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).
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.
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’.
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.
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.
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.
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.
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.
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:
