The receiver of sound can thus infer information not just about the source of sound but also about the environment’s complexity. The spectrum of broadband sound changes, too, as acoustic energy at high frequencies is more readily scattered and absorbed than energy at low frequencies. All of these phenomena depend on the frequency of sound. Sound bends as the ocean is layered with pressure, temperature, and salinity changing as a function of depth, and with freshwater inputs. Sound is scattered by scatterers in the water (such as gas bubbles or fish swim bladders). Some sound may travel through the seafloor and radiate back into the water some distance away. Sounds may be reflected at the sea surface and seafloor. Anthropogenic sources of sound include ships, boats, fish-finding echosounders, oil rigs, gas wells, subsea mines, dredgers, trenchers, pile drivers, naval sonar, seismic surveys, underwater explosions, etc.Īs these sounds travel from their source through the environment, they may follow multiple propagation paths. Biotic sound sources include singing whales, chorusing fishes, feeding urchins, and crackling crustaceans. Natural, geophysical, abiotic sound sources include wind blowing over the ocean surface, rain falling onto the ocean surface, waves breaking on the beach, polar ice breaking under pressure and temperature influences, subsea volcanoes erupting, subsea earthquakes rumbling along the seafloor, etc. These sounds may be grouped by origin: abiotic, biotic, and anthropogenic. Given that sound may propagate over very long ranges with little loss, a myriad of sounds is commonly heard at any one place. Furthermore, sound can be detected from all directions, providing omnidirectional alerting of activities happening in the environment. And while water transports chemicals, chemoreception is most effective over short ranges, where chemical concentration is high. Visual sensing based on sun- or moonlight is limited to the upper few meters of water. This is because water conducts sound very well (i.e., fast and far), while light propagates poorly under water. Whether the topic is the function of humpback whale song, echolocation in wild bottlenose dolphins, the masking of grey whale sounds by ship noise, the role of chorusing in fish spawning behavior, the effects of seismic surveying on benthic organisms, or the capability of an echosounder to track a school of fish, the way in which sound propagates through the ocean affects how we can use sound to study animals, how sound we produce impacts animals, and how animals use sound.Īquatic fauna has evolved to use sound for environmental sensing, navigation, and communication. It is imperative that bioacousticians who work in aquatic environments have a basic understanding of sound propagation under water. It concludes with a few practical examples of modeling propagation loss for whale song and a seismic airgun array. It provides an overview of publicly available sound propagation software (including wavenumber integration and parabolic equation models). This chapter explains Snell’s law, reflection and transmission coefficients, and Lloyd’s mirror. These are needed to develop the most common concepts of sound propagation under water: ray tracing and normal modes. It introduces the concept of the layered ocean, presenting temperature, salinity, and resulting sound speed profiles. The chapter begins by deriving the sonar equation for a number of scenarios, including animal acoustic communication, communication masking by noise, and acoustic surveying of animals. ![]() There are common misconceptions about sound propagation in water, such as “low-frequency sound does not propagate in shallow water,” “over hard seafloors, all sound is reflected, leading to cylindrical spreading,” and “over soft seafloors, sound propagates spherically.” This chapter aims to remove common misconceptions and empowers the reader to comprehend sound propagation phenomena in a range of environments and appreciate the limitations of widely used sound propagation models. It is converted into heat by exciting molecular vibrations. ![]() It is transmitted into the seafloor and partially lost from the water. It scatters off rough surfaces (such as the sea surface and the seafloor) and off reflectors within the water column (e.g., gas bubbles, fish swim bladders, and suspended particles). Rather, it reflects, refracts, and diffracts. Sound does not propagate along straight-line transmission paths. Sound propagation under water is a complex process.
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