Localizing sources using a network of asynchronous compact arrays (PDF)

Urazghildiiev, I.R. and D.E. Hannay

IEEE J. Ocean. Eng. 45(3): 1091-1098 (2019)

DOI: 10.1109/JOE.2019.2915913

This paper considers the problem of passive acoustic localization of sources using a network of stationary compact arrays of acoustic sensors providing azimuth and elevation measurements of detected sounds. The maximum-likelihood estimator and Cramér-Rao bounds are derived. The localization accuracy is evaluated using statistical simulations and in situ tests. The plots of azimuth, range, and position estimation accuracy are presented. Test results demonstrated that superefficient position estimates with an error lower than the Cramér-Rao bounds can be achieved for surface vessels and other sound sources with known depths.

Potential benefits of vessel slowdowns on endangered southern resident killer whales (PDF)

Joy, R., D. Tollit, J. Wood, A. MacGillivray, Z. Li, K. Trounce, and O. Robinson

Frontiers in Marine Science 6: 344 (2019)

DOI: 10.3389/fmars.2019.00344

A voluntary commercial vessel slowdown trial was conducted through 16 nm of shipping lanes overlapping critical habitat of at-risk southern resident killer whales (SRKW) in the Salish Sea. From August 7 to October 6, 2017, the trial requested piloted vessels to slow to 11 knots speed-through-water. […] Slowdown results were compared to ‘Baseline’ noise of the same region, matched across lunar months. […] A regional vessel noise model predicted noise for a range of traffic volume and vessel speed scenarios for a 1133 km^2 ‘Slowdown region’ containing the 16 nm of shipping lanes. A temporally and spatially explicit simulation model evaluated the changes in traffic volume and speed on SRKW in their foraging habitat within this Slowdown region. The model tracked the number and magnitude of noise-exposure events that impacted each of 78 (simulated) SRKW across different traffic scenarios. These disturbance metrics were simplified to a cumulative effect termed ‘potential lost foraging time’ that corresponded to the sum of disturbance events described by assumptions of time that whales could not forage due to noise disturbance. […]

Slowing deep-sea commercial vessels reduces underwater radiated noise (PDF)

MacGillivray, A.O., Z. Li, D.E. Hannay, K.B. Trounce and O.M. Robinson

J. Acoust. Soc. Am. 146: 340-351 (2019)

DOI: 10.1121/1.5116140

During 2017, the Vancouver Fraser Port Authority's Enhancing Cetacean Habitat and Observation program carried out a two-month voluntary vessel slowdown trial to determine whether slowing to 11 knots was an effective method for reducing underwater radiated vessel noise. The trial was carried out in Haro Strait, British Columbia, in critical habitat of endangered southern resident killer whales. During the trial, vessel noise measurements were collected next to shipping lanes on two hydrophones inside the Haro Strait slowdown zone, while a third hydrophone in Strait of Georgia measured vessels noise outside the slowdown zone. Vessel movements were tracked using the automated identification system (AIS), and vessel pilots logged slowdown participation information for each transit. An automated data processing system analyzed acoustical and AIS data from the three hydrophone stations to calculate radiated noise levels and monopole source levels (SLs) of passing vessels. Comparing measurements of vessels participating in the trial with measurements from control periods before and after the trial showed that slowing down was an effective method for reducing mean broadband SLs for five categories of piloted commercial vessels: containerships (11.5 dB), cruise vessels (10.5 dB), vehicle carriers (9.3 dB), tankers (6.1 dB), and bulkers (5.9 dB).

Sound exposure level as a metric for analyzing and managing underwater soundscapes (PDF)

Martin, S.B., C. Morris, K. Bröker, and C. O’Neill

J. Acoust. Soc. Am. 146: 135-149 (2019)

DOI: 10.1121/1.5113578

The auditory frequency weighted daily sound exposure level (SEL) is used in many jurisdictions to assess possible injury to the hearing of marine life. Therefore, using daily SEL to describe soundscapes would provide baseline information about the environment using the same tools used to measure injury. Here, the daily SEL from 12 recordings with durations of 18–97 days are analyzed to: (1) identify natural soundscapes versus environments affected by human activity, (2) demonstrate how SEL accumulates from different types of sources, (3) show the effects of recorder duty cycling on daily SEL, (4) make recommendations on collecting data for daily SEL analysis, and (5) discuss the use of the daily SEL as an indicator of cumulative effects. The autocorrelation of the one-minute sound exposure is used to help identify soundscapes not affected by human activity. Human sound sources reduce the autocorrelation and add low-frequency energy to the soundscapes. To measure the daily SEL for all marine mammal auditory frequency weighting groups, data should be sampled at 64 kHz or higher, for at least 1 min out of every 30 min. The daily autocorrelation of the one-minute SEL provides a confidence interval for the daily SEL computed with duty-cycled data.

Systematic source level measurements of whale watching vessels and other small boats (PDF)

Wladichuk, J.L., D.E. Hannay, A.O. MacGillivray, Z. Li, and S. Thornton

Journal of Ocean Technology 14(3): 108-126 (2019)

JOT Article page

Marine mammals rely heavily on sound for foraging, communicating, and navigating. As noise in the ocean increases, their ability to perform these important life functions can be affected. In the past decade, numerous studies have expanded our awareness of the effects of anthropogenic noise on marine life. Improving our knowledge of how sound impacts marine mammals is particularly important in coastal waters where the spatial distributions of vessels and marine mammals overlap, as exemplified by the critical habitat for the endangered Southern Resident killer whale (Orcinus orca). The impacts of small vessel traffic (including the commercial and recreational whale watching that is directed on this population) has been difficult to assess as there is a data gap for small vessel noise emissions. In this study, two autonomous marine acoustic recorders were deployed in transboundary Haro Strait (British Columbia, Canada, and Washington State, USA) from July to October 2017 to measure sound levels produced by whale watching vessels and other small boats. […]

The presence of large whale species in Clayoquot Sound and its offshore waters

Burnham, R.E., D.A. Duffus, and X. Mouy

Continental Shelf Research 177: 15–23 (2019)

DOI: 10.1016/j.csr.2019.03.004

Large whale populations in the northeast Pacific were severely reduced by whaling, with many showing limited recovery. Their use of offshore waters and limited knowledge of life histories has hindered studies focused on estimating population numbers and mapping habitat use. Acoustic recordings, using vocalizations as a marker of whale presence, may be the first step in re-establishing baseline knowledge of species presence over time and space. Recordings from both stationary and mobile platforms, covering waters from coastal to shelf-break and offshore waters, show spatial segregation in the dominant species recorded. Inshore recordings are dominated by more coastally-focused species, whereas fin (Balaenoptera physalus) blue (Balaenoptera musculus) and sperm whales (Physeter macrocephalus) are primarily heard in the shelf-break zones. Calls tentatively described for sei whales (Balaenoptera borealis) are also noted. Calls matching those previously described to these species as breeding and foraging calls were found. Acoustic monitoring surveys like this study are needed to better map presence and habitat use of these rare and endangered species, ultimately leading to the identification and protection of areas important to population recovery.

Western North Atlantic humpback whale fall and spring acoustic repertoire: Insight into onset and cessation of singing behavior (PDF)

Kowarski, K., H. Moors-Murphy, E. Maxner, and S. Cerchio

J. Acoust. Soc. Am. 145: 2305–2316 (2019)

DOI: 10.1121/1.5095404

Humpback whale songs have been described worldwide and studies exploring non-song vocal behavior continue to expand; however, studies on the transition periods when whales shift to and from the seasonal behavioral state of singing are lacking and may be potentially informative regarding the proximal factors controlling the onset and offset of humpback whale male singing. Acoustic recorders collected data off eastern Canada continuously from the Bay of Fundy in the fall of 2015 and near-continuously off northeast Nova Scotia in the spring of 2016. Humpback whale acoustic occurrence and behavior were identified by systematically reviewing a subset of acoustic recordings for presence before analyzing the highest quality recordings for behavior. The onset of singing in the fall was gradual over a period of about three weeks with an intermediate form, termed “song fragment,” occurring prior to full songs. In comparison, singing in the spring seemed to end abruptly with few song fragments. Song fragments could be produced by juveniles learning to sing for the first time or mature males preparing for breeding activities prior to migrating to southern breeding grounds. The authors propose an alternative hypothesis that the timing and manner of transitions could be driven by physiological processes similar to those documented in songbirds.

Acoustic characterization of exploration drilling in the Chukchi and Beaufort seas

Austin, M.E., D.E. Hannay, and K.C. Bröker

Journal of the Acoustical Society of America 144: 115–123 (2018)

DOI: 10.1121/1.5044417

This paper characterizes underwater sound levels produced by three drilling units during offshore exploration drilling at three sites in the Beaufort and Chukchi seas. Received levels and spectra are reported as functions of distance during drilling and excavation of mudline cellars (MLCs). Sound levels emitted during MLC excavation exceeded those during drilling at all three sites, although this operation was much shorter in duration. Drilling sounds exhibited tones below 2 kHz, with harmonics present to 10 kHz, while MLC excavation sounds were broadband in character. Drilling sounds varied substantially between the three operations, whereas MLC excavation sounds were more consistent in amplitude and spectral distribution. Estimates of broadband and 1/3-octave band source levels were computed from measurements at 1 km range. The broadband drilling source levels were 168.6 dB re 1 μPa m for the Kulluk drilling unit, 174.9 dB re 1 μPa m for the drillship Noble Discoverer, and 170.1 dB re 1 μPa m for the semi-submersible Polar Pioneer. The received levels measured at 1 km during MLC excavation yielded source level estimates that were more consistent among sources: 191.8, 193.0, and 193.3 dB re 1 μPa for the Discoverer, Kulluk, and Polar Pioneer, respectively.

Assessing vessel slowdown for reducing auditory masking for marine mammals and fish of the western Canadian Arctic

Pine, M.K., D.E. Hannay, S.J. Insley, W.D. Halliday, and F. Juanes

Marine Pollution Bulletin 135: 290-302 (2018)

DOI: 10.1016/j.marpolbul.2018.07.031

Vessel slowdown may be an alternative mitigation option in regions where re-routing shipping corridors to avoid important marine mammal habitat is not possible. We investigated the potential relief in masking in marine mammals and fish from a 10 knot speed reduction of container and cruise ships. The mitigation effect from slower vessels was not equal between ambient sound conditions, species or vessel-type. Under quiet ambient conditions, a speed reduction from 25 to 15 knots resulted in smaller listening space reductions by 16–23%, 10–18%, 1–2%, 5–8% and 8% respectively for belugas, bowheads, bearded seals, ringed seals, and fish, depending on vessel-type. However, under noisy conditions, those savings were between 9 and 19% more, depending on the species. This was due to the differences in species' hearing sensitivities and the low ambient sound levels measured in the study region. Vessel slowdown could be an effective mitigation strategy for reducing masking.

Behavioral responses of harbor porpoises (Phocoena phocoena) to sonar playback sequences of sweeps and tones (3.5-4.1 kHz) (PDF)

Kastelein, R.A., L. Helder-Hoek, S. Van de Voorde, S. de Winter, S. Janssen, and M.A. Ainslie

Aquatic Mammals 44: 389-404 (2018)

DOI: 10.1578/AM.44.4.2018.389

Naval sonar signals may affect the behavior of harbor porpoises (Phocoena phocoena). The 53C sonar system produces 1,600 ms sonar signals in the 3.5 to 4.1 kHz band, each consisting of a sweep immediately followed by two tones which are separated by a 100 ms silence. Effects of sound pressure level (SPL) and duty cycle on the behavioral responses of two harbor porpoises to these sounds were investigated. Respiration rate, distance to the transducer, swimming speed, and the number of jumps during sound exposure and baseline periods were compared. Harbor porpoises were exposed to 30-min playbacks of 53C sonar sounds at five average received SPLs (Lrecs) with a duty cycle of 2.7%, and at six Lrecs with a duty cycle of 96%, under low ambient noise conditions. They did not respond to the sounds when the duty cycle was 2.7%, even at the maximum Lrec (143 dB re 1 μPa). When the duty cycle was 96%, only Porpoise 06 increased his respiration rate when the Lrec was ≥119 dB re 1 μPa, and he moved away from the transducer only at an Lrec of 143 dB re 1 μPa. At the same Lrec and duty cycle, the effect of 53C sonar sounds on harbor porpoise behavior was weaker than that of 1 to 2 kHz, 6 to 7 kHz, and 25 kHz sonar signals observed in previous studies.

Noise exposure from commercial shipping for the southern resident killer whale population

Cominelli, S., R. Devillers, H. Yurk, A. MacGillivray, L. McWhinnie, and R. Canessa

Marine Pollution Bulletin 136: 177-200 (2018)

DOI: 10.1016/j.marpolbul.2018.08.050

This study assesses vessel-noise exposure levels for Southern Resident Killer Whales (SRKW) in the Salish Sea. Kernel Density Estimation (KDE) was used to delineate SRKW summer core areas. Those areas were combined with the output of a regional cumulative noise model describing sound level variations generated by commercial vessels (1/3-octave-bands from 10 Hz to 63.1 kHz). Cumulative distribution functions were used to evaluate SRKW's noise exposure from 15 vessel categories over three zones located within the KDE. Median cumulative noise values were used to group categories based on the associated exposure levels. Ferries, Tugboats, Vehicle Carriers, Recreational Vessels, Containers, and Bulkers showed high levels of exposure (Leq−50th > 90 dB re 1 μPa) within SRKW core areas. Management actions aiming at reducing SRKW noise exposure during the summer should target the abovementioned categories and take into consideration the spatial distribution of their levels of exposure, their mechanical and their operational characteristics.

Passive acoustic detection and estimation of the number of sources using compact arrays (PDF)

Urazghildiiev, I.R. and D.E. Hannay

Journal of the Acoustical Society of America 143: 2825-2833 (2018)

DOI: 10.1121/1.5037361

The problem of estimating the number of sound-producing sources detected using a compact array of hydrophones is addressed. Closed form expressions representing the techniques of automatic detection and estimation of the number of callers are given. Their performance is evaluated on a year-long dataset (1 October 2015–6 October 2016) containing humpback whale and killer whale calls collected in the Strait of Georgia, near Vancouver, British Columbia. Manual verification of the automatic detections produced by the approach required ∼40 h.

Population consequences of disturbance by offshore oil and gas activity for endangered sperm whales (Physeter macrocephalus)

Farmer, N.A., K. Baker, D.G. Zeddies, S.L. Denes, D.P. Noren, L.P. Garrison, A. Machernis, E.M. Fougères, and M. Zykov

Biological Conservation 227: 189-204 (2018)

DOI: 10.1016/j.biocon.2018.09.006

Sperm whale (Physeter macrocephalus) populations are still recovering from massive population declines associated with commercial whaling operations. The species continues to face a suite of contemporary threats, including pollution, ship strikes, fisheries interactions, habitat loss and degradation, oil spills, and anthropogenic noise. The sperm whale stock in the northern Gulf of Mexico was exposed to oil from the Deepwater Horizon (DWH) oil spill and is exposed to high levels of anthropogenic noises generated by geological and geophysical (G&G) surveys for hydrocarbon deposits. Population impacts from oil and gas activities were predicted from models that incorporated two stressors: (i) oil exposure from DWH and (ii) noise from G&G surveys. Oil exposure was projected to reduce survival and reproductive success, causing a mean stock decline of 26% by 2025. Additionally, exposure to underwater noise can adversely impact whale hearing, communication, foraging efficiency, and disturb essential behaviors. Exposures to G&G survey noise were determined by simulating individual movements through three-dimensional sound fields generated by different survey methods. Behavioral disturbance was evaluated as reduced foraging opportunities under four dose-response functions. Bioenergetic models tracked the depletion of reserves in blubber, muscle, and viscera. All simulations suggested significant reductions in relative fitness of reproductive females were a likely consequence of persistent disturbances to foraging behaviors. Under a 160 dB SPL unweighted dose-response function, up to 4.4 ± 0.3% of the stock may reach terminal starvation due to behavioral disturbance associated with future G&G surveys, leading to abortions, calf abandonment, and up to 25% greater stock declines beyond those predicted from DWH oil exposure. Uncertainty in our results emphasizes a need for further controlled exposure experiments to generate behavioral disturbance dose-response curves and detailed evaluation of individual resilience following disturbance events. Given our focus on a limited suite of threats and need for field verification of these modeled impacts, precautionary management application of our results is recommended for this endangered species.

Signals from the deep: Spatial and temporal acoustic occurrence of beaked whales off western Ireland (PDF)

Kowarski, K., J. Delarue, B. Martin, J. O’Brien, R. Meade, O.Ó. Cadhla, and S. Berrow

PLoS ONE 13: e0199431 (2018)

DOI: 10.1371/journal.pone.0199431

Little is known of the spatio-temporal occurrence of beaked whales off western Ireland, limiting the ability of Regulators to implement appropriate management and conservation measures. To address this knowledge gap, static acoustic monitoring was carried out using eight fixed bottom-mounted autonomous acoustic recorders: four from May to December 2015 on Ireland’s northern slope and four from March to November 2016 on the western and southern slopes. Recorders ran for 205 to 230 days, resulting in 4.09 TB of data sampled at 250 kHz which could capture beaked whale acoustic signals. Zero-crossing-based automated detectors identified beaked whale clicks. A sample of detections was manually validated to evaluate and optimize detector performance. Analysis confirmed the occurrence of Sowerby’s and Cuvier’s beaked whales and Northern bottlenose whales. Northern bottlenose whale clicks occurred in late summer and autumn, but were too few to allow further analysis. Cuvier’s and Sowerby’s clicks occurred at all stations throughout the monitoring period. There was a significant effect of month and station (latitude) on the mean daily number of click detections for both species. Cuvier’s clicks were more abundant at lower latitudes while Sowerby’s were greater at higher latitudes, particularly in the spring, suggesting a spatial segregation between species, possibly driven by prey preference. Cuvier’s occurrence increased in late autumn 2015 off northwest Porcupine Bank, a region of higher relative occurrence for each species. Seismic airgun shots, with daily sound exposure levels as high as 175 dB re 1 μPa^2·s, did not appear to impact the mean daily number of Cuvier’s or Sowerby’s beaked whale click detections. This work provides insight into the significance of Irish waters for beaked whales and highlights the importance of using acoustics for beaked whale monitoring.

Singing through winter nights: Seasonal and diel occurrence of humpback whale (Megaptera novaeangliae) calls in and around the Gully MPA, offshore eastern Canada (PDF)

Kowarski, K., C. Evers, H. Moors-Murphy, B. Martin, and S.L. Denes

Marine Mammal Science 34: 169-189 (2018)

DOI: 10.1111/mms.12447

Humpback whale use of areas off eastern Canada is poorly understood, a knowledge gap that could impact future conservation efforts. We describe the acoustic occurrence of humpback whales in and around the Gully Marine Protected Area (MPA), an eastern Scotian Shelf submarine canyon. Near-continuous acoustic recordings sampling at 16 kHz were collected from the MPA and nearby slope areas from October 2012 to September 2014 using near-bottom recorders. In an offshore region where humpbacks were thought to be rare, we observed calls from October to June with a peak in song and nonsong calls in December and January. This suggests that some individuals occur in Canadian waters in winter and the Gully region may be a North Atlantic humpback whale migratory corridor. Calls were predominantly songs indicating potential mating activities. Song and nonsong calls occurred more at sunset and during hours of darkness than during daylight. This study improves our understanding of the seasonal occurrence of humpback whales on the Scotian Slope and, more specifically, their use of an offshore protected area.

Underwater noise from pile driving of conductor casing at a deep-water oil platform (PDF)

MacGillivray, A.

Journal of the Acoustical Society of America 143: 450-459 (2018)

DOI: 10.1121/1.5021554

Underwater noise from impact pile driving of 512 -m-long conductor casings was measured at a deep-water offshore oil platform in the Santa Barbara Channel. Beamforming measurements, obtained with a vertical array, confirmed that the primary wave front generated by hammering the conductor casing was a Mach cone propagating at an angle of 17.6° below the horizontal. Analysis of the processed array data also revealed the presence of high-frequency secondary waves at angles steeper than 45° below the horizontal. These secondary waves, which appeared to be generated near the sea-surface, dominated the acoustic spectrum of the pulses at frequencies above 1 kHz. Shallow hydrophone measurements outside the Mach cone showed clear evidence of a surface shadow zone, which was caused by the strong downward directivity of the source. Although reflected waves, diffraction, and secondary waves still produced sound inside the surface shadow zone, sound levels were 10–15 dB lower in this region. Long-term hydrophone measurements showed that there was little difference (±1 dB) in mean sound levels from impact hammering of different conductors installed at the same platform over three months.

Unknown beaked whale echolocation signals recorded off eastern New Zealand (PDF)

Giorli, G., K.T. Goetz, T. Kimberly, J. Delarue, E. Maxner, K.A. Kowarski, S. Bruce Martin, and C. McPherson

Journal of the Acoustical Society of America 143:EL285-EL291 (2018)

DOI: 10.1121/1.5032127

The echolocation signals of most beaked whale species are still unknown. In fact, out of the 22 species comprising the family Ziphiidae, only the echolocation pulses for 7 species have been clearly described. This study describes two distinct beaked whale echolocation signals recorded in the Cook Strait region using passive acoustic technology. These signals differ from previously described Ziphiid species clicks. A description of the time-frequency characteristics of the two signals is provided. Understanding the characteristics of these signals is necessary to correctly identify species from their echolocation signals and enables future monitoring of beaked whales using passive acoustics techniques.

Using a stationary compact array of acoustic sensors to estimate the motion parameters of sources (PDF)

Urazghildiiev, I.R. and D.E. Hannay

IEEE J. Ocean. Eng. 43: 1134–1142 (2018)

DOI: 10.1109/JOE.2017.2772984

Compact arrays of acoustic sensors can provide bearing measurements of detected sounds. An important application for compact arrays is estimating the motion parameters of sources. This work considers the problem of bearings-only estimating of the positions and heading angles of vessels in the presence of bearing and speed measurement errors. Statistical simulations are used to evaluate the dependence of bearing estimation accuracy on the duration of the observation interval and the variance of speed errors. Plots of range and heading estimation errors obtained for 171 vessel passes with known positions are presented. Test results demonstrate that the accuracy of the estimating range depends strongly on speed error variance.

Bowhead whale localization using time-difference-of-arrival data from asynchronous recorders (PDF)

Warner, G.A., S.E. Dosso, and D.E. Hannay

Journal of the Acoustical Society of America 141: 1921-1935 (2017)

DOI: 10.1121/1.4978438

This paper estimates bowhead whale locations and uncertainties using nonlinear Bayesian inversion of the time-difference-of-arrival (TDOA) of low-frequency whale calls recorded on onmi-directional asynchronous recorders in the shallow waters of the northeastern Chukchi Sea, Alaska. A Y-shaped cluster of seven autonomous ocean-bottom hydrophones, separated by 0.5–9.2 km, was deployed for several months over which time their clocks drifted out of synchronization. Hundreds of recorded whale calls are manually associated between recorders. The TDOA between hydrophone pairs are calculated from filtered waveform cross correlations and depend on the whale locations, hydrophone locations, relative recorder clock offsets, and effective waveguide sound speed. A nonlinear Bayesian inversion estimates all of these parameters and their uncertainties as well as data error statistics. The problem is highly nonlinear and a linearized inversion did not produce physically realistic results. Whale location uncertainties from nonlinear inversion can be low enough to allow accurate tracking of migrating whales that vocalize repeatedly over several minutes. Estimates of clock drift rates are obtained from inversions of TDOA data over two weeks and agree with corresponding estimates obtained from long-time averaged ambient noise cross correlations. The inversion is suitable for application to large data sets of manually or automatically detected whale calls.

Characteristics of seismic survey pulses and the ambient soundscape in Baffin Bay and Melville Bay, West Greenland (PDF)

Martin, S.B., M.-N.R. Matthews, J.T. MacDonnell, and K. Bröker

Journal of the Acoustical Society of America 142: 3331-3346 (2017)

DOI: 10.1121/1.5014049

In 2012 a seismic survey campaign involving four vessels was conducted in Baffin Bay, West Greenland. Long-distance (150 km) pre-survey acoustic modeling was performed in accordance with regulatory requirements. Four acoustic recorders, three with hydrophones at 100, 200, and 400 m depths, measured ambient and anthropogenic sound during the survey. Additional recordings without the surveys were made from September 2013 to September 2014. The results show that (1) the soundscape of Baffin Bay is typical for open ocean environments and Melville Bay's soundscape is dominated by glacial ice noise; (2) there are distinct multipath arrivals of seismic pulses 40 km from the array; (3) seismic sound levels vary little as a function of depth; (4) high fidelity pre-survey acoustic propagation modeling produced reliable results; (5) the daily SEL did not exceed regulatory thresholds and were different using Southall, Bowles, Ellison, Finneran, Gentry, Greene, Kastak, Ketten, Miller, Nachtigall, Richardson, Thomas, and Tyack [(2007) Aquat. Mamm. 33, 411–521] or NOAA weightings [National Marine Fisheries Service (2016). NOAA Technical Memorandum NMFS-OPR-55, p. 178]; (6) fluctuations of SPL with range were better described by additive models than linear regression; and (7) the survey increased the 1-min SPL by 28 dB, with most of the energy below 100 Hz; energy in the 16 000 Hz octave band was 20 dB above the ambient background 6 km from the source.