In an acoustic pulse-echo ranging system, a high impedance piezoelectric transducer element is connected via a transformer to a transmitter, and also directly to a high impedance input of a receiver through a network, in parallel with the transducer element which includes a limiting resistor having a value which is low compared with the impedance of the transducer and a pair of oppositely connected diodes connected across the receiver input, the diodes being in series with the resistor and conducting to complete a damping circuit for large amplitude signals during transmission, and to limit the amplitude of signals applied to the receiver. When the signals are not of sufficient amplitude to cause the diodes to conduct, the damping circuit is normally rendered ineffective, but a further circuit is provided in parallel with the diodes to selectively switch into the network a further smaller resistance which acts both to form an attenuator at the receiver input in conjunction with the damping resistor, and to restore the damping effect. By this means, the dynamic range required by the receiver is much reduced, and effective damping can be applied selectively during reception to reduce the effect of ringing at short ranges. A tuned circuit formed with the transducer by the transformer through which the transmitter drives the transducer is tuned by means of an air gap in the transformer core, which may be adjustable.