clusters or overlapping signals. Fish and whale signals recorded near Hawaii in February-March 2020 were clustered with DEC, GMM, and conventional clustering. DEC features demonstrated the highest accuracy of 77.5% on a small, manually labeled dataset for classifying signals into fish and whale clusters.

A Modeling Comparison of the Potential Effects on Marine Mammals from Sounds Produced by Marine Vibroseis and Air Gun Seismic Sources

Concerns about the potential environmental impacts of geophysical surveys using air gun sources, coupled with advances in geophysical surveying technology and data processing, are driving research and development of commercially viable alternative technologies such as marine vibroseis (MV). MV systems produce controllable acoustic signals through volume displacement of water using a vibrating plate or shell. MV sources generally produce lower acoustic pressure and reduced bandwidth (spectral content) compared to air gun sources, but to be effective sources for geophysical surveys they typically produce longer duration signals with short inter-signal periods. Few studies have evaluated the potential effects of MV system use on marine fauna. In this desktop study, potential acoustic exposure of marine mammals was estimated for MV and air gun arrays by modeling the source signal, sound propagation, and animal movement in representative survey scenarios. In the scenarios, few marine mammals could be expected to be exposed to potentially injurious sound levels for either source type, but fewer were predicted for MV arrays than air gun arrays. The estimated number of marine mammals exposed to sound levels associated with behavioral disturbance depended on the selection of evaluation criteria. More behavioral disturbance was predicted for MV arrays compared to air gun arrays using a single threshold sound pressure level (SPL), while the opposite result was found when using frequency-weighted sound fields and a multiple-step, probabilistic, threshold function.

Underwater noise characterization of down-the-hole pile driving activities off Biorka Island, Alaska

Although down-the-hole (DTH) pile driving is increasingly used for in-water pile installation, the characteristics of underwater noise from DTH pile driving is largely undocumented and unstudied. This study presents a comprehensive analysis of the noise characteristics during DTH pile driving of two steel pipe piles in shallow waters off southeast Alaska. The results showed that single-strike sound exposure levels measured at 10 m were 147 and 145 dB re 1 μPa²s with a total of 21,742 and 38,631 hammer strikes, with cumulative sound exposure levels to install each pile at 192 and 191 dB re 1 μPa²s, respectively. Though noise levels from a single strike was lower than impact pile driving of a similar pile, the cumulative sound exposure levels are likely comparable due to the much higher striking rate.

Introduction to the special issue on the effects of sound on aquatic life

The effects of anthropogenic (man-made) underwater sound on aquatic life have become an important environmental issue. One of the focal ways to present and to share knowledge on the topic has been the international conference on The Effects of Noise on Aquatic Life ("Aquatic Noise"). The conferences have brought together people from diverse interests and backgrounds to share information and ideas directed at understanding and solving the challenges of the potential effects of sound on aquatic life. The papers published here and in a related special issue of Proceedings of Meetings on Acoustics present a good overview of the many topics and ideas covered at the meeting. Indeed, the growth in studies on anthropogenic sound since the first meeting in 2007 reflects the increasing use of oceans, lakes, rivers, and other waterways by humans. However, there are still very substantial knowledge gaps about the effects of sound on all aquatic animals, and these gaps lead to there being a substantial need for a better understanding of the sounds produced by various sources and how these sounds may affect animals.

Application of kurtosis to underwater sound

Regulations for underwater anthropogenic noise are typically formulated in terms of peak sound pressure, root-mean-square sound pressure, and (weighted or unweighted) sound exposure. Sound effect studies on humans and other terrestrial mammals suggest that in addition to these metrics, the impulsiveness of sound (often quantified by its kurtosis β) is also related to the risk of hearing impairment. Kurtosis is often used to distinguish between ambient noise and transients, such as echolocation clicks and dolphin whistles. A lack of standardization of the integration interval leads to ambiguous kurtosis values, especially for transient signals. In the current research, kurtosis is applied to transient signals typical for high-power underwater noise. For integration time (t₂-t₁), the quantity (t₂-t₁)/β is shown to be a robust measure of signal duration, closely related to the effective signal duration, τ_eff for sounds from airguns, pile driving, and explosions. This research provides practical formulas for kurtosis of impulsive sounds and compares kurtosis between measurements of transient sounds from different sources.

Evaluating the predictive strength of underwater noise exposure criteria for marine mammals

The aim of underwater noise exposure criteria in a regulatory context is to identify at what received levels noise-induced effects are predicted to occur, so that those effects may be appropriately considered in an evaluation or mitigation context under the respective regulatory regime. Special emphasis has been given to hearing related impairment of marine mammals due to their high sensitivity to and reliance on underwater sound. Existing regulations of underwater noise show substantial qualitative and quantitative discrepancies. A dataset acquired during an experiment that induced temporary threshold shift (TTS) in a harbor porpoise (Phocoena phocoena) from Lucke, Siebert, Lepper, and Blanchet [(2009). J. Acoust. Soc. Am. 125, 4060-4070] was reanalyzed to see if various exposure criteria predicted TTS differently for high-frequency cetaceans. This provided an unambiguous quantitative comparison of predicted TTS levels for the existing noise exposure criteria used by regulatory bodies in several countries. The comparative evaluation of the existing noise exposure criteria shows substantial disagreement in the predicted levels for onset for auditory effects. While frequency-weighting functions evolved to provide a better representation of sensitivity to noise exposure when compared to measured results at the criteria's onset, thresholds remain the most important parameter determining a match between criteria and measured results.