Or sonar device; Device that detects objects under water by means of sonar—by using reflected sound waves. Most boats are equipped with echo sounders to measure the water depth beneath them. An echo sounder consists of a transmitter, which emits an ultrasonic pulse (see ultrasound), and a receiver, which detects the pulse after reflection from the seabed. The time between transmission and receipt of the reflected signal is a measure of the depth of water. Fishing boats also use echo sounders to detect shoals of fish. echo-sounder