Measuring the efficiency/acoustic power of a transducer
To measure the efficiency of an acoustic transducer requires 2 basic measurements.  One is the electrical input power which can be measured with standard electrical meters.  The second is the acoustic pressure level (usually called source level) generated by the transducer when excited by the electrical power which is a measure of the acoustic output power. This requires a calibrated hydrophone (a known value of how many volts output per acoustic pressure impinging on the hydrophone) located in the far field of the transducer.  The formula for the expected source level is the following:

 Source Level (dB) = 171.6 +10log(Pin) + DI


The units of source level are dB with respect to 1 micropascal at 1 yard, Pin is in watts and DI is in dB. If the hydrophone distance from the transducer is not 1 yard, the measured pressure must be modified by 20logR where R is the distance in yards.
DI is the directivity index of the transducer which is a measure of the directionality of the acoustic field generated by the transducer.  If the transducer generates an omni-directional pattern (uniform in all directions) then the DI = 0 dB. There are standard formulas for calculating the DI of transducers that can be modeled either as a line or a circle.  In general , the DI is a function of the area of the radiating face of the transducer and the frequency.  Since the transducer will have an efficiency of less than 100%, the measured source level will be less than calculated from the above formula.  If it is 3 dB less, than the transducer efficiency is 50%.  Note that acoustic measurements are not that accurate compared to electrical measurements so it is possible to measure an efficiency of over 100% if the transducer is very efficient.