Humminbird HDR 650 - Manual (Page 5)

How Sonar Works
Sonar technology is based on sound waves. The HDR 650 Digital Depth Sounder
uses sonar to determine depth directly below the transducer. Your HDR 650
Digital Depth Sounder consists of two components: the HDR 650 sonar unit and
the transducer. The sonar unit contains the transmitter and receiver, as well as
the user controls and display. The transducer is mounted beneath the water
surface and converts electrical energy from the transmitter into mechanical
pulses or sound waves. The transducer also receives the reflected sound waves
and converts them back into electrical signals for display on the sonar unit.
Sonar is very fast. A sound wave can travel from the surface to a depth of 240 ft
(70 m) and back again in less than 1/4 of a second. It is unlikely that your boat
can "outrun" this sonar signal.
1 How Sonar Works