After the use of liquid pressure to measure the depth, it was thought of using the propagation of sound in water.
A special echo sounder was built in Germany, said "a bowler,", in which a small pear-shaped bomb was thrown into the sea, using a tube. The bowler, maintained vertical by a fin, acquired a speed of two meters per second and exploded when hitting the bottom.
Counting the seconds between the launch and the explosion at sea, the depth was obtained by multiplying by two the number of seconds counted.
The results were satisfactory for depths up to 100 meters and low browsing speed, otherwise the reception was delayed and weakened.
Echo sounders were then produced, with which the depth is obtained according to the time an acoustic signal emitted by the sounder takes to come back after being reflected from the bottom. The signal can be produced by an explosion or by mechanical vibration of an transmitter applied to the hull.
However, since the waves propagate in all directions, the sound was subject to severe disturbances requiring the use of very intense sources. The measurements could also be distorted when the waves were reflected from a surface other than the seabed.
Was then invented the emission wave beam, that, like the light of a projector, is directed only toward the bottom, thus arriving at the modern ultrasonic probes, that measure the depth by reflection of vibrations at very high frequencies. The first ones used the piezoelectric properties of quartz, and were followed by magnetostrictive oscillators electrically driven.