&gt; The transducers used appear to be of the piezo-electric type, those have a fairly high internal resistance and the voltages produced by those devices would be relatively high, the currents correspondingly tiny.<p>Exactly. These transducers look just like low-cost piezos used for ultrasonic ranging in air (e.g. car parking sensors, alarms, etc.). I have not seen any in that package that would be made for any significant amount of power. High power transducers look quite different.