Echoes...

If you stand in front of a highwall or cliff and shout the sound of your voice may come back to you slightly later,which is an echo.The sound wave travels from your mouth to the wall or cliff,bounce off it and return to your ears.You may hear a lot of echoes in large places like cathedrals and under wide domes.The sound waves bounce around several times.The echoes lengthed the original sound so that it takes times to die away.

Uses...

• When you are in a concert hall,you hear sounds bouncing from the walls and ceiling as well as sounds coming from the stage.These sounds mix together and give the music a good quality of sound.The shape of the hall and the materials on its wall and ceiling greatly affect the quality of the sound.Builders of walls have to study acoustics,the science of sound.

• Sound waves travel faster through hard rocks than through soft rocks.Hard underground rocks also give a different type of echo to soft rocks and pockets of gas or liquid.The shape and density old rock information often give clues about the minerals they contain.Surveyors looking for particular underground rock information often use echoes to find them.This technique is called echo sounding

A similar technique to echo sounding that can be used for underwater surveying.SONAR ,or Sound Navigation Ranging,is used to map the ocean floor and to locate schools of fish or submarines.When locating a moving object,the direction of the movement can be found by studying the frequency of the echo waves.

On the case of the death of John Kennedy Jr,son of John F. Kennedy,his body was located by the National Transportation Safety Board (NTSB).The body was found during the night(21th July) in about 30m of water along with a "significance part of a wreckage" of his single-engine airplane which clashed on 16th July 99.The search for the three bodies had been focused on an area off Martha's Vineyard.Divers searching for wreckage on 20th July honed in on two target-sites within a few hundred metres of where radar information indicated the plane had hit the water.                                                                                                                                              Source:The Straits Times , Singapore (22nd July)