Page 28 - Spring 2018

Jennifer L. Miksis-Olds
Postal:
School of Marine Science and Ocean Engineering University of New Hampshire 24 Colovos Road Durham, New Hampshire 03824 USA
Email:
j.miksisolds@unh.edu
Bruce Martin
Postal:
JASCO Applied Sciences 32 Troop Avenue - Suite 32 Dartmouth, Nova Scotia B3B 1Z1 Canada
Email:
bruce.martin@jasco.com
Peter L. Tyack
Postal:
Sea Mammal Research Unit Scottish Oceans Institute School of Biology University of St Andrews Fife, St Andrews KY16 8LB Scotland, United Kingdom
Email:
plt@st-andrews.ac.uk
Exploring the Ocean Through Soundscapes
Listening to underwater soundscapes helps us understand how
ocean physics and the biology of marine communities are responding to a dynamically changing ocean.
It is a clear afternoon, and you are looking out at the skyline from the highest point within 100 km. From this vantage point, you can see for “miles and miles,” but the only sounds you can hear are the people with you, a few birds, insects, and the wind. Now, if you went to an equivalent point in the ocean to stand on the mid-Atlantic ridge overlooking the ocean’s abyssal plain, you would still have 1,200 m of inky black water above and around you. Listening through a hydro- 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 whale calls, trapped in the deep sound channel, would arrive from thousands
of kilometers away. Every few seconds, the sound channel would also bring you energy pulses from oil and gas seismic surveys arriving from Brazil, Africa, the North Sea, and Newfoundland.
Underwater acoustic research has revealed the amazing physics of how sound propagates in the ocean, primarily motivated by using sound to detect oil and gas under the Earth’s crust or for naval applications. Along the way, we learned that 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 biodiversity, animal density, and ecosystem status and health. This article intro- duces the idea of an underwater soundscape, successes in using the soundscape to understand marine ecology, the modeling of soundscapes, and ocean sound as an essential ocean variable (EOV).
Underwater Soundscapes
A great deal of information related to ocean dynamics and human activities can be gained simply by listening to the ambient-sound field. This acoustic landscape, 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- corder are characterized by our typical engineering measurements such as sound 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.