# Underwater Acoustics

My Communications coursework will be on non-radio communications. My
chosen topic is underwater acoustics. The applications of underwater
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 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?. [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.
Low Power
Useful bandwidth for many applications (compressed video, sonar,
data telemetry)
Useful range
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, 3×10^8 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.

Underwater Acoustics 7.7 of 10 on the basis of 3153 Review.