We have set up and are using an apparatus for the measurement of total sound power output of a piezoelectric transducer radiating into water. This apparatus combines the better features of previously used methods which depend on radiation pressure. The input is modulated at a low frequency and the output power is intercepted by an absorbing target which experiences a force at the modulating frequency. The target is mounted on the armature of an electromagnetic microphone provided with an independent coil through which a current at the modulating frequency is adjusted in amplitude and phase, either manually or automatically by feedback, to arrest the motion of the armature. Under these conditions the force depends only on the current, and the apparatus can be calibrated using direct current and dead weights. It is thus absolute. In practice, the carrier frequency is swept over any part of the range 1–80 MHz while a recording of power output versus frequency is made. Examples of curves from normal and defective transducers will be shown. Other applications, as in medical dosimetry will be discussed.