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.

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