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The applications of underwater acoustics and their advantages and disadvantages

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Introduction

Non-radio communications- Underwater Acoustics.

Engineering Systems: Communications ELA012

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.

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

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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.

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Middle

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.

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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’.

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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.

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Conclusion

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

        Sankesh Waghray A361261

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