Noise-induced hearing loss in marine mammals: A review of temporary threshold shift studies from 1996 to 2015

Behavioral reactions of harbor porpoises to impact pile driving noise are predicted by the auditory frequency weighted sound pressure level

Offshore impact pile driving is a major source of high level underwater noise that can disturb marine mammal behavior tens of kilometers away. Projects involving pile driving are therefore subject to environmental impact assessments, which include modelling of the spatial extent of the behavioral disturbance. Reliable predictions about behavioral reaction distances require robust estimates of the minimum received levels of noise above which animals are likely to respond. Studies of reactions of harbor porpoises to pile driving noise in the wild and playback in captivity were identified, and reaction thresholds were extracted. Thresholds were weighted with the auditory frequency weighting function for VHF-cetaceans, the functional hearing group to which porpoises belong. The thresholds derived from playback studies to animals in captivity could be frequency weighted directly, whereas thresholds from exposure to noise from actual pile driving activities were weighted via a range-dependent weighting factor. Seven studies of porpoise reactions provided a first estimate of a behavioral reaction threshold as a VHF-weighted received level (Lp,125 ms,VHF) in the range 95-115 dB re 1 μPa.

Comment on "Similar susceptibility to temporary hearing threshold shifts despite different audiograms in harbor porpoises and harbor seals" [J. Acoust. Soc. Am. 155, 396-404 (2024)] (L)

Gransier and Kastelein [J. Acoust. Soc. Am. 155, 396-404 (2024)] present a review of selected studies on temporary threshold shift (TTS) in seals and porpoises. In contrast to the conclusion made in the paper, the results presented are fully consistent with the current understanding that sound exposure level is the best overall predictor of TTSs in marine mammals. If all available TTS studies on seals and porpoises exposed to narrowband noise are included, there is support neither for the conclusion that seals and porpoises are equally susceptible to TTSs nor for their claim that audiograms are poor predictors of the frequency dependence of TTS susceptibility.

An easy to implement empirical approach for estimating underwater sound transmission loss during pile driving in Florida

Underwater noise data were collected from 84 pile drives during bridge construction at various sites in Florida. These data were used to develop an empirically based model for underwater transmission loss associated with root mean squared, peak, and sound exposure level values. The model was verified using readings from other datasets as well as data from this study, and it appeared to reproduce reported transmission loss coefficient values well when data were curated to match data used in the empirical model's development and limited to situations where robust data were used in model development. As such, the model described here has some limitations, but in the context of pile driving in Florida where most piles are of similar dimensions and driven in similar water depths, especially during impact pile driving concrete piles, it may represent a useful design tool that engineers can use to predict underwater noise due to pile driving without the need to sample sound at multiple locations during driving.

Characterizing the underwater soundscape at the site of a proposed port in northeast Iceland

Finnafjörður is a small fjord in northeast Iceland, where the planned construction of a large port has the potential to meaningfully change the marine soundscape and ecosystem. In this study, we used one year (2021/22) of passive acoustic recordings to characterize the pre-construction soundscape, including broadband and decidecade sound pressure levels (SPL), frequency-weighted sound exposure levels, seasonal and diel variability and identified regular types of sound. Finnafjörður is relatively quiet with median decidecade levels centered between 25 Hz and 50 kHz of 74.5 to 86.3 dB re 1 μPa. Wind and rain dominate ambient SPL, while anthropogenic sources only occasionally contributed to the soundscape. Regular biological sound sources include humpback whales, toothed whales, and fish. This baseline soundscape description can be used for noise management during port construction, to monitor future changes in the region, and to act as a framework for comprehensive impact assessments as ports are developed globally.

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.

Current species protection does not serve its porpoise-Knowledge gaps on the impact of pressures on the Critically Endangered Baltic Proper harbour porpoise population, and future recommendations for its protection

Successful management requires information on pressures that threaten a species and areas where conservation actions are needed. The Baltic Proper harbour porpoise population was first listed as Critically Endangered by the International Union for the Conservation of Nature in 2008. Now, 16 years later, there is no change in conservation status despite ample conservation policy calling for its protection and an urgent need for management action to protect this population. Here, we provide an overview of the current status of the population, highlight knowledge gaps on the impact of pressures, and make recommendations for management of anthropogenic activities. Based on an exceeded limit for anthropogenic mortality, the high concentrations of contaminants in the Baltic Sea, combined with reductions in prey availability and increases in underwater noise, it is inferred that this population is likely still decreasing in size and conservation action becomes more urgent. As bycatch and unprotected underwater explosions result in direct mortality, they must be reduced to zero. Inputs of contaminants, waste, and existing and emerging noise sources should be minimised and regulated. Additionally, ecosystem-based sustainable management of fisheries is paramount in order to ensure prey availability, and maintain a healthy Baltic Sea. Stranding networks to routinely assess individuals for genetic population assignment and health need to be expanded, to identify rare samples from this population. Knowledge is still scarce on the population-level impact of each threat, along with the cumulative impact of multiple pressures on the population. However, the current knowledge and management instruments are sufficient to apply effective protection for the population now. While bycatch is the main pressure impacting this population, urgent conservation action is needed across all anthropogenic activities. Extinction of the Baltic Proper harbour porpoise population is a choice: decision-makers have the fate of this genetically and biologically distinct marine mammal population in their hands.

Narrowband noise induces frequency-specific underwater temporary threshold shifts in freshwater turtles

Freshwater turtles exhibit temporary threshold shifts (TTS) when exposed to broadband sound, but whether frequency-restricted narrowband noise induces TTS was unknown. Underwater TTS was investigated in two freshwater turtle species (Emydidae) following exposures to 16-octave narrowband noise (155-172 dB re 1 μPa2 s). While shifts occurred in all turtles at the noise center frequency (400 Hz), there were more instances of TTS and greater shift magnitudes at 12 octave above the center frequency, despite considerably lower received levels. These frequency-specific data provide new insight into how TTS manifests in turtles and expand empirical models to predict freshwater turtle TTS.

Echolocating Bats Have Evolved Decreased Susceptibility to Noise-Induced Temporary Hearing Losses

Glenis Long championed the application of quantitative psychophysical methods to understand comparative hearing abilities across species. She contributed the first psychophysical studies of absolute and masked hearing sensitivities in an auditory specialist, the echolocating horseshoe bat. Her data demonstrated that this bat has hyperacute frequency discrimination in the 83-kHz range of its echolocation broadcast. This specialization facilitates the bat's use of Doppler shift compensation to separate echoes of fluttering insects from concurrent echoes of non-moving objects. In this review, we discuss another specialization for hearing in a species of echolocating bat that contributes to perception of echoes within a complex auditory scene. Psychophysical and behavioral studies with big brown bats show that exposures to long duration, intense wideband or narrowband ultrasonic noise do not induce significant increases in their thresholds to echoes and do not impair their ability to orient through a naturalistic sonar scene containing multiple distracting echoes. Thresholds of auditory brainstem responses also remain low after intense noise exposures. These data indicate that big brown bats are not susceptible to temporary threshold shifts as measured in comparable paradigms used with other mammals, at least within the range of stimulus parameters that have been tested so far. We hypothesize that echolocating bats have evolved a decreased susceptibility to noise-induced hearing losses as a specialization for echolocation in noisy environments.

Frequency-dependent temporary threshold shifts in the Eastern painted turtle (Chrysemys picta picta)

Testudines are a highly threatened group facing an array of stressors, including alteration of their sensory environment. Underwater noise pollution has the potential to induce hearing loss and disrupt detection of biologically important acoustic cues and signals. To examine the conditions that induce temporary threshold shifts (TTS) in hearing in the freshwater Eastern painted turtle (Chrysemys picta picta), three individuals were exposed to band limited continuous white noise (50-1000 Hz) of varying durations and amplitudes (sound exposure levels ranged from 151 to 171 dB re 1 μPa2 s). Control and post-exposure auditory thresholds were measured and compared at 400 and 600 Hz using auditory evoked potential methods. TTS occurred in all individuals at both test frequencies, with shifts of 6.1-41.4 dB. While the numbers of TTS occurrences were equal between frequencies, greater shifts were observed at 600 Hz, a frequency of higher auditory sensitivity, compared to 400 Hz. The onset of TTS occurred at 154 dB re 1 μPa2 s for 600 Hz, compared to 158 dB re 1 μPa2 s at 400 Hz. The 400-Hz onset and patterns of TTS growth and recovery were similar to those observed in previously studied Trachemys scripta elegans, suggesting TTS may be comparable across Emydidae species.

Dolphin short-term auditory fatigue and self-mitigation

Auditory brainstem responses (ABRs) were measured at 57 kHz in two dolphins warned of an impending intense tone at 40 kHz. Over the course of testing, the duration of the intense tone was increased from 0.5 to 16 s to determine if changes in ABRs observed after cessation of the intense sound were the result of post-stimulatory auditory fatigue or conditioned hearing attenuation. One dolphin exhibited conditioned hearing attenuation after the warning sound preceding the intense sound, but little evidence of post-stimulatory fatigue after the intense sound. The second dolphin showed no conditioned attenuation before the intense sound, but auditory fatigue afterwards. The fatigue was observed within a few seconds after cessation of the intense tone: i.e., at time scales much shorter than those in previous studies of marine mammal noise-induced threshold shifts, which feature measurements on the order of a few minutes after exposure. The differences observed between the two individuals (less auditory fatigue in the dolphin that exhibited the conditioned attenuation) support the hypothesis that conditioned attenuation is a form of "self-mitigation."

Similar susceptibility to temporary hearing threshold shifts despite different audiograms in harbor porpoises and harbor seals

When they are exposed to loud fatiguing sounds in the oceans, marine mammals are susceptible to hearing damage in the form of temporary hearing threshold shifts (TTSs) or permanent hearing threshold shifts. We compared the level-dependent and frequency-dependent susceptibility to TTSs in harbor seals and harbor porpoises, species with different hearing sensitivities in the low- and high-frequency regions. Both species were exposed to 100% duty cycle one-sixth-octave noise bands at frequencies that covered their entire hearing range. In the case of the 6.5 kHz exposure for the harbor seals, a pure tone (continuous wave) was used. TTS was quantified as a function of sound pressure level (SPL) half an octave above the center frequency of the fatiguing sound. The species have different audiograms, but their frequency-specific susceptibility to TTS was more similar. The hearing frequency range in which both species were most susceptible to TTS was 22.5-50 kHz. Furthermore, the frequency ranges were characterized by having similar critical levels (defined as the SPL of the fatiguing sound above which the magnitude of TTS induced as a function of SPL increases more strongly). This standardized between-species comparison indicates that the audiogram is not a good predictor of frequency-dependent susceptibility to TTS.

Communication in Cook Inlet beluga whales: Describing the vocal repertoire and masking of calls by commercial ship noise

Many species rely on acoustic communication to coordinate activities and communicate to conspecifics. Cataloging vocal behavior is a first step towards understanding how individuals communicate information and how communication may be degraded by anthropogenic noise. The Cook Inlet beluga population is endangered with an estimated 331 individuals. Anthropogenic noise is considered a threat for this population and can negatively impact communication. To characterize this population's vocal behavior, vocalizations were measured and classified into three categories: whistles (n = 1264, 77%), pulsed calls (n = 354, 22%), and combined calls (n = 15, 1%), resulting in 41 call types. Two quantitative analyses were conducted to compare with the manual classification. A classification and regression tree and Random Forest had a 95% and 85% agreement with the manual classification, respectively. The most common call types per category were then used to investigate masking by commercial ship noise. Results indicate that these call types were partially masked by distant ship noise and completely masked by close ship noise in the frequency range of 0-12 kHz. Understanding vocal behavior and the effects of masking in Cook Inlet belugas provides important information supporting the management of this endangered population.

Auditory masking in odobenid and otariid carnivoresa)

As the only living species within the odobenid lineage of carnivores, walruses (Odobenus rosmarus) have no close relatives from which auditory information can be extrapolated. Sea lions and fur seals in the otariid lineage are the nearest evolutionary outgroup. To advance understanding of odobenid and otariid hearing, we conducted behavioral testing with two walruses and one California sea lion (Zalophus californianus). Detection thresholds for airborne sounds were measured from 0.08 to at least 16 kHz in ambient noise conditions and then re-measured in the presence of octave-band white masking noise. Walruses were more sensitive than the sea lion at lower frequencies and less sensitive at higher frequencies. Critical ratios for the walruses ranged from 20 dB at 0.2 kHz to 32 dB at 10 kHz, while critical ratios for the sea lion ranged from 16 dB at 0.2 kHz to 35 dB at 32 kHz. The masking values for these species are comparable to one another and to those of terrestrial carnivores, increasing by about 3 dB per octave with increasing frequency. Despite apparent differences in hearing range and sensitivity, odobenids and otariids have a similar ability to hear signals in noisy conditions.

Temporary noise-induced underwater hearing loss in an aquatic turtle (Trachemys scripta elegans)

Noise pollution in aquatic environments can cause hearing loss in noise-exposed animals. We investigated whether exposure to continuous underwater white noise (50-1000 Hz) affects the auditory sensitivity of an aquatic turtle Trachemys scripta elegans (red-eared slider) across 16 noise conditions of differing durations and amplitudes. Sound exposure levels (SELs) ranged between 155 and 193 dB re 1 μPa2 s, and auditory sensitivity was measured at 400 Hz using auditory evoked potential methods. Comparing control and post-exposure thresholds revealed temporary threshold shifts (TTS) in all three individuals, with at least two of the three turtles experiencing TTS at all but the two lowest SELs tested, and shifts up to 40 dB. There were significant positive relationships between shift magnitude and exposure duration, amplitude, and SEL. The mean predicted TTS onset was 160 dB re 1 μPa2 s. There was individual variation in susceptibility to TTS, threshold shift magnitude, and recovery rate, which was non-monotonic and occurred on time scales ranging from < 1 h to > 2 days post-exposure. Recovery rates were generally greater after higher magnitude shifts. Sound levels inducing hearing loss were comparatively low, suggesting aquatic turtles may be more sensitive to underwater noise than previously considered.

Bottlenose dolphin temporary threshold shift following exposure to 10-ms impulses centered at 8 kHza)

Studies of marine mammal temporary threshold shift (TTS) from impulsive sources have typically produced small TTS magnitudes, likely due to much of the energy in tested sources lying below the subjects' range of best hearing. In this study of dolphin TTS, 10-ms impulses centered at 8 kHz were used with the goal of inducing larger magnitudes of TTS and assessing the time course of hearing recovery. Most impulses had sound pressure levels of 175-180 dB re 1 μPa, while inter-pulse interval (IPI) and total number of impulses were varied. Dolphin TTS increased with increasing cumulative sound exposure level (SEL) and there was no apparent effect of IPI for exposures with equal SEL. The lowest TTS onset was 184 dB re 1 μPa2s, although early exposures with 20-s IPI and cumulative SEL of 182-183 dB re 1 μPa2s produced respective TTS of 35 and 16 dB in two dolphins. Continued testing with higher SELs up to 191 dB re 1 μPa2s in one of those dolphins, however, failed to result in TTS greater than 14 dB. Recovery rates were similar to those from other studies with non-impulsive sources and depended on the magnitude of the initial TTS.

Temporary threshold shift in bottlenose dolphins exposed to steady-state, 1/6-octave noise centered at 0.5 to 80 kHza)

Temporary threshold shift (TTS) was measured in bottlenose dolphins after 1-h exposures to 1/6-octave noise centered at 0.5, 2, 8, 20, 40, and 80 kHz. Tests were conducted in netted ocean enclosures, with the dolphins free-swimming during noise exposures. Exposure levels were estimated using a combination of video-based measurement of dolphin position, calibrated exposure sound fields, and animal-borne archival recording tags. Hearing thresholds were measured before and after exposures using behavioral methods (0.5, 2, 8 kHz) or behavioral and electrophysiological [auditory brainstem response (ABR)] methods (20, 40, 80 kHz). No substantial effects of the noise were seen at 40 and 80 kHz at the highest exposure levels. At 2, 8, and 20 kHz, exposure levels required for 6 dB of TTS (onset TTS exposures) were similar to previous studies; however, at 0.5 kHz, onset TTS was much lower than predicted values. No clear relationships could be identified between ABR- and behaviorally measured TTS. The results raise questions about the validity of current noise exposure guidelines for dolphins at frequencies below ∼1 kHz and how to accurately estimate received noise levels from free-swimming animals.

Acoustic Monitoring of Professionally Managed Marine Mammals for Health and Welfare Insights

Research evaluating marine mammal welfare and opportunities for advancements in the care of species housed in a professional facility have rapidly increased in the past decade. While topics, such as comfortable housing, adequate social opportunities, stimulating enrichment, and a high standard of medical care, have continued to receive attention from managers and scientists, there is a lack of established acoustic consideration for monitoring the welfare of these animals. Marine mammals rely on sound production and reception for navigation and communication. Regulations governing anthropogenic sound production in our oceans have been put in place by many countries around the world, largely based on the results of research with managed and trained animals, due to the potential negative impacts that unrestricted noise can have on marine mammals. However, there has not been an established best practice for the acoustic welfare monitoring of marine mammals in professional care. By monitoring animal hearing and vocal behavior, a more holistic view of animal welfare can be achieved through the early detection of anthropogenic sound sources, the acoustic behavior of the animals, and even the features of the calls. In this review, the practice of monitoring cetacean acoustic welfare through behavioral hearing tests and auditory evoked potentials (AEPs), passive acoustic monitoring, such as the Welfare Acoustic Monitoring System (WAMS), as well as ideas for using advanced technologies for utilizing vocal biomarkers of health are introduced and reviewed as opportunities for integration into marine mammal welfare plans.

Controllable acoustic deterrent based on the warning signals generated by nonel detonators

Acoustic deterrents are a practical strategy to mitigate the impact of underwater noise on marine mammals. However, their safety and effectiveness are still debatable. This study proposes a controllable acoustic deterrence method to protect marine mammals threatened by underwater blasting noise. The method creates strong-randomness warning signals using nonel detonators and establishes an escape time for animals protected. Combining the BELLHOP ray-based acoustic model with the marine environmental parameters and animals' auditory characteristics, we built a prediction model to establish a link between the acoustic fields and the adjustable source parameters, and provide a Risk zone and Deterrent zone for animals. The simulation and experimental results demonstrated that the root mean squared error between the simulated and measured sound pressure spectral density levels did not exceed 4.5 dB and the coefficient of determination remained at approximately 0.8, indicating that the new deterrent is an effective method with good controllable performances.

Relationship between occupational noise exposure and hypertension: Cross-sectional evidence from real-world

Background

Occupational noise is one of the most common and prevalent occupational hazards worldwide and may induce adverse auditory and/or non-auditory health effects. However, the relationship between occupational noise exposure and hypertension is controversial and has long been debated.

Methods

Based on large sample cross-sectional data from all registered occupational health examination units from 2021 to 2022 (N = 101,605), this study aimed to analyze the prevalence of hearing loss and hypertension and to explore the influencing factors of hypertension of workers in Wuhan. Descriptive statistics, univariate analyses and multivariate analyses were used. Forest plot and nomograms were constructed for the visualization of predictive results. The ROC curve, AUC, C-index and calibration curves were used to assess the predictive accuracy and validity. DCA was performed to evaluate the net benefit that workers could receive.

Results

Higher rate of high-frequency hearing loss (25.3%), speech frequency hearing loss (8.8%), ECG abnormalities (31.9%) and hypertension (21.0%) were found in workers exposed to occupational noise in Wuhan. Occupational noise exposure (OR = 1.09, 95% CI: 1.01-1.18, p = 0.04), growth of age (OR: 1.07, 95% CI: 1.07-1.07, p < 0.001), overweight (OR: 1.82, 95% CI: 1.73-1.92, p < 0.001), obesity (OR: 3.62, 95% CI: 3.42-3.83, p < 0.001), hyperglycemia (OR: 1.84, 95% CI: 1.73-1.96, p < 0.001), hypercholesterolemia (OR = 1.34; 95% CI 1.22-1.48; p < 0.001), ECG abnormalities (OR = 1.11; 95% CI 1.07-1.15; p < 0.001) and family history of hypertension (OR = 1.69; 95% CI 1.58-1.81; p < 0.001) were risk factors of hypertension for workers. Male workers had a relatively higher hypertension risk than female workers (OR = 1.61; 95% CI 1.54-1.69; p < 0.001). Ear protective measures could not reduce the risk of hypertension in workers. Our nomogram has good predictive accuracy and validity. A dynamic nomogram to predict the workers' risk of hypertension was established publicly available online.

Conclusion

Occupational noise exposure may elevate workers' hypertension risk. More effective and relevant prevention measures should be taken. Our nomogram may help identify high-risk workers and facilitate timely interventions.

Mechanisms of auditory masking in marine mammals

Anthropogenic noise is an increasing threat to marine mammals that rely on sound for communication, navigation, detecting prey and predators, and finding mates. Auditory masking is one consequence of anthropogenic noise, the study of which is approached from multiple disciplines including field investigations of animal behavior, noise characterization from in-situ recordings, computational modeling of communication space, and hearing experiments conducted in the laboratory. This paper focuses on laboratory hearing experiments applying psychophysical methods, with an emphasis on the mechanisms that govern auditory masking. Topics include tone detection in simple, complex, and natural noise; mechanisms for comodulation masking release and other forms of release from masking; the role of temporal resolution in auditory masking; and energetic vs informational masking.

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.

Thresholds for noise induced hearing loss in harbor porpoises and phocid seals

Intense sound sources, such as pile driving, airguns, and military sonars, have the potential to inflict hearing loss in marine mammals and are, therefore, regulated in many countries. The most recent criteria for noise induced hearing loss are based on empirical data collected until 2015 and recommend frequency-weighted and species group-specific thresholds to predict the onset of temporary threshold shift (TTS). Here, evidence made available after 2015 in light of the current criteria for two functional hearing groups is reviewed. For impulsive sounds (from pile driving and air guns), there is strong support for the current threshold for very high frequency cetaceans, including harbor porpoises (Phocoena phocoena). Less strong support also exists for the threshold for phocid seals in water, including harbor seals (Phoca vitulina). For non-impulsive sounds, there is good correspondence between exposure functions and empirical thresholds below 10 kHz for porpoises (applicable to assessment and regulation of military sonars) and between 3 and 16 kHz for seals. Above 10 kHz for porpoises and outside of the range 3-16 kHz for seals, there are substantial differences (up to 35 dB) between the predicted thresholds for TTS and empirical results. These discrepancies call for further studies.

Marine compressed air source array primary acoustic field characterization from at-sea measurements

The primary acoustic field of a standard seismic survey source array is described based on a calibrated dataset collected in the Gulf of Mexico. Three vertical array moorings were deployed to measure the full dynamic range and bandwidth of the acoustic field emitted by the compressed air source array. The designated source vessel followed a specified set of survey lines to provide a dataset with broad coverage of ranges and departure angles from the array. Acoustic metrics relevant to criteria associated with potential impacts on marine life are calculated from the recorded data. Sound pressure levels from direct arrivals exhibit large variability for a fixed distance between source and receiver; this indicates that the distance cannot be reliably used as a single parameter to derive meaningful exposure levels for a moving source array. The far-field acoustic metrics' variations with distance along the true acoustic path for a narrow angular bin are accurately predicted using a simplified model of the surface-affected source waveform, which is a function of the direction. The presented acoustic metrics can be used for benchmarking existing source/propagation models for predicting acoustic fields of seismic source arrays and developing simplified data-supported models for environmental impact assessments.

A Risk-Based Model Using Communication Distance Reduction for the Assessment of Underwater Continuous Noise: An Application to the Bottlenose Dolphin (Tursiops truncatus) Inhabiting the Spanish North Atlantic Marine Demarcation

Over the last decade, national authorities and European administrations have made great efforts to establish methodological standards for the assessment of underwater continuous noise, especially under the requirements set by the Marine Strategy Framework Directive (MSFD). Through the MSFD implementation across EU Member States Marine Reporting Units (MRUs), it is intended to establish the Good Environmental Status (GES) whether it is achieved or not. The evaluation of the Sound Pressure Level (SPL) at the local or regional scale for 1/3 octave band of 63 Hz and 125 Hz and the identification of long temporary trends were considered to be a priority due to the valuable information they can offer in relation to continuous low-frequency noise. Nevertheless, the methodology to determine threshold values from which to evaluate the GES has become difficult to define, and new approaches and considerations are currently being discussed by groups of experts, such as the technical subgroup on underwater acoustics (TGnoise) and regional commissions (e.g., OSPAR). This work presents a methodology to perform the assessment of a given area, providing a risk index that is related to potential appearance of masking effect due to the underwater noise produced by marine traffic. The risk index is hinged on the calculation of area under curves defined by the density of animals and a variable related to underwater noise SPL, defined as percentage of communication distance reduction. At this stage, the methodology presented does not consider physiological or behavioral mechanisms to overcome the masking by animals. The methodology presented has been applied to the bottlenose dolphin (Tursiops truncatus) inhabiting the ABIES—NOR marine demarcation to illustrate the possible use of risk-based models to manage marine areas related to human pressures, such as marine traffic, with the potential adverse impact on a given species (e.g., masking effect).

Predicting the population consequences of acoustic disturbance, with application to an endangered gray whale population

Acoustic disturbance is a growing conservation concern for wildlife populations because it can elicit physiological and behavioral responses that can have cascading impacts on population dynamics. State-dependent behavioral and life history models implemented via Stochastic Dynamic Programming (SDP) provide a natural framework for quantifying biologically meaningful population changes resulting from disturbance by linking environment, physiology, and metrics of fitness. We developed an SDP model using the endangered western gray whale (Eschrichtius robustus) as a case study because they experience acoustic disturbance on their summer foraging grounds. We modeled the behavior and physiological dynamics of pregnant females as they arrived on the feeding grounds and predicted the probability of female and offspring survival, with and without acoustic disturbance and in the presence/absence of high prey availability. Upon arrival in mid-May, pregnant females initially exhibited relatively random behavior before they transitioned to intensive feeding that resulted in continual fat mass gain until departure. This shift in behavior co-occurred with a change in spatial distribution; early in the season, whales were more equally distributed among foraging areas with moderate to high energy availability, whereas by mid-July whales transitioned to predominate use of the location that had the highest energy availability. Exclusion from energy-rich offshore areas led to reproductive failure and in extreme cases, mortality of adult females that had lasting impacts on population dynamics. Simulated disturbances in nearshore foraging areas had little to no impact on female survival or reproductive success at the population level. At the individual level, the impact of disturbance was unequally distributed across females of different lengths, both with respect to the number of times an individual was disturbed and the impact of disturbance on vital rates. Our results highlight the susceptibility of large capital breeders to reductions in prey availability, and indicate that who, where, and when individuals are disturbed are likely to be important considerations when assessing the impacts of acoustic activities. This model provides a framework to inform planned acoustic disturbances and assess the effectiveness of mitigation strategies for large capital breeders.

Evidence of Hearing Loss and Unrelated Toxoplasmosis in a Free-Ranging Harbour Porpoise (Phocoena phocoena)

Simple Summary

Evidence of hearing impairment was identified in a female harbour porpoise (Phocoena phocoena) on the basis of inner ear analysis. The animal live stranded on the Dutch coast at Domburg in 2016 and died a few hours later. Ultrastructural examination of the inner ear revealed evidence of sensory cell loss, which is compatible with noise exposure. In addition, histopathology also revealed multifocal necrotising protozoal encephalitis. A diagnosis of toxoplasmosis was confirmed by positive staining of tissue with anti-Toxoplasma gondii antibodies; however, T. gondii tachyzoites were not observed histologically in any of the examined tissues. This is the first case of presumptive noise-induced hearing loss and demonstration of T. gondii cysts in the brain of a free-ranging harbour porpoise from the North Sea.

Abstract

Evidence of hearing impairment was identified in a harbour porpoise (Phocoena phocoena) on the basis of scanning electron microscopy. In addition, based on histopathology and immunohistochemistry, there were signs of unrelated cerebral toxoplasmosis. The six-year old individual live stranded on the Dutch coast at Domburg in 2016 and died a few hours later. The most significant gross lesion was multifocal necrosis and haemorrhage of the cerebrum. Histopathology of the brain revealed extensive necrosis and haemorrhage in the cerebrum with multifocal accumulations of degenerated neutrophils, lymphocytes and macrophages, and perivascular lymphocytic cuffing. The diagnosis of cerebral toxoplasmosis was confirmed by positive staining of protozoa with anti-Toxoplasma gondii antibodies. Tachyzoites were not observed histologically in any of the examined tissues. Ultrastructural evaluation of the inner ear revealed evidence of scattered loss of outer hair cells in a 290 µm long segment of the apical turn of the cochlea, and in a focal region of ~ 1.5 mm from the apex of the cochlea, which was compatible with noise-induced hearing loss. This is the first case of concurrent presumptive noise-induced hearing loss and toxoplasmosis in a free-ranging harbour porpoise from the North Sea.

Seal bomb explosion sound source characterization

Small explosive charges, called seal bombs, used by commercial fisheries to deter marine mammals from depredation and accidental bycatch during fishing operations, produce high level sounds that may negatively impact nearby animals. Seal bombs were exploded underwater and recorded at various ranges with a calibrated hydrophone to characterize the pulse waveforms and to provide appropriate propagation loss models for source level (SL) estimates. Waveform refraction became important at about 1500 m slant range with approximately spherical spreading losses observed at shorter ranges. The SL for seal bombs was estimated to be 233 dB re 1 μPa m; however, for impulses such as explosions, better metrics integrate over the pulse duration, accounting for the total energy in the pulse, including source pressure impulse, estimated as 193 Pa m s, and sound exposure source level, estimated as 197 dB re 1 μPa² m² s over a 2 ms window. Accounting for the whole 100 ms waveform, including the bubble pulses and sea surface reflections, sound exposure source level was 203 dB re 1 μPa² m² s. Furthermore, integrating the energy over an entire event period of multiple explosions (i.e., cumulative sound exposure level) should be considered when evaluating impact.

Turning Scientific Knowledge into Regulation: Effective Measures for Noise Mitigation of Pile Driving

Pile driving is one of the most intense anthropogenic noise sources in the marine environment. Each foundation pile may require up to a several thousand strokes of high hammer energy to be driven to the embedded depth. Scientific evidence shows that effects on the marine environment have to be anticipated if mitigation measures are not applied. Effective mitigation measures to prevent and reduce the impact of pile driving noise should therefore be part of regulation. The role of regulators is to demonstrate and assess the applicability, efficiency and effectiveness of noise mitigation measures. This requires both, scientific knowledge on noise impacts and the consideration of normative aspects of noise mitigation. The establishment of mitigation procedures in plans and approvals granted by regulatory agencies includes several stages. Here, we outline a step-wise approach in which most of the actions described may be performed simultaneously. Potential measures include the appropriate maritime spatial planning to avoid conflicts with nature conservation, site development for offshore wind farms to avoid undesirable activities in time and space, coordination of activities to avoid cumulative effects, and the application of technical noise abatement systems to reduce noise at the source. To increase the acceptance of noise mitigation applications, technical measures should fulfil a number of requirements: (a) they are applicable and affordable, (b) they are state-of-the-art or at least advanced in development, (c) their efficiency can be assessed with standardised procedures. In this study, the efficiency of noise mitigation applied recently in offshore wind farm construction projects in the German North Sea is explained and discussed with regard to the regulation framework, including the technical abatement of impulsive pile driving noise.

Influence of environmental and anthropogenic acoustic cues in sea-finding of hatchling leatherback (Dermochelys coriacea) sea turtles

Although the visual and geomagnetic orientation cues used by sea turtle hatchlings during sea-finding have been well studied, the potential for auditory stimuli to act as an orientation cue has not been explored. We investigated the response of sea turtle hatchlings to natural and anthropogenic noises present on their nesting beaches during sea-finding. The responses of hatchling leatherback sea turtles, Dermochelys coriacea, collected from the Sandy Point National Wildlife Refuge, St. Croix, were measured in the presence of aerial acoustic sounds within hatchlings' hearing range of 50 to 1600 Hz. The highest sound energy produced by beach waves occurs at frequencies 50-1000 Hz, which overlaps with the most sensitive hearing range of hatchling leatherbacks (50-400 Hz). Natural beach wave sounds, which have highest sound energy at frequencies of 50-1000 Hz, may be masked by human conversations (85-650 Hz) and vehicle traffic noise (60-8000 Hz). In the presence of three stimuli, a) beach wave sounds (72.0 dB re: 20 μPa), b) human conversation (72.4 dB re: 20 μPa), and c) vehicle traffic noise (71.1 dB re: 20 μPa), hatchlings exhibited no phonotaxic response (wave sounds: mean angle = 152.1°, p = 0.645; human conversation: mean angle = 67.4°, p = 0.554; traffic noise: mean angle = 125.7°, p = 0.887). These results may be due to the hatchlings being unable to localize sounds in the experimental arena. Visual and auditory cues may also converge to affect sea-finding orientation. Future studies should focus on the localization ability of sea turtles and on the role that sound may play in orientation when combined with other sensory and environmental cues.

How Loud Can you go? Physical and Physiological Constraints to Producing High Sound Pressures in Animal Vocalizations

Sound is vital for communication and navigation across the animal kingdom and sound communication is unrivaled in accuracy and information richness over long distances both in air and water. The source level (SL) of the sound is a key factor in determining the range at which animals can communicate and the range at which echolocators can operate their biosonar. Here we compile, standardize and compare measurements of the loudest animals both in air and water. In air we find a remarkable similarity in the highest SLs produced across the different taxa. Within all taxa we find species that produce sound above 100 dBpeak re 20 μPa at 1 m, and a few bird and mammal species have SLs as high as 125 dBpeak re 20 μPa at 1 m. We next used pulsating sphere and piston models to estimate the maximum sound pressures generated in the radiated sound field. These data suggest that the loudest species within all taxa converge upon maximum pressures of 140–150 dBpeak re 20 μPa in air. In water, the toothed whales produce by far the loudest SLs up to 240 dBpeak re 1 μPa at 1 m. We discuss possible physical limitations to the production, radiation and propagation of high sound pressures. Furthermore, we discuss physiological limitations to the wide variety of sound generating mechanisms that have evolved in air and water of which many are still not well-understood or even unknown. We propose that in air, non-linear sound propagation forms a limit to producing louder sounds. While non-linear sound propagation may play a role in water as well, both sperm whale and pistol shrimp reach another physical limit of sound production, the cavitation limit in water. Taken together, our data suggests that both in air and water, animals evolved that produce sound so loud that they are pushing against physical rather than physiological limits of sound production, radiation and propagation.

Temporal variation of the underwater soundscape in Jiaotou Bay, an Indo-Pacific humpback dolphin (Sousa chinensis) habitat off Hainan Island, China

The underwater soundscape is an important ecological element affecting numerous aquatic animals, in particular dolphins, which must identify salient cues from ambient ocean noise. In this study, temporal variations in the soundscape of Jiaotou Bay were monitored from February 2016 to January 2017, where a population of Indo-Pacific humpback dolphins (Sousa chinensis) has recently been a regular sighting. An autonomous acoustic recorder was deployed in shallow waters, and 1/3-octave band sound pressure levels (SPLs) were calculated with central frequencies ranging from 25 Hz to 40 kHz, then were grouped into 3 subdivided bands via cluster analysis. SPLs at each major band showed significant differences on a diel, fishing-related period, seasonal, and tidal phase scale. Anthropogenic noise generated by passing ships and underwater explosions were recorded in the study area. The fish and dolphin acoustic activities both exhibited diel and seasonal variations, but no tidal cycle patterns. A negative significant relationship between anthropogenic sound detection rates and dolphin detection rates were observed, and fish detection rates showed no effect on dolphin detection rates, indicating anthropogenic activity avoidance and no forced foraging in dolphins in the study area. The results provide fundamental insight into the acoustic dynamics of an important Indo-Pacific humpback dolphin habitat within a coastal area affected by a rapid increase in human activity, and demonstrate the need to protect animal habitat from anthropogenic noises.

How could operational underwater sound from future offshore wind turbines impact marine life?

Offshore wind farms are part of the transition to a sustainable energy supply and both the total numbers and size of wind turbines are rapidly increasing. While the impact of underwater sound related to construction work has been in the focus of research and regulation, few data exist on the potential impact of underwater sound from operational wind farms. Here, we reviewed published sound levels of underwater sound from operational wind farms and found an increase with size of wind turbines expressed in terms of their nominal power. This trend was identified in both broadband and turbine-specific spectral band sound pressure levels (SPLs). For a nominal power of 10 MW, the trends in broadband SPLs and turbine-specific spectral band SPLs yielded source levels of 170 and 177 dB re 1 μPa m, respectively. The shift from using gear boxes to direct drive technology is expected to reduce the sound level by 10 dB. Using the National Oceanic Atmospheric Administration criterion for behavioral disruption for continuous noise (i.e., level B), a single 10 MW direct drive turbine is expected to cause behavioral response in marine mammals up to 1.4 km distance from the turbine, compared to 6.3 km for a turbine with gear box.

Bottom trawling noise: Are fishing vessels polluting to deeper acoustic habitats?

The impact of bottom trawling noise was quantified on two surrounding marine acoustic habitats using fixed mooring acoustic recorders. Noise during trawling activity is shown to be considerably louder than ambient noise and a nearby underway research vessel. Estimated source levels were above cetacean damage thresholds. Measurements at a submarine canyon indicated potential noise focussing, inferring a role for such features to enhance down slope noise propagation at continental margin sites. Modelled sound propagates more efficiently when sourced from trawling gear dragging along the seabed relative to the vessel as a surface source. Results are contextualised with respect to marine mammal harm, to other anthropogenic ocean noise sources, topography and seasons. Noise energy emitted by bottom trawling activity is a source of pollution that requires further consideration, in line with other pervasive trawling pressures on marine species and seabed habitats, especially in areas of heightened ecological susceptibility.

Temporary hearing threshold shift in harbor seals (Phoca vitulina) due to one-sixth-octave noise bands centered at 0.5, 1, and 2 kHz

This study concludes a larger project on the frequency-dependent susceptibility to noise-induced temporary hearing threshold shift (TTS) in harbor seals (Phoca vitulina). Here, two seals were exposed to one-sixth-octave noise bands (NBs) centered at 0.5, 1, and 2 kHz at several sound exposure levels (SELs, in dB re 1 μPa²s). TTSs were quantified at the center frequency of each NB, half an octave above, and one octave above, at the earliest within 1-4 min after exposure. Generally, elicited TTSs were low, and the highest TTS_1-4 occurred at half an octave above the center frequency of the fatiguing sound: after exposure to the 0.5-kHz NB at 210 dB SEL, the TTS_1-4 at 0.71 kHz was 2.3 dB; after exposure to the 1-kHz NB at 207 dB SEL, the TTS_1-4 at 1.4 kHz was 6.1 dB; and after exposure to the 2-kHz NB at 215 dB SEL, TTS_1-4 at 2.8 kHz was 7.9 dB. Hearing always recovered within 60 min, and susceptibility to TTS was similar in both seals. The results show that, for the studied frequency range, the lower the center frequency of the fatiguing sound, the higher the SEL required to cause the same TTS.

Combining Cochlear Analysis and Auditory Evoked Potentials in a Beluga Whale With High-Frequency Hearing Loss

Correlations between inner ear morphology and auditory sensitivity in the same individual are extremely difficult to obtain for stranded cetaceans. Animals in captivity and rehabilitation offer the opportunity to combine several techniques to study the auditory system and cases of hearing impairment in a controlled environment. Morphologic and auditory findings from two beluga whales (Delphinapterus leucas) in managed care are presented. Cochlear analysis of a 21-year-old beluga whale showed bilateral high-frequency hearing loss. Specifically, scanning electron microscopy of the left ear revealed sensory cell death in the first 4.9 mm of the base of the cochlea with scar formation. Immunofluorescence microscopy of the right ear confirmed the absence of hair cells and type I afferent innervation in the first 6.6 mm of the base of the cochlea, most likely due to an ischemia. Auditory evoked potentials (AEPs) measured 1.5 years prior this beluga's death showed a generalized hearing loss, being more pronounced in the high frequencies. This individual might have had a mixed hearing loss that would explain the generalized hearing impairment. Conversely, based on AEP evaluation, her mother had normal hearing and subsequent cochlear analysis did not feature any apparent sensorineural pathology. This is believed to be the first study to compare two cochlear analysis techniques and hearing sensitivity measurements from AEPs in cetaceans. The ability to combine morphological and auditory data is crucial to validate predictions of cochlear frequency maps based on morphological features. In addition, our study shows that these three complementary analysis techniques lead to comparable results, thus improving our understanding of how hearing impairment can be detected in stranding cases.

Evaluating temporary threshold shift onset levels for impulsive noise in seals

The auditory effects of single- and multiple-shot impulsive noise exposures were evaluated in a bearded seal (Erignathus barbatus). This study replicated and expanded upon recent work with related species [Reichmuth, Ghoul, Sills, Rouse, and Southall (2016). J. Acoust. Soc. Am. 140, 2646-2658]. Behavioral methods were used to measure hearing sensitivity before and immediately following exposure to underwater noise from a seismic air gun. Hearing was evaluated at 100 Hz-close to the maximum energy in the received pulse, and 400 Hz-the frequency with the highest sensation level. When no evidence of a temporary threshold shift (TTS) was found following single shots at 185 dB re 1 μPa² s unweighted sound exposure level (SEL) and 207 dB re 1 μPa peak-to-peak sound pressure, the number of exposures was gradually increased from one to ten. Transient shifts in hearing thresholds at 400 Hz were apparent following exposure to four to ten consecutive pulses (cumulative SEL 191-195 dB re 1 μPa² s; 167-171 dB re 1 μPa² s with frequency weighting for phocid carnivores in water). Along with these auditory data, the effects of seismic exposures on response time, response bias, and behavior were investigated. This study has implications for predicting TTS onset following impulsive noise exposure in seals.

Long-term and large-scale spatiotemporal patterns of soundscape in a tropical habitat of the Indo-Pacific humpback dolphin (Sousa chinensis)

Little is known about the characteristics of ambient sound in shallow waters southwest of Hainan Island, China, a tropical habitat of the Indo-Pacific humpback dolphin. The spatiotemporal patterns of soundscape in this area were thus studied and described here. Acoustic data collected from February 2018 to February 2019 at ten monitoring sites, spanning ~200 km of the coastline, were analyzed. The ambient sound characteristics in the investigated area showed significant spatiotemporal variations. Sound levels centered at 0.5 and 1 kHz were higher during dusk and night than other times of the day at all monitoring sites except for one. Higher sound levels at frequencies above 8 kHz were documented during autumn and winter at all sites except for three of them. Biological and anthropogenic sound sources including soniferous fishes, snapping shrimps, dolphins, ships, pile-driving activities, and explosions were identified during spectrogram analyses of a subsample of the dataset. The shipping noise was frequently detected throughout the monitoring sites. Spatiotemporal variations of the soundscape in the investigated waters provided baseline information on the local marine environment, which will be beneficial to the protection of the vulnerable Indo-Pacific humpback dolphin population recently discovered in the investigated waters.

Predicting the exposure of diving grey seals to shipping noise

There is high spatial overlap between grey seals and shipping traffic, and the functional hearing range of grey seals indicates sensitivity to underwater noise emitted by ships. However, there is still very little data regarding the exposure of grey seals to shipping noise, constraining effective policy decisions. Particularly, there are few predictions that consider the at-sea movement of seals. Consequently, this study aimed to predict the exposure of adult grey seals and pups to shipping noise along a three-dimensional movement track, and assess the influence of shipping characteristics on sound exposure levels. Using ship location data, a ship source model, and the acoustic propagation model, RAMSurf, this study estimated weighted 24-h sound exposure levels (10-1000 Hz) (SEL_w). Median predicted 24-h SEL_w was 128 and 142 dB re 1 μPa²s for the pups and adults, respectively. The predicted exposure of seals to shipping noise did not exceed best evidence thresholds for temporary threshold shift. Exposure was mediated by the number of ships, ship source level, the distance between seals and ships, and the at-sea behaviour of the seals. The results can inform regulatory planning related to anthropogenic pressures on seal populations.

Lack of reproducibility of temporary hearing threshold shifts in a harbor porpoise after exposure to repeated airgun sounds

Noise-induced temporary hearing threshold shift (TTS) was studied in a harbor porpoise exposed to impulsive sounds of scaled-down airguns while both stationary and free-swimming for up to 90 min. In a previous study, ∼4 dB TTS was elicited in this porpoise, but despite 8 dB higher single-shot and cumulative exposure levels (up to 199 dB re 1 μPa²s) in the present study, the porpoise showed no significant TTS at hearing frequencies 2, 4, or 8 kHz. There were no changes in the study animal's audiogram between the studies or significant differences in the fatiguing sound that could explain the difference, but audible and visual cues in the present study may have allowed the porpoise to predict when the fatiguing sounds would be produced. The discrepancy between the studies may have resulted from self-mitigation by the porpoise. Self-mitigation, resulting in reduced hearing sensitivity, can be achieved via changes in the orientation of the head, or via alteration of the hearing threshold by processes in the ear or central nervous system.

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.

Application of damped cylindrical spreading to assess range to injury threshold for fishes from impact pile driving

Environmental risk assessment for impact pile driving requires characterization of the radiated sound field. Damped cylindrical spreading (DCS) describes propagation of the acoustic Mach cone generated by striking a pile and predicts sound exposure level (L_E) versus range. For known water depth and sediment properties, DCS permits extrapolation from a measurement at a known range. Impact assessment criteria typically involve zero-to-peak sound pressure level (L_p,pk), root-mean-square sound pressure level (L_p,rms), and cumulative sound exposure level (L_E,cum). To facilitate predictions using DCS, L_p,pk and L_p,rms were estimated from L_E using empirical regressions. Using a wind farm construction scenario in the North Sea, DCS was applied to estimate ranges to recommended thresholds in fishes. For 3500 hammer strikes, the estimated L_E,cum impact ranges for mortal and recoverable injury were up to 1.8 and 3.1 km, respectively. Applying a 10 dB noise abatement measure, these distances reduced to 0.29 km for mortal injury and 0.65 km for recoverable injury. An underlying detail that produces unstable results is the averaging time for calculating L_p,rms, which by convention is equal to the 90%-energy signal duration. A stable alternative is proposed for this quantity based on the effective signal duration.

Underwater Noise Emission Due to Offshore Pile Installation: A Review

The growing demand for renewable energy supply stimulates a drastic increase in the deployment rate of offshore wind energy. Offshore wind power generators are usually supported by large foundation piles that are driven into the seabed with hydraulic impact hammers or vibratory devices. The pile installation process, which is key to the construction of every new wind farm, is hindered by a serious by-product: the underwater noise pollution. This paper presents a comprehensive review of the state-of-the-art computational methods to predict the underwater noise emission by the installation of foundation piles offshore including the available noise mitigation strategies. Future challenges in the field are identified under the prism of the ever-increasing size of wind turbines and the emerging pile driving technologies.

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.

Estimating the effects of pile driving sounds on seals: Pitfalls and possibilities

Understanding the potential effects of pile driving sounds on marine wildlife is essential for regulating offshore wind developments. Here, tracking data from 24 harbour seals were used to quantify effects and investigate sensitivity to the methods used to predict these. The Aquarius pile driving model was used to model source characteristics and acoustic propagation loss (16 Hz-20 kHz). Predicted cumulative sound exposure levels (SELcums) experienced by each seal were compared to different auditory weighting functions and damage thresholds to estimate temporary (TTS) and permanent (PTS) threshold shift occurrence. Each approach produced markedly different results; however, the most recent criteria established by Southall et al. [(2019) Aquat. Mamm. 45, 125-232] suggests that TTS occurrence was low (17% of seals). Predictions of seal density during pile driving made by Russell et al. [(2016) J. Appl. Ecol. 53, 1642-1652] were compared to distance from the wind farm and predicted single-strike sound exposure levels (SELss) by multiple approaches. Predicted seal density significantly decreased within 25 km or above SELss (averaged across depths and pile installations) of 145 dB re 1 μPa²⋅s. However, there was substantial variation in SELss with depth and installation, and thus in the predicted relationship with seal density. These results highlight uncertainty in estimated effects, which should be considered in future assessments.

Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear

Previous studies have demonstrated that some species of odontocetes can be conditioned to reduce hearing sensitivity when warned of an impending intense sound; however, the underlying mechanisms remain poorly understood. In the present study, conditioned hearing attenuation was elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 28 kHz. Testing was performed in air, with sounds presented via contact transducers. Hearing was assessed via noninvasive measurement of monaural auditory nerve responses (ANR) and binaural auditory brainstem responses (ABR). ABRs/ANRs were measured in response to 40-kHz tone bursts, over 2 to 3-s time intervals before and after the conditioned and unconditioned stimuli. Results showed reductions in ABR/ANR amplitude and increases in latency after pairing the warning and more intense tones. Monaural ANRs from the left and right ears were attenuated by similar amounts when the warning and more intense sounds were preferentially applied to the right ear. The data support a neural mechanism operating at the level of the cochlea and/or auditory nerve and suggest the involvement of neural projections that can affect the contralateral ear.

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

Regulations designed to mitigate the effects of man-made sounds on marine mammal hearing specify maximum daily sound exposure levels. The limits are lower for impulsive than non-impulsive sounds. The regulations do not indicate how to quantify impulsiveness; instead sounds are grouped by properties at the source. To address this gap, three metrics of impulsiveness (kurtosis, crest factor, and the Harris impulse factor) were compared using values from random noise and real-world ocean sounds. Kurtosis is recommended for quantifying impulsiveness. Kurtosis greater than 40 indicates a sound is fully impulsive. Only sounds above the effective quiet threshold (EQT) are considered intense enough to accumulate over time and cause hearing injury. A functional definition for EQT is proposed: the auditory frequency-weighted sound pressure level (SPL) that could accumulate to cause temporary threshold shift from non-impulsive sound as described in Southall, Finneran, Reichmuth, Nachtigall, Ketten, Bowles, Ellison, Nowacek, and Tyack [(2019). Aquat. Mamm. 45, 125-232]. It is known that impulsive sounds change to non-impulsive as these sounds propagate. This paper shows that this is not relevant for assessing hearing injury because sounds retain impulsive character when SPLs are above EQT. Sounds from vessels are normally considered non-impulsive; however, 66% of vessels analyzed were impulsive when weighted for very-high frequency mammal hearing.

Temporary hearing threshold shift in harbor seals (Phoca vitulina) due to a one-sixth-octave noise band centered at 32 kHz

Two female harbor seals were exposed for 60 min to a continuous one-sixth-octave noise band centered at 32 kHz at sound pressure levels of 92 to 152 dB re 1 μPa, resulting in sound exposure levels (SELs) of 128 to 188 dB re 1 μPa²s. This was part of a larger project determining frequency-dependent susceptibility to temporary threshold shift (TTS) in harbor seals over their entire hearing range. After exposure, TTSs were quantified at 32, 45, and 63 kHz with a psychoacoustic technique. At 32 kHz, only small TTSs (up to 5.9 dB) were measured 1-4 min (TTS_1-4) after exposure, and recovery was within 1 h. The higher the SEL, the higher the TTS induced at 45 kHz. Below ∼176 dB re 1 μPa²s, the maximum TTS_1-4 was at 32 kHz; above ∼176 dB re 1 μPa²s, the maximum TTS_1-4 (up to 33.8 dB) was at 45 kHz. During one particular session, a seal was inadvertently exposed to an SEL of ∼191 dB re 1 μPa²s and at 45 kHz, her TTS_1-4 was >45 dB; her hearing recovered over 4 days. Harbor seals appear to be equally susceptible to TTS caused by sounds in the 2.5-32 kHz range.

Effects of multiple exposures to pile driving noise on harbor porpoise hearing during simulated flights-An evaluation tool

Exploitation of renewable energy from offshore wind farms is substantially increasing worldwide. The majority of wind turbines are bottom mounted, causing high levels of impulsive noise during construction. To prevent temporary threshold shifts (TTS) in harbor porpoise hearing, single strike sound exposure levels (SEL_SS) are restricted in Germany by law to a maximum of 160 dB re 1 μPa²s at a distance of 750 m from the sound source. Underwater recordings of pile driving strikes, recorded during the construction of an offshore wind farm in the German North Sea, were analyzed. Using a simulation approach, it was tested whether a TTS can still be induced under current protective regulations by multiple exposures. The evaluation tool presented here can be easily adjusted for different sound propagation, acoustic signals, or species and enables one to calculate a minimum deterrence distance. Based on this simulation approach, only the combination of SEL_SS regulation, previous deterrence, and soft start allow harbor porpoises to avoid a TTS from multiple exposures. However, deterrence efficiency has to be monitored.