Exploring the Ocean Through Soundscapes Jennifer L. Miksis-Olds Listening to underwater soundscapes helps us understand how Postal: ocean physics and the biology of marine communities are responding School of Marine Science to a dynamically changing ocean. and Ocean Engineering It is a clear afternoon, and you are looking out at the skyline from the highest University of New Hampshire point within 100 km. From this vantage point, you can see for “miles and miles,” 24 Colovos Road but the only sounds you can hear are the people with you, a few birds, insects, Durham, New Hampshire 03824 and the wind. Now, if you went to an equivalent point in the ocean to stand on USA the mid-Atlantic ridge overlooking the ocean’s abyssal plain, you would still have Email: 1,200 m of inky black water above and around you. Listening through a hydro- [email protected] phone, the sounds you hear would be extraordinarily rich. Crustaceans would be heard scratching at the rock and deepwater corals. Sperm, beaked, and pilot whales would be searching for food using echolocating click trains. Blue and fin Bruce Martin whale calls, trapped in the deep sound channel, would arrive from thousands Postal: of kilometers away. Every few seconds, the sound channel would also bring you JASCO Applied Sciences energy pulses from oil and gas seismic surveys arriving from Brazil, Africa, the 32 Troop Avenue - Suite 32 North Sea, and Newfoundland. Dartmouth, Nova Scotia B3B 1Z1 Underwater acoustic research has revealed the amazing physics of how sound Canada propagates in the ocean, primarily motivated by using sound to detect oil and gas Email: under the Earth’s crust or for naval applications. Along the way, we learned that [email protected] marine life has capitalized on ocean physics and evolved the use of sound as a pri- mary sensory modality for interacting with the environment. We are now listening in on the underwater conversations and using passive acoustics to assess marine Peter L. Tyack biodiversity, animal density, and ecosystem status and health. This article intro- duces the idea of an underwater soundscape, successes in using the soundscape to Postal: understand marine ecology, the modeling of soundscapes, and ocean sound as an Sea Mammal Research Unit essential ocean variable (EOV). Scottish Oceans Institute School of Biology Underwater Soundscapes University of St Andrews A great deal of information related to ocean dynamics and human activities can Fife, St Andrews KY16 8LB be gained simply by listening to the ambient-sound field. This acoustic landscape, Scotland, United Kingdom or soundscape, is the sum of multiple sound sources that all arrive at the location of a receiving animal or acoustic recorder. The sounds measured at an acoustic re- Email: corder are characterized by our typical engineering measurements such as sound [email protected] pressure levels, weighted sound exposure levels (SELs), roughness, and kurtosis. The percept of sounds to marine life depends on the relative contribution of each source, source direction, propagation through the environment, behavioral con- text, hearing capabilities of the listener, and history of the listener with similar sounds (Figure 1). Underwater soundscapes are dynamic; they vary in space and time and within and between habitats. Sound in the deep ocean propagates such great distances underwater that soundscapes are influenced not only by local conditions but also by much more distant sound sources than in air. The underwater soundscape is 26 | Acoustics Today | Spring 2018 | volume 14, issue 1 ©2018 Acoustical Society of America. All rights reserved. from marine mammals, fishes, and invertebrates). In Fig- ure 2, the single-headed arrows show that the soundscape is directly influenced in a single direction by anthropogenic and abiotic factors and the double-headed arrows indicate that the soundscape is not only influenced by but also influ- ences the biological soundscape component. Consequently, the underwater soundscape is not merely a physical param- eter of the environment to be measured and quantified. The soundscape depends on the listener and has a feedback loop where changes in the soundscape have the potential to im- pact acoustic behavior and biotic factors that influence the behavioral ecology of the ecosystem and ultimately further alter the soundscape (Figure 2). Figure 2. Soundscape presented within the context of acoustic ecol- ogy. Green boxes and arrows, natural factors: behavioral ecology, acoustic behavior, and abiotic and biotic factors contributing to (out- Figure 1. Top: soundscape is composed of “sound,” the physi- going arrows) or impacted by (incoming arrows) the soundscape cal measurements of the sound field, and the “scape” that conveys (blue box and arrows); red box and arrows, interactions and in- how all of the sound sources overlap and are perceived by the lis- fluences of human activity related to or impacting the soundscape. Adapted from van Opzeeland and Miksis-Olds (2012, Figure 1). tener White boxes with Si(1-7), signals from sound sources in the environment with different sizes and orientations meant to convey different source types; black boxes, physical and perceptual char- Soundscape analysis is performed on live-data streams or acteristics of the sound signals by the listener. L , sound level in Aeq recordings of received pressure signals from passive acoustic decibels equivalent to the total A-weighted sound energy measured recorders deployed on the ocean bottom or moored in the over a stated period of time. From Jennings and Cain (2013). Bot- tom: graphic representation of the multiple ocean sources contrib- water column. The recordings allow us to observe marine uting to an ocean soundscape. From NOAA’s Ocean Noise Strategy. habitats without the confounding effects of human presence Available at http://acousticstoday.org/nefsc. or sampling biases. Recordings of the full range of ocean sounds have a wide bandwidth (150 kHz or more), last for composed of contributions (Figure 2) from human activity periods of months to years, and often are collected at mul- (e.g., shipping, fishing vessels, seismic airgun surveys), nat- tiple locations that the researchers compare for similarities ural abiotic or geophysical processes (e.g., wind, rain, ice), and differences (listen to real-time soundscapes recorded in and acoustic contributions from biological sources (e.g., different ocean locations at http://www.listentothedeep.com). sound produced from animal movement and vocalizations These datasets are called five-dimensional because they have Spring 2018 | Acoustics Today | 27 Exploring the Ocean Through Soundscapes Figure 3. Five minutes of acoustic data from a seismic vessel and air- gun array passing over a directional hydrophone. Top: sound pressure level time series. Bottom: spectrogram where color indicates direction of arrival shown by the color wheel (yellow, North; blue, South). The sensor was 60 cm from the seabed in 110 m of water. Note that the vessel direction color changes before the impulses from the airgun ar- ray because the array was ~100 m behind the vessel. time, frequency, amplitude, latitude, and longitude. The goal of soundscape analysis is extracting information from the recordings to identify which sources are present, the source amplitudes, how sources interact, and how animals in the en- vironment may perceive and respond to the sounds. In recent years, some research teams have started making directional soundscape recordings that increase the data to six dimen- sions by adding the direction of arrival (Figure 3). Direc- tional pressure sensors in deep water also offer the potential to measure particle motion. Unfortunately, this methodology does not extend to accurate measurements of particle mo- tion near the sea surface, at the seabed, or in shallow water because it is not linearly related to pressure in these regions. Particle motion, as opposed to pressure, is the component of sound sensed by most fishes and marine invertebrates. Its measurement and perception is a subject that needs extensive investigation (Hawkins and Popper, 2017). Passive acoustic monitoring (soundscape) data can be se- Figure 4. Examples of soundscape data presentations using an lectively broken down to gain a greater understanding of 11-month dataset recorded 20 m off the seabed in 1,280 m of water off the sources shaping the temporal, spatial, and spectral pat- Newfoundland, Canada. A: time series of 1-hour band-limited sound terns of the acoustic environment (Mann, 2012; Au and pressure levels (SPL). B: long-term spectral average of the complete Lammers, 2016; e.g., Figure 4). There are a wide variety of dataset. C: daily distributions of 1-minute acoustic complexity index acoustic measures and presentation formats in the marine (ACI) values for the frequency band of 40-200 Hz. Values below 1 indicate lower complexity (i.e., continuous sound sources) and values soundscape literature related to the foci of each study. For above 1 indicate higher complexity (impulsive sound sources). D: dis- example, studies of soundscape patterns and trends tend to tribution of 1-minute power spectral densities including exceedance utilize measures of sound pressure levels (Figure 4, A and percentiles (i.e., 5% of the power spectral density values exceeded the B) and sound level exceedance percentiles (sound level that L5 line). Orange dashed ellipses, presence of seismic survey signals; is exceeded N% of the time during a specified time period; black solid