echo    It  is the reflected sound, coming back to the listener after  an  interval ( 0.1 s), so that it is heard distinctly. The phenomenon  also  occurs  in  transmission lines. It causes ghost images in TV. Echoes are used to locate underwater objects in  submarines  and  ships by measuring the time delay between transmitted pulse and the reflected echo . The technique is called sonar (sound navigation ranging). High frequency sound called ultrasound* is used in sonar.