The acoustic power flowing from a monopole source is proportional to the radiation resistance presented to the source. This value is determined by the modal structure and the acoustic impedance of the walls of the chamber. The rotating sound diffuser modulates the modal structure and, with it, the radiation resistance. Comparison of the time average of the radiation resistance as the monopole source location is changed with and without the diffuser rotating indicates the effectiveness of a given diffuser design in removing the dependence of acoustic power output on the source position. The rotating sound diffuser evaluated was a symmetrical biconical surface of revolution rotating at speeds up to 30 rev/min. Fifty percent of the surface area of the diffuser was covered with ribbed glass fiber, the remainder was left uncovered. In order to measure the radiation resistance, an acoustic impedance head was fabricated from a modified loudspeaker. Displacement of the diaphragm was sensed using a non-contacting fiber-optic sensor. The resulting pressure generated in front of the diaphragm was measured by a condenser microphone. Instrumentation normally used for the measurement of complex mechanical impedance was adapted to record the real part of the radiation impedance.