Measurement of acoustic power output in the active control of sound

Abstract

Active sound control can be used to obtain large global reductions in low-frequency harmonic enclosed sound fields. The secondary sources of sound in such applications have typically been adjusted to minimize practical approximations to the total acoustic potential energy in the enclosure. Minimizing the total acoustic power output of the primary and secondary sources has recently been theoretically shown to give very similar results. In this paper, some of the physical consequences of minimizing total power output are discussed, specifically the implications this has on the power output of the secondary sources. In the single channel case, it is found that the acoustic power output of the secondary source is exactly zero when adjusted to minimize the total power output of both sources. A method of adjusting the secondary source to achieve this objective is presented, in which the volume velocity of the secondary source is adjusted to be out of phase with that of the primary source, and the secondary source amplitude is gradually increased. The amplitude is adjusted using measurements of the acoustic power output of the secondary source only. This initially becomes negative as the amplitude is increased, indicating that the secondary is absorbing power, but then increases until it is exactly zero at the amplitude required to minimize the total power output. This method has been verified by experiments in an enclosure in which the acoustic power output of two loudspeakers acting as the primary and secondary sources were monitored, together with the sum of the squared pressures at 32 microphone locations in the enclosure.