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Ocean Acoustics

Ocean Acoustics. Ocean is largely opaque to EM radiation … but sound propagates well Consider: propagation speed refraction, reflection scattering, attenuation range Applications. Crowd sourcing whale song analysis http://whale.fm

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Ocean Acoustics

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  1. Ocean Acoustics Ocean is largely opaque to EM radiation … but sound propagates well Consider: propagation speed refraction, reflection scattering, attenuation range Applications Crowd sourcing whale song analysis http://whale.fm Sound in the ocean: http://www.whoi.edu/oceanus/viewTopic.do?o=read&id=83&type=11

  2. In 1826, Jean Daniel Colladon used a church bell under water to prove that sound traveled faster through water than air. In lake Geneva, assisted by Charles-Francois Sturm, a mathematician, the two split apart by 10 miles in boats. One struck the bell underwater and ignited gunpowder above the surface at the same time. The other started a timer at the flash of the gunpowder and stopped the timer when the sound arrived, using a trumpet-like instrument to detect the underwater noise. Despite their simple devices, they calculated the speed of sound underwater at 1435 m/s.

  3. Speed of sound in seawater is about 1500 m s-1 Sound propagates by compression and rarefaction of a compressible medium. Water is not very compressible, so the speed is much higher than in air (340 m s-1) Affected by: Pressure Temperature Salinity  Constant sound speedmeans no refraction

  4. Since sound velocity is not constant, sound rays follow curved paths. They are refracted following the relationship given by Snell’s Law: It can be shown in the case of a constant sound velocity gradient, dc/dz, that the sound ray describes an arc of a circle whose radius is:

  5. SOund Frequency And Ranging (SOFAR) channel RAFOS float

  6. Transmission loss From a point source, sound spreads spherically and intensity falls proportional to R2 Some sound reaches the bottom and is reflected, so the transmission loss is decreased Over very long distances, with much of the energy trapped in the sound channel, the spreading is cylindrical and transmission loss is proportional to R

  7. Sound intensity in decibels • Sound Level intensity is described in non-dimensional units (relative to a reference) on a log scale in decibels • SL = 10 log ( I/Iref ) = 20 log ( p/pref ) • Factor of 10 because it is decibels, not bels • Factor of 20 because intensity I is proportional to p2 • A 10 dB increase means an order of magnitude increase in sound intensity • The reference pressure in water is 1 microPascal

  8. baleen whales 10-50 Hz human voice 80-3000 Hz

  9. Attenuation loss Attenuation is the combination of absorption and scattering. Absorption: due to molecular properties of water and increases with square of the frequency Scattering: lack of homogeneity of the ocean – temperature microstructure, air bubbles, plankton, particulates … Essentially all attenuation is by absorption at frequencies > 500 Hz For comparison, attenuation of 10 kHz sound in moist air is about 160 dB/km; in the ocean the attenuation loss for the same frequency is about 1 dB/km. 1000 Hz: air 5 db/km, water 0.05 db/km

  10. Air is more compressible than water, and air bubbles have natural resonant frequencies dependent on their size. Fish have air-filled sacs called swim bladders that are roughly ellipsoidal in shape and which can present a resonant target to sonar. Normalized acoustical cross-section for an air bubble in water, and for a rigid sphere of the same radius, R. The peak corresponds to a natural resonant frequency of 52 kHz.

  11. The differences in swim bladders cause differences in the return echo of a sonar signal. Echo signatures for specific species can be determined and used to identify fish. Fish finder display

  12. Strait of Georgia - East Node - 24 Hours - Zooplankton Acoustic Profiler (ZAP) 8 pm 8 am http://venus.uvic.ca/index.php

  13. Strait of Georgia - East Node – 24 hours data from Zooplankton Acoustic Profiler (ZAP) 8 pm 8 am http://www.venus.uvic.ca/?page_id=43

  14. http://www.stripertracker.org Acoustic transmitters (tags) for this project produce sound at 67 KHz (above audible range for humans) Acoustic tags are surgically implanted in striped bass. Have a wax coating, to prevent immune system from rejecting it. Two-year battery life. Transmitter "pings" an identifying code detectable by hydrophones up to 500 m away. Hydrophones (underwater microphones) are hung from buoys at fixed listening checkpoints, throughout the Mullica River/Great Bay Estuary. Checkpoints are placed mostly at narrow passages, such as Little Egg Inlet, so that fish must pass through them in order to move up or down river, or into or out of the coastal ocean. Environmental data from REMUS AUVs and moored instruments are compared to the striped bass' movement patterns

  15. Acoustic Tomography A network of acoustic sources and receivers has multiple horizontal paths (in addition to multiple rays in sound channel between any given S-R pair). Travel times along the many paths are used to invert for the 3-dimensional sound speed pattern – which can be related to 3-D maps of ocean temperature.

  16. Acoustic Thermometry of Ocean Climate Changes in acoustic travel times over very long distances are used to monitor seasonal and interannual ocean temperature change related to changing climate. http://atoc.ucsd.edu Proposed global ATOC system

  17. 'Brinicle' ice finger of death filmed in Antarctic http://www.bbc.co.uk/nature/15835014

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