Human's SONAR:
The main component of SONAR equipment is an electroacoustic transducer. The transducer enables two phenomena to occur. It first converts electric energy into acoustic energy. This is done under similar principles as a microphone. After sound waves have been sent into the water and are returned as echoes, the transducer converts into a hydrophone which then listens for the echo and changes the acoustic energy into electric energy. This electric energy is then interpreted to discover information about the object.
The original signal sent into the water enters, as a narrow beam, at a speed of 1500m/s. The sound is then propagated in the ocean by five major ways. The spreading of sound can be disrupted by:
1. Absorption: This occurs in two ways: internal friction and dissociation of dissolved salts into ions. Internal friction depends on the viscosity of the ocean water and involves the conversion of sound energy into heat energy. Whereas dissociation involves the conversion of sound energy into chemical energy because the equilibrium condition of the molecules of salts in the water are shifted. This type of propagation accounts for 90% of absorption.
2. Spreading: This is caused by a drop of intensity with distance. Spreading occurs with light and radio waves also. Spreading can be calculated by the inverse-square law I = I1
r2 Where I is the intensity at a range of r yards, and I1 is the intensity at one yard from the source.3. Scattering: This is basically what the name says it is: the scattering of sound when it comes in contact with suspended matter, marine organisms, or the thermal micro-structure of the ocean. This type of scattering results in a very small loss of sound. Ship's wakes or large schools of fish can produce a high loss by scattering. Another consequence of scattering is that the sound may be sent off in many different directions, called "shadow" regions, where the sound would not usually go.
4. Reflection: Both the surface and bottom of the ocean cause a reflection of sound. Usually any type of reflection is unfavourable because the sound is scattered and can be in variable phases. However, in shallow water, if over-top of sand and rock, (which are very good reflectors), multiple reflections from the surface and bottom channel together causing a good SONAR condition because the shadow zones are eliminated.
5. Refraction: Is an effect of the layers with different sonic velocities. These layers bend a sound toward the lowest velocity. This is another cause of interference to the sound travel.
Along with these five disturbances, temperature and depth also have a large effect; however, pressure and salinity (amount of salt in the water) are usually constant and do not have much effect. Another factor which has a large effect is the strength of a target. This refers to its reflecting power. The target must be stronger than the other signals it may be receiving. These other signals include self-noise which is generated by motion of the ship carrying sonar and will increase as the speed of the ship increases, or reverberation, which is the signals from organisms in the water. Lastly, ambient sea noises affect how well the sonar signals reach the target. Ambient sea noises refer to the machinery on the listening ship or motion through the water.
Despite all these factors, SONAR does work! It is important for the searchers on the ships or submarines to know the approximate distances, ranges and the strength of the returning echoes. The distance the beam travels is calculated using the following equation:
Once the sound waves have made contact with an object (target) they reflect back to the transducer as echoes. The strength of the echo depends on several variables, and can be figured out using the following equation:
Where:
T = is the target strength (this depends on the nature, example: size, location, etc. of the target and thearea surrounding it)* The result is squared because the same loss factor is incurred by returning echoes
