Abstract: An echo sounder has a transducer for transmitting and receiving pulses of energy, a rotary arm display and a meter or digital display. The rotary arm is arranged to cause the transducer to transmit a pulse of energy on each occasion it passes a datum position. The motor of the rotary arm display may be switched off while simultaneously energizing an oscillator having an output frequency equal to that of the rotational speed of the motor prior to deenergization.

Abstract: An echo sounder has conventional transducer means for transmitting and receiving pulses of energy and display means for providing a display indicative of the period between the transmission of a pulse of energy and the receipt of an echo pulse. The echo sounder includes suppressor means defining a variable alarm window during which period alone echo pulses are passed to an alarm device. In an embodiment designed for marine applications the display means comprises a rotor arm carrying a first light emitting device connected to the transducer means and a second light emitting device energised during the alarm window. A motor is provided for rotating the rotary arm which co-operates with a scale. An alarm device responds to each echo signal occurring during the variable alarm window.