Underwater acoustics
Hydroacoustics can be used to detect the depth of a water body (bathymetry), as well as the presence or absence, abundance, distribution, size, and behavior of underwater plants[1] and animals.
The next major step in the development of underwater acoustics was made by Daniel Colladon, a Swiss physicist, and Charles Sturm, a French mathematician.
In 1826, on Lake Geneva, they measured the elapsed time between a flash of light and the sound of a submerged ship's bell heard using an underwater listening horn.
The sinking of Titanic in 1912 and the start of World War I provided the impetus for the next wave of progress in underwater acoustics.
[5] The development of both active ASDIC and passive sonar (SOund Navigation And Ranging) proceeded apace during the war, driven by the first large scale deployments of submarines.
In 1919, the first scientific paper on underwater acoustics was published,[6] theoretically describing the refraction of sound waves produced by temperature and salinity gradients in the ocean.
Near the surface of the sea losses can occur in a bubble layer or in ice, while at the bottom sound can penetrate into the sediment and be absorbed.
[9] At high frequency (above about 1 kHz) or when the sea is rough, some of the incident sound is scattered, and this is taken into account by assigning a reflection coefficient whose magnitude is less than one.
At the equator and temperate latitudes in the ocean, the surface temperature is high enough to reverse the pressure effect, such that a sound speed minimum occurs at depth of a few hundred meters.
The EPWI is defined as the magnitude of the intensity of a plane wave of the same RMS pressure as the true acoustic field.
Though acoustic propagation modelling generally predicts a constant received sound level, in practice there are both temporal and spatial fluctuations.
Because of the non-linearity there is a dependence of sound speed on the pressure amplitude so that large changes travel faster than small ones.
For the same numerical value of SPL, the intensity of a plane wave (power per unit area, proportional to mean square sound pressure divided by acoustic impedance) in air is about 202×3600 = 1 440 000 times higher than in water.
Biological sources include cetaceans (especially blue, fin and sperm whales),[31][32] certain types of fish, and snapping shrimp.
[34] Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman.
[36] Bottom loss has been measured as a function of grazing angle for many frequencies in various locations, for example those by the US Marine Geophysical Survey.
These differences include:[38] The lowest audible SPL for a human diver with normal hearing is about 67 dB re 1 μPa, with greatest sensitivity occurring at frequencies around 1 kHz.
[41] Guidelines for exposure of human divers to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre.
Acoustic communications form an active field of research [52][53] with significant challenges to overcome, especially in horizontal, shallow-water channels.
Moreover, the low speed of sound causes multipath propagation to stretch over time delay intervals of tens or hundreds of milliseconds, as well as significant Doppler shifts and spreading.
Often acoustic communication systems are not limited by noise, but by reverberation and time variability beyond the capability of receiver algorithms.
The fidelity of underwater communication links can be greatly improved by the use of hydrophone arrays, which allow processing techniques such as adaptive beamforming and diversity combining.
Unlike most radio signals which are quickly absorbed, sound propagates far underwater and at a rate that can be precisely measured or estimated.
By sending a sound wave ahead of a ship, the theory went, a return echo bouncing off the submerged portion of an iceberg should give early warning of collisions.
[58] Using a refined echo sounder, the German survey ship Meteor made several passes across the South Atlantic from the equator to Antarctica between 1925 and 1927, taking soundings every 5 to 20 miles.
[59] Important contributions to acoustical oceanography have been made by: The earliest and most widespread use of sound and sonar technology to study the properties of the sea is the use of a rainbow echo sounder to measure water depth.
As technology advances, the development of high resolution sonars in the second half of the 20th century made it possible to not just detect underwater objects but to classify them and even image them.
[60][61][62] Due to its excellent propagation properties, underwater sound is used as a tool to aid the study of marine life, from microplankton to the blue whale.
An acoustic transmitter is attached to the fish (sometimes internally) while an array of receivers listen to the information conveyed by the sound wave.
[63] Pistol shrimp create sonoluminescent cavitation bubbles that reach up to 5,000 K (4,700 °C) [64] A neutrino is a fundamental particle that interacts very weakly with other matter.
