Underwater Acoustics

Underwater Acoustics

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My Communications coursework will be on non-radio communications. My
chosen topic is underwater acoustics. The applications of underwater
acoustics and their advantages and disadvantages will be studied.

All forms of non-radio communications are based on waves. Waves are
generally a disturbance in a surface, transferring energy from A to B.
Waves can be mechanical vibrations travel through a medium. For
example: water, sound. These waves are called mechanical waves.
Progressive waves are created from a point and energy is distributed
to the surroundings. For example: dropping a pebble in the middle of a
pond causes energy to be distributed outwards. All waves can be
classed into two categories:

* Transverse waves: In Transverse waves the direction of the
particle movement is perpendicular to the direction of the wave.

[IMAGE]

[IMAGE]


* Longitudinal waves: The particles in longitudinal waves travel in
the same direction as the direction of the wave.

[IMAGE]


[IMAGE][IMAGE]Waves that can travel underwater without getting too
distorted are used for comunicating underwater. Sound waves fill this
criteria as they can travel long distances without getting distored
too much. Sound waves are longitudnal and mechanical waves. They are
longitudinal because when they travel they create an area of
compression and then rarefractions within the air. A sound wave, like
any other wave is introduced into a medium by a vibrating object. The
motion of the particles in the medium in which a sound wave vibrates
back and forth is measured by the frequency. The frequency of a wave
is measured as the number of complete back-and-forth vibrations of a
particle of the medium per second. Unit of frequency is Hertz (Hz).
The frequency of a wave can be altered by increasing the number of
vibrations per second. [IMAGE]Increasing the frequency, increases the
pitch of the wave. Any sound that can heard by a human ear is called
an infrasound (20Hz to 20000Hz). Above this range the sound is known

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as ultrasound. Ultrasound is used to communicate underwater because
its can travel long distance without a lot of distortion.

Water is an ideal medium for the transmission of sound. Speed of sound
in air is about 330m/s, but in water the speed is 1500m/s which is 4.5
times faster that in air. This means using sound as a communication
tool underwater is better than using it in air. The speed can the
altered by the effect of tempertaure, pressure and the salinity of the
water. Increasing the temperature by 1oC, increases the speed by 4m/s
in water. The deeper a sound wave tavel into the water the faster it
goes because of the increase in pressure. A wave traveling a kilometer
deeper will be travelling about 17m/s faster. Salinity of water is the
total amount of salt dissolved in seawater; the units most often used
are parts per thousand (ppt.) or ‘psu’. An average salinity value for
seawater is 35ppt. Increasing the salinity by 1 psu increases the
speed by 1.4m/s. So the speed of sound can be altered to suit the
needs.

A sound wave doesn’t stop when it reaches an obstacle. It has some
very useful properties like reflection, diffraction and transmission
through a medium.

When a sound wave is reflected of a surface, it ‘bounces’ of it and
changes direction. The angle of incidence (i) is equal to the angle of
reflection (r).

[IMAGE]


So when a sound waves hits the sea bed or hits an obstacle in the sea
it will reflect of it. But this is only for flat surfaces. The waves
behave differently when the hit circular surfaces. When the waves hit
a circular object the reflect into a center focal point. So two waves
hiting a semi-circular object at oppostie ends will both reflect toa
focal point at the center of the circle.

[IMAGE]

Waves can also changes their path by diffracting around an obstacle or
when they go through an opening. This can be observed when sound can
be heard around a corner. This property increases a waves ‘reach’.

[IMAGE]

When a wave hits a different medium to what it is already travelling
in, it can either speed up or slow down depending on the medium. Sound
waves speed up when they travel from air to water. But underwater,
waves slow down when then hit shallow water because they move away
from the normal. They travel fastest in deepwater.

There are two important areas where underwater communication uses
acoustics.These are:

* Monitoring marine-life and communication between marine life.

* SONAR.

Its is very important to monitor marine-life in the water for many
different reason. Over fishing can cause a species of fish to become
extint. Also if there is a sudden decrease in a population for certain
species of fishes it can tell us that there is a high amount of
fishing activity going on or there is another predator in the sea.

Most marine life are sound producers and they can be divided into
three different groups; invertebrates, fish and cetaceans.
Invertebrates have three separate branches which include; crustaceans,
mollusks and sea urchins. Crustaceans like crabs produce a click sound
using their claws. These sounds are produced during feeding or just
when the crabs are moving. Mollusks like octopus can produce rasping
noises during feeding and they can also produce a popping noise when
they swim. Sea urchins like crustaceans produce a clicking noise
during movement and feeding due to their moving legs and spine. The
basic sounds produced by an invertebrate are clicking sounds, which
can be classed as a short pulse lasting for a second. These can be
heard in the audible spectrum of 20Hz to 20000Hz. By following the
clicking or the rasping sounds we can monitor the invertebrates’
habitat; where they feed and what kind of conditions they need to
survive.

Fishes emit three types of noises, which are stridulatory,
swim-bladder vibrations and hydrodynamic sounds. Stridulatory are
created when fishes move their fins or gnash their teeth. Normally
these sounds are heard during feeding but they can also be emitted
when a fish is captured or when its frightened. The noises normally
resonate at about 100Hz frequency. So they can be differentiated with
other noises, which might have been transmitted at a different
frequency.

[IMAGE]Many species of fishes have special vibrating muscles near
their swim-bladder, which have a fundamental frequency from 100 to
500Hz. The exact value depends on the species. The physical movement
of a fish through water produces a displacement and a pressure wave.
This wave is not periodic but has similar properties to sound waves,
hence it can be deducted by a hydrophone. This hydrodynamic noise is
generally low frequency and often sounds like seismic disturbance. The
noises can be deducted at the surface and using the findings we can
See the fish activity and also their breeding patterns. During
breeding seasons, more noises will be heard from the sea, telling us
that the population has increased. A fish can also hear a predator
coming. Their hearing is sensitive to frequency about 500 to 600 Hz.
Water currents are received by a series of minute organs scattered
all over the body. These microscopic bundles of hair cells are usually
just under the skin cells.

Cetaceans produce two classes of sound; echo locating clicks and
harmonic whistles of cries. Almost all whales posses the ability of
echo locating clicks and the basic broad-band noise click is less than
0.01 second in duration. The clicks contain a lot of ultrasonic energy
over 100kHz. The clicks are produced by a series of specialized air
chambers within the nasal passage. The repetition of clicks varies
with the target distance. Most of the cetaceans emit high-pitched
whistles or squeals who pitch varies from 1k to 10k Hz. The purpose of
these whistles is to be involved with communication with other members
of the same species and also to state the emotion of the species.

So marine life uses sound waves to communicate with each other and we
can also use sound waves to study the habitat of the marine life. Many
marine mammals rely on sound for communication, navigation, or
detection of predators and prey. Whales may use sound to attract
mates, repel rivals, communicate within a social group or between
groups, navigate, or find food. Presumably, disruption of any of these
biologically important functions could interfere with normal
activities and behavior, thereby affecting the reproductive success of
individuals and the sizes of a populations. Sound, particularly
low-frequency sound, propagates very efficiently underwater; some
human activities could affect quite large areas of the ocean.
Behavioral effects could have serious consequences for populations, if
they involved large-scale effects disruption of migration, feeding,
breeding, or other critical activities. So it is important to monitor
marine life in order to prevent some rare species from extinction and
also to learn about different species.

[IMAGE][IMAGE]One of the biggest applications of underwater acoustics
is the use of sound, navigation and ranging (SONAR). It was developed
for tracking enemy submarines during World War II. A sonar system
consists of a transmitter, transducer, receiver and display.

[IMAGE]An electrical impulse is transmitted out into the water using a
transmitting device. The electrical impulse is then converted into a
sound wave by the transducer and then the wave is sent out to the
bottom of the ocean, to calculate the depth. When the wave reaches the
floor of the ocean, it rebounds and the echo strikes the transducer
again. A transducer can convert and electrical impulse to a sound wave
vice versa. When the transducer receives the rebounded signal, it
converts it into an electrical signal again. This signal is then
amplified by the receiver and sent to the display. The time lapse can
be calculated since the speed of sound is constant underwater. Since
this happens many times per second, a continuous line is drawn across
the display, showing the bottom signal. In addition, echoes returned
from any object in the water between the surface and bottom are also
displayed. By knowing the speed of sound through water (4800 feet per
second) and the time it takes for the echo to be received, the unit
can show the depth of the water and any fish in the water.

There are four factors to a good sonar unit; High power transmitter,
efficient transducer, sensitive receiver and high resolution/contrast
display. High transmitter power increases the chance of a good echo in
deep water. The transducer must not only be able to withstand the high
power from the transmitter, but it also has to convert the electrical
power into sound energy with little loss in signal strength. At the
other extreme, it has to be able to detect the smallest of echoes
returning from deep water. The receiver also has an extremely wide
range of signals it has to deal with. It must dampen the extremely
high transmit signal and amplify the small signals returning from the
transducer. The display must have high resolution and good contrast to
be able to show all of the detail crisply and clearly.[IMAGE]

Typically, a 50 kHz sonar can penetrate water to deeper depths than
higher frequencies. This is due to water's natural ability to absorb
sound waves. The rate of absorption is greater for higher frequency
sound than it is for lower frequencies. For shallow water, higher
frequencies are used.

The type of water also affects its operation to a large degree. Sound
waves travel easily in a clear freshwater environment. In salt water
however, sound is absorbed and reflected by suspended material in the
water.

Sonar is also used for underwater communication between submarines.
The same method of transmitter, transducer, receiver and display are
used. An electrical signal from a submarine is converted to a sound
wave and then sent out to another submarine. This wave is received and
then converted back into an electrical signal. The signal is then
displayed as a message.

[IMAGE]During the war allies communicated with each other underwater
using sonar. Submarines can be found using passive acoustics or active
acoustics. In passive acoustics, submarines can be detected by the
sounds that they make. These sounds travel through the water for great
distances. Receivers, in form of hydrophones, are placed on the floor
of the ocean in order to pick up any noises made by enemy submarines.
The hydrophones are connected to shore stations where the acoustic
data are analyzed. Submarines themselves are equipped with passive
sonar systems that are used to detect and determine the relative
position of underwater acoustic sources. Actives acoustics is using
the method that is used to find fishes in water. By transmitting a
sound pulse, they can determine the direction of the echoes that
return from objects hit by the sound. They can also measure the time
it takes for echoes to return and calculate the distance to the object
causing the echo.

The advantages of using sonar are that things underwater can be seen
very clearly. The depth of the ocean can be known accurately allowing
us to learn more about the earth. The disadvantage is that the signal
can get distorted as it has to travel long distances.

Acoustic systems have several advantages and disadvantages. The
advantages are:

* Low Power

* Useful bandwidth for many applications (compressed video, sonar,
data telemetry)

* Useful range

and the disadvantages are:

* The sea is acoustically noisy- engines, active sonar systems and
even marine life are all potential interferers

* The sea is very reverberant, in shallow water an underwater
'handclap' could still be audible 0.5 seconds later.

* The path travelled by the acoustic wave is not necessarily the
straight line between the source and the receiver- surface and
seabed reflections, as well as diffraction due to temperature
differences, can bend the wave in unexpected and constantly
changing directions.

* If the source or destination are moving (and it is unusual to have
anything at sea which is perfectly still), then the Doppler effect
will 'stretch' or 'shrink' the transmitted signal.

* Acoustic waves travel slower in water than the electromagnetic
waves discussed above, approximately 1500m/s for sound, 3x10^8 m/s
for light, RF, and cable connections

Some of these disadvantages can be removed by using digital signal
processing:

* Digital filtering removes or reduces the unwanted noise signals

* Digital processing can be used to 'ignore' reverberance and echoes

* Array processing can be used to electronically 'steer' the
receiver to point towards the best signal.

* Processing techniques have been developed to calculate and
compensate for significant Doppler effect.

Underwater acoustics are an alternative to radio communications. Sound
waves are used for this form of communication because they travel
really well underwater. DSP has also ensured that the signal is
clearer and there is less loss in signal. An obvious improvement would
be to try to increase the strength of the sound waves. This way they
can reach further without getting too distorted.

References:

Books:

* Computational Ocean Acoustics- Jensen, Kuerman, Porter and Schmidt

* Underwater acoustics- Albers

* Underwater acoustics- R.W.B. Stephens

Websites:

* www.members.aol.com/nicholash/waves/waves.htm

* www.physicsclassroom.com/class/waves

* www.oceanlink.island.net/oinfo/acoustics.html

* www.lowrence.com/tutorials/sonar/sonar_tutorial_02.asp

* http://omp.gso.uri.edu/dosits/people/defense/1.htm

* http://flipper.ncl.ac.uk/intro.html
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