Navigation and Seamanship

3.1.7.1. Sound in water

Sound is partly reflected when it passes from one medium to another having different density and elasticity. The amount of reflection depends on the difference between then properties of the materials. Air and water are materials of greatly different density and elasticity and their interface or the layer at which they meet is almost a complete barrier sound. Air bubbles / rocks gives strong echos. Soft, muddy bottoms gives weaker echos. Fish with a swim bladder / air bladder give better echos than fish without, because the difference in density between the water and the gas in the swim bladder is greater than that between water and fish flesh. Regular changes in depth, salinity and temperature affect the propagation of sound.

Echo-sounders transmit a pulse of acoustic energy down towards the seabed and measure the total time taken for it to travel through the water, i.e. the outwards and return journey. If the measured time is one second and it is known that the speed of acoustic waves is 1500 m/s, the depth is obviously (1500 x 1)/2 metres = 750 m. Similarly, the two way travel time of sound wave is 5 seconds, then the depth is 1500 x 5/2 =7500/2=3750 m.

The echo sounder was originally designed to measure depth of water and then used to find the fishes beneath the vessel. By using the same principle as the echosounder, equipments such as SONAR and net sonde were developed. Regarding the use of the echosounder for fish finding, the interpretation of the echo-tracers has to be repeatedly taught and exercised. For example, there is a big difference between echo traces received from a hard bottom (rocks, coral) and a soft bottom (sand, mud). Different types of fish give varying echo traces.