Ocean Noise Levels Research Articles

The effect of ocean fronts on acoustic wave propagation in the Celtic Sea

Underwater noise is now classed as pollution in accordance with the Marine Strategy Framework Directive. Noise from shipping is a major contributor to the ambient noise levels in ocean, particularly at low (<300Hz) frequencies. This paper studies patterns and seasonal variations of underwater noise in the Celtic Sea by using a coupled ocean model (POLCOMS) and an acoustic model (HARCAM) in the year 2010. Two sources of sound are considered: (i) representing a typical large cargo ship and (ii) noise from pile-driving activity. In summer, when the source of sound is on the onshore side of the front, the sound energy is mostly concentrated in the near-bottom layer. In winter, the sound from the same source is distributed more evenly in the vertical. The difference between the sound level in summer and winter at 10m depth is as high as 20dB at a distance of 40km. When the source of sound is on the seaward side of the front, the sound level is nearly uniform in the vertical. The transmission loss is also greater (~ 16 dB) in the summer than in the winter for shallow source while it is up to ~20dB for deep source at 30km.

Decreasing the radiated acoustic and vibration noise of both prop-driven and buoyancy-driven autonomous underwater vehicles

Our previously published results from decreasing the radiated acoustic and vibration noise of a mid-size, prop-driven autonomous underwater vehicle (AUV) show self noise levels recorded at sea by an AUV-mounted hydrophone array that are at, or below, typical background ocean noise levels across the frequency band above 200 Hz. The remaining noise below 200 Hz is primarily vibration induced. The modifications required to achieve this 20–50 dB reduction in propulsion and steering system noise levels will be reviewed in this talk. In addition, at-sea measurements of the acoustic noise radiated by the large (30 L) buoyancy engine on a 20-ft wing span flying wing autonomous underwater glider are presented. Whereas a prop-driven system operates continuously, the buoyancy-driven propulsion system has a very low duty cycle of a few percent; it is on only for about 3 min during each dive cycle. Onboard self noise from the glider's internal fluid-based roll control system far exceeds that from an aileron-based system. However, the former system provides control authority at or near neutral buoyancy. [Work supported by the Office of Naval Research and BP.]

Modeling Low-Frequency Anthropogenic Noise in the Oceans: A Comparison of Propagation Models

AbstractIncreased human activities in the marine environment in recent years have caused a significant general increase in ocean noise level, raising concerns over its impact on marine life. Anthropogenic noise in the oceans covers a wide range of frequencies down to 10 Hz and can propagate over distances up to several tens of kilometers. Many environmental factors affect the propagation conditions and make it impossible to express the propagation with a simple equation. Mathematical/numerical modeling is required to predict and evaluate the impact of man-made noise on marine life. The study compares transmission loss versus range calculated by a wavenumber integration model, a model based on the parabolic equation (PE), and a ray-tracing model. The models are tested and compared for a number of different scenarios with fluid and elastic bottom layers in range-dependent environments. Each case is analyzed and interpreted at several frequencies from 15 to 250 Hz. The simulation results showed excellent agreement between the PE and the ray result for the transmission loss over a range of 10 km in a relative steep upslope elastic bottom environment and for frequencies down to about 25 Hz. In another test, comparisons over a 50-km upslope continental shelf scenario resulted in good agreement for the fluid bottom for frequencies above 100 Hz and poorer for 50 Hz. Over the elastic high loss boom, the two results showed the same general trend but with differences in details. Overall, the ray-tracing model appears to be an acceptable and efficient choice providing reliable predictions, even at low frequencies, in shallow water.

Wind dependence of shallow water ambient noise in a biologically rich temperate coastal area

The Iroise Marine Natural Park, created in 2007, is the first French natural marine park. This archipelago located in Western Brittany is a shallow water area that comprises 11 islands and hosts a rich variety of marine life, including seaweed fields, benthic organisms, endangered seals, and cetaceans. Three underwater autonomous recorders were moored at 10-m depth and sampled at 32 kHz from June 2011 to November 2011. Here we report on the dependency of shallow water ambient noise level on wind speed in a biologically rich environment. First we extract the ambient noise level in presence of transient sounds produced by benthic organisms by removing instantaneous sound pressure levels higher than a threshold computed using the kurtosis of the raw 10 sec time series. We then show that the ambient noise level allows to extract environmental information such as wind speed and biological rhythms, and that both are explaining 90% of its variance. Dependence of ambient noise and ocean noise level to wind speed at several frequencies are compared to reference work by Wenz (1962) and previous shallow water studies. Finally, we discuss how data assimilation coupling measured ambient noise and environmental parameters can help monitor marine ecosystems.

Acoustic quality of critical habitats for three threatened whale populations

Habitat loss is a leading cause of biodiversity loss in terrestrial ecosystems. For marine species that rely on acoustic cues to navigate, find food or select mates, sound is a key element of their environment. Chronic forms of human-generated ocean noise have the potential to mask communication signals over substantial fractions of their functional areas for substantial fractions of the year, which makes acoustic masking a qualitatively similar stressor to habitat loss. International policy decisions on chronic ocean noise are evolving, which creates an opportunity to advance the scientific foundation of decision-making. We measured ocean noise levels at 12 sites, chosen for current and predicted intensities of anthropogenic activities and importance to three endangered whale species in Canada's Pacific Ocean: fin, humpback and killer whales. Canada includes sound as a key element of resident killer whale critical habitat, but not for other species. In the frequency bands that killer whales use for social communication, noise levels were highest in legally designated killer whale critical habitats. In contrast, noise levels were generally lower in habitats known to be important for baleen whales, but these quieter areas are not yet given special legal protection. These noise levels translate into potentially serious fractions of lost opportunities for acoustic communication. Median noise levels are high enough to reduce the communication spaces for fin, humpback and killer whales under typical (median) conditions by 1, 52 and 62%, respectively, and 30, 94 and 97% under noisy conditions. As countries begin to articulate their policies to protect acoustic attributes of marine habitats under their jurisdiction, we recommend quantifying loss of communication space, but quantitative targets need to be set. We see two ways forward. Managers could specify limits of acceptable change in terms of population-level impacts, which can be modelled through effects from communication masking and/or disturbance on prey intake. Alternatively, managers can specify targets reflecting amount of habitat to protect for each species, adjusting upward to account for habitat effectively lost from chronic ocean noise.

Is the ocean really getting louder?

In 1975 Donald Ross indicated a long term trend of low frequency anthropogenic noise increase of 0.55dB/year between 1958 and 1975. This trend in ocean ambient noise levels due to expansion in global shipping has yielded an increase in the ambient noise floor of the ocean that is anywhere from 6dB to 12dB higher than what it was in 1958 (depending on location). What became known as the “Ross Prediction” did not incorporate other anthropogenic sources of noise such as navigation and communication signals, noise from offshore fossil fuel exploration and extraction, and the noises from other marine industrial enterprises. There is a concern that the increase in ambient noise is masking biologically significant sounds, although the evidence for this is still scarce and somewhat speculative. Meanwhile perhaps 90 percent of the biomass of complex vertebrates has been removed from the ocean since 1850 due to industrialized whaling and fishing operations. This paper examines whether the ocean ambient noise floor may have been significantly higher in 1800 than in the 1958 baseline year of the “Ross Prediction,” and speculates that ambient noise levels may be less of a biological aggravator than the particular characteristics of a noise source.

Noiseonomics: The relationship between ambient noise levels in the sea and global economic trends

In recent years, the topic of noise in the sea and its effects on marine mammals has attracted considerable attention from both the scientific community and the general public. Since marine mammals rely heavily on acoustics as a primary means of communicating, navigating, and foraging in the ocean, any change in their acoustic environment may have an impact on their behavior. Specifically, a growing body of literature suggests that low-frequency, ambient noise levels in the open ocean increased approximately 3.3 dB per decade during the period 1950–2007. Here we show that this increase can be attributed primarily to commercial shipping activity, which in turn, can be linked to global economic growth. As a corollary, we conclude that ambient noise levels can be directly related to global economic conditions. We provide experimental evidence supporting this theory and discuss its implications for predicting future noise levels based on global economic trends.

Study on hull vibration and underwater radiation noise induced by propeller excitation considering shafting alignment

The recent increases in the number, size, speed and horsepower of commercial ships led the ocean ambient noise levels at low frequencies increased. Accordingly, the analysis of ship hull vibration control and noise reduction needs to be paid more attention now. In this paper, a finite element model of bulk carrier including shafting is built. Considering the shafting alignment, the vibration and underwater radiation noise characteristics of hull structure caused by propeller excite force are calculated. Finite element method (FEM) is employed to simulate the vibration response of the hull structure due to the propeller excitation in consideration of fluid-structure interaction. Then the outer surface of the finite element model is treated as a boundary element model. Finite element solutions of the hull surface vibratory velocity are further used as velocity boundary condition of the hull boundary element model for consequent underwater radiated noise calculation with boundary element method (BEM). In addition, the characteristics of transfer force and radiated sound power of hull structure are obtained. Comparing with the numerical results of considering shafting alignment and without, the effect of shafting alignment on propeller-induced ship hull vibration and underwater radiation noise is analyzed.

Low frequency deep ocean ambient noise trend in the Northeast Pacific Ocean

Concern about effects of anthropogenic noise on marine life has stimulated new studies to establish present-day ocean noise levels and compare them to noise levels from previous times. This paper reports on the trend in low-frequency (10-400 Hz) ambient noise levels and presents measurements made using a calibrated multi-element volume array at deep ocean sites in the Northeast Pacific from 1978 to 1986. The experiments provided spectral noise levels as well as horizontal and vertical noise directionality. The data presented here provide evidence that the trend derived from 1960s data extended to around 1980, but has since continued at a lower rate.

Potential causes of increasing low‐frequency ocean noise levels.

A simple expression for the global average ambient noise is derived in terms of parameters representing the characteristics of the sound sources, the number of such sources and the propagation properties of the deep ocean, and used to explore possible causes of increasing low frequency noise in the deep ocean. A 5 dB increase in noise level over 35 years, reported by [Andrew et al., ARLO 3, 65-70 (2002)], is explained by the increase in the total number of ocean-going ships and their average tonnage, with no evidence of any significant change in source level during that time. The theoretical effect of increasing transparency to low frequency sound, which has been proposed as a possible cause of part of this increasing noise [Hester et al., Geophysical Research Letters 35, L19601 (2008)], is less than 1 dB to date. Predicted changes in the 21st century might be positive or negative depending on the relative contributions from the change in pH (which is expected to increase globally averaged noise) and the change in sea surface temperature (expected to decrease noise).

Wind-generated ambient noise in a topographically isolated basin: A pre-industrial era proxy

During the mid-1980s, calibrated measurements of ambient noise and wind speed were made in the Tongue of the Ocean in the Bahamas to quantify the spectra and statistics of wind-generated noise. This deep basin is topographically isolated from the Atlantic Ocean and, therefore, largely acoustically decoupled from the Atlantic Ocean deep sound channel. The quantitative effects of contaminating (non-surface wind-generated) noise sources within the basin were eliminated by careful measurement and robust statistical analysis methodologies. Above 500 Hz, the spectral slopes are approximately -5 dB per octave and independent of wind speed. Below 500 Hz, the ambient noise is no longer a linear function of wind speed. Below 100 Hz and for wind speeds greater than 18.5 knots (kt), the ambient noise is independent of frequency. The minimum observed ambient noise level falls 13 dB below Urick's "light shipping" level at 30 Hz and 2-5 dB below Wenz's sea state zero level through the wind-dominated portion of the spectrum. The basin's geographical isolation and the rigorous measurement and analysis methodologies employed make this two-decade-old data set a reasonable and justified proxy for pre-industrial era ocean noise levels in the 20 Hz to 20 kHz frequency band.

Individual right whales call louder in increased environmental noise

The ability to modify vocalizations to compensate for environmental noise is critical for successful communication in a dynamic acoustic environment. Many marine species rely on sound for vital life functions including communication, navigation and feeding. The impacts of significant increases in ocean noise levels from human activities are a current area of concern for the conservation of marine mammals. Here, we document changes in calling behaviour by individual endangered North Atlantic right whales (Eubalaena glacialis) in increased background noise. Right whales, like several bird and primate species, respond to periods of increased noise by increasing the amplitude of their calls. This behaviour may help maintain the communication range with conspecifics during periods of increased noise. These call modifications have implications for conservation efforts for right whales, affecting both the way whales use sound to communicate and our ability to detect them with passive acoustic monitoring systems.

Trends in low‐frequency deep ocean ambient noise levels: New results from old data.

Concern about effects of anthropogenic sound generated by ships, seismic exploration, naval operations, and ocean acoustic research on marine life has stimulated new studies of marine animal bioacoustics. An underlying issue in assessing the noise impact establishes the present‐day background noise level and compares it to noise levels from previous times. The most generally accepted prediction of noise level trend at low frequencies (below 500 Hz) is due to Ross, who used noise levels from the 1950s and the mid‐1960s to predict an increase of about 3 dB/decade. This paper addresses the issue of the trend in low‐frequency ambient noise levels in the ocean and presents measurements taken in the intermediate years between the mid‐1960s and the present time. The measurements were made using a calibrated multi‐element volume array (MEVA) at deep ocean sites in the Northeast Pacific from 1978 to 1986. The array provided spectral noise levels and horizontal and vertical directionalities. Compared to the 1965 va...

Cook Inlet beluga whale echolocations at the Port of Anchorage expansion.

Ocean noise levels increase as a result of human activities. As a result, the effects of anthropogenic noise on marine mammals are a growing concern. A genetically distinct population of beluga whales in Cook Inlet was listed as endangered under the Endangered Species Act in 2008. NOAA has identified many potential threats to the Cook Inlet beluga whale population, including coastal development and anthropogenic noise. The Port of Anchorage Marine Terminal Redevelopment Project, currently underway in Upper Cook Inlet, involves several types of water construction activities, including pile driving. However, the effect of pile driving noise on beluga whales is unknown. A passive acoustic array of sonobuoys was deployed during 20 days in August and September 2009 to acoustically detect the presence of beluga whales in the vicinity of in‐water pile driving at the Port project site. Beluga whale echolocation clicks were detected on days when the whales were also visually sighted by the on‐site visual observers...

Decreasing the radiated acoustic and vibration noise of a mid-size, prop-driven, autonomous underwater vehicle.

Previously published efforts at decreasing the radiated acoustic and vibration noise of the propulsion system of an Odyssey IIb autonomous underwater vehicle (AUV) manufactured by Bluefin Robotics, Inc. resulted in noise levels recorded by an AUV-mounted hydrophone array that were at or below typical background ocean noise levels across much of the frequency band from 300 Hz to 10 kHz [IEEE J. Ocean. Eng. 30, 179–187]. The modifications required to achieve this 20–50-dB reduction in propulsion noise levels will be reviewed in this talk. Recently, these modifications have been incorporated into the Bluefin 21 AUV at the Scripps Institution of Oceanography. In addition, the stepper motors in the linear actuators used to steer this AUV’s vectored-thrust tail cone in depth and heading have been replaced. At-sea measurements show that the high-level, broadband transients that previously occurred every 2–3 s due to these actuators are no longer visible in the hydrophone data. Eliminating these sources of self-noise allow the vehicle to be used in marine biological studies without vehicle noise disturbing either the acoustic measurements themselves or the habitats under observation. [Work supported by the Office of Naval Research and British Petroleum.]

Ocean acidification and its impact on ocean noise level: An analysis using empirical and physical models of acoustic transmission loss.

Over the next several hundred years, ocean acidification is expected to reduce average ocean pH. It has been reported that, given a 0.3 reduction in pH from 8.1 to 7.8, a 40% reduction in the acoustic absorption coefficient at low frequencies (less than 1 kHz) could result, suggesting a significant increase in ocean noise. Such extrapolations are based on the corresponding percentage change in the absorption loss only and ignore other dominant acoustic energy loss mechanisms, such as seabed attenuation, as well as the effect of ocean waveguide physics and the depth dependence of ocean pH. Presented here is an analysis of the impact of the reduction in ocean pH on ocean noise level using empirical and physical models of acoustic transmission loss. Three ocean acoustic environments are analyzed to elucidate the expected change in ocean noise level as a function of frequency: shallow water, a surface duct, and the deep ocean. Results show a negligible change in ocean noise in the shallow water and surface duct environments for all frequencies. In the deep ocean, the maximum change in ocean noise level occurs at approximately 2 kHz, with no significant change below 800 Hz.

Ocean noise level change in response to ocean acidification.

It has been known for several decades that reduction in ocean pH (increase in acidity) would have an impact on ocean sound content and propagation loss. Net flux of carbon dioxide into the ocean has already reduced pH in the upper ocean, and as a result has reduced absorption in those same waters. This trend will likely continue. Based on forecasts of oceanic pH profiles and sound sources, changes in ocean sound content (ambient noise) can be computed. In general, sound energy loss can be partitioned into intrinsic dissipation (the chemical relaxation absorption process) and dissipation in the seafloor, with the boundary loss being analogous to sub-unity reflectivity at the boundaries of a room or concert hall. Here, expected deep ocean noise level changes in various frequency bands from 0.1 to 10 kHz are presented. A model is used that includes basic propagation physics (including the sound channel), depth variability of absorption, loss of sound energy into seafloor, and distributed acoustic sources. At low frequency, little sound is absorbed, so absorption effects are muted. At higher frequencies, sound does not propagate far, also minimizing absorption effects. In the center of the band, there can be sizable effects.

Historical ambient noise levels in a topographically isolated basin

During the late 1970s and early 1980s, careful measurements of ambient noise were made in the Tongue of the Ocean (TOTO) in the Bahamas. The measurements were made using a vertical array of five omni‐directional hydrophones moored in 2200 m water depth with an average hydrophone depth of 122 m. Data were collected within a 20 Hz ‐ 20 kHz band at ten specific 1/3 octave band frequencies, and then reported in spectrum level as a function of surface wind speed. Records contaminated by anthropogenic sources (e.g., local vessel traffic), biological sources (e.g., marine mammal vocalizations) and other sources (e.g., rain) were eliminated to allow analysis of background ocean ambient noise only. TOTO is a deep basin, topographically isolated from the Atlantic Ocean, and therefore largely acoustically decoupled from the Atlantic Ocean deep sound channel. The basin's geographical isolation and the controlled data collection methodology employed over a four‐year period make this three‐decade‐old data set a potential proxy for preindustrial era ocean noise levels. The environmental setting, data collection methodology, analysis methodology and results are presented and discussed within the context of historical ocean ambient noise levels.

High-resolution repeat bathymetric mapping using a mid-size autonomous underwater vehicle

Shipborne bathymetric surveys south of Santa Barbara, CA indicate that massive slope failures have occurred along the northern flank of the Santa Barbara Basin. This region is seismically active and has the highest sedimentation rates along the California coast due to rapid erosion of the Transverse Ranges. A set of geodetic monitoring systems is being developed to study the character and deformation of the ocean bottom in this region, particularly that of a prominent east-west trending crack aligned with the head scarp of one of the underwater landslides. One component is a high-resolution repeat bathymetric mapping system composed of a multibeam sonar mounted inside a Bluefin 21 AUV. Navigation to the decimeter level is provided by an inertial navigation system and accurate depth sensor, and by a long baseline (LBL) acoustic transponder system previously developed to monitor tectonic plate motions. The quality of the data, particularly that from the high-precision LBL system, is enhanced significantly by the decrease in the radiated acoustic and vibration noise of the AUV to levels at, or below, ocean background noise levels through modifications to the AUV propulsion system. [Work sponsored by British Petroleum and the Office of Naval Research.]

Long-term trends in ocean noise

The effect of ocean noise, particularly from anthropogenic sources such as Navy sonar systems, on marine mammal hearing, behavior, and reproduction is of great concern. However, relatively few studies exist to make a definitive conclusion regarding rising ambient noise levels, due largely to the dearth of information over sufficiently long time periods for a sufficient number of geographical locations. A few studies have estimated that ocean noise is rising at a rate of approximately 3–5 dB per decade at some frequencies in some locations. This study is based upon ambient noise measurements from U.S. Navy sonobuoys deployed by operational units between 1993 and 2004. Data and analysis are presented for three areas of the ocean—the Northwestern Atlantic Ocean, the Greenland-Iceland-United Kingdom (GIUK) gap, and the Mediterranean Sea. Analysis of this data demonstrates a less significant increase, and in some cases, a decrease, in ocean noise for these three ocean areas at frequencies between 50 and 2000 Hz. Ocean noise levels in this study are comparable to measured ocean noise levels reported in the literature during the 1960’s.