Minimization of acoustic power in free space using quadrupole sound sources

Low-frequency noise is difficult to suppress by passive control using a sound absorbing material. Active noise control is expected to be an effective method against low-frequency noise. Minimizing the total acoustic power emitted by a sound source causes a zero-control power phenomenon. In the zero-control power phenomenon, the absorption and blowout of sound energy around the control sound source (control sound source) are balanced, and the time average of the control power come into zero. Thus, control power suppression may be able to minimize total acoustic power. Previous studies have assumed that the sound source is a monopole sound source, or a piston embedded in an infinite baffle, so there are restrictions on the sound source. In this research, the sound sources are regarded as quadrupole sound source. Numerical computations are performed based on the derived control law to clarify the effect of the position and the rotation angle of the quadrupole on the acoustic power. In addition, the relationship between the wavelength and the suppression effect is examined from a numerical point of view.