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 Sound in the ocean:

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.

 Constant sound speed means no refraction 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 Here z is positive increasing downward

In the case of a constant sound velocity gradient, dc/dz, a sound ray initially follows the arc of a circle whose radius is: Since sound velocity is not constant, sound rays follow curved paths. They are refracted following the relationship given by Snell’s Law:

SOund Frequency and Ranging (SOFAR) channel

SOund Frequency And Ranging (SOFAR) channel

SOFAR float Floats were tracked using a network of hydrophones around the western North Atlantic that was deployed in the 1950s as part of system to determine the splashdown location of missiles from Cape Canaveral. The missiles would release a small explosive charge that detonates once it reaches the depth of the SOFAR channel. From the arrival times of this signal at three or more hydrophones the splashdown point could be located to <1 km accuracy.

From a point source, sound spreads spherically and intensity falls proportional to R 2 Transmission loss 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

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/I ref ) = 20 log ( p/p ref ) Factor of 10 because it is decibels, not bels Factor of 20 because intensity I is proportional to p 2 A 10 dB increase means an order of magnitude increase in sound intensity The reference pressure in water is 1 microPascal

human voice Hz baleen whales Hz

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 Hz: air 5 db/km, water 0.05 db/km Attenuation loss

Some ocean chemical reactions are influenced by sound, by influencing acidity Sound waves exert pressure as they pass through water Low-frequency sound waves cause borate to lose an OH - group and become boric acid. Pressure “squeezes” borate ions [B(OH) 4 - ] into boric acid [B(OH) 3 ], a more compact molecule. This process absorbs energy. “What that essentially means is that the water is squishy,” Tim Duda says. “When you push on it with a sound wave, it gives. Boric acid quickly changes back to borate, replenishing the borate (but absorbing no sound)

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. 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.

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

Strait of Georgia - East Node - 24 Hours - Zooplankton Acoustic Profiler (ZAP) 8 pm 8 am

Strait of Georgia - East Node – 24 hours data from Zooplankton Acoustic Profiler (ZAP) 8 pm 8 am

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

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.

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. Proposed global ATOC system