Actively created quiet zones by parametric loudspeaker as control sources in the sound field

In active noise control system, the omnidirectional loudspeakers are utilized as the control source to suppress the noise in the targeted area. However, the sound pressure levels of noise remote from the targeted area in the sound field may actually increase. Based on the parametric theory, the parametric loudspeaker generates highly directional audible sound due to the nonlinear interaction of ultrasound waves with neighboring frequencies. As a result of the high directivity, the parametric loudspeakers can be used as the control source to reduce the sound pressure levels at the control points without noise augment in other areas. In this study, the application of the parametric loudspeaker as a secondary source in active noise control system is discussed. The power outputs of both the primary and secondary sources are analyzed by the impedance-based approach. It is found that when the total power output of the two sources is minimized, the secondary source absorbs the acoustic power of the primary source because of the weak coupling between the two sources. Under such circumstances the minimization of the power output of the primary source is the same as the maximization of the power absorption of the secondary source. Furthermore, the quiet zone generated by the control system in a pure tone sound field is examined as well. Results from computer simulation and experimental measurements show that there is a quiet zone along the axial direction of the secondary source, when the two sources are arranged in a line. There is negligible increase of sound pressure in the sound field and the total acoustic power is reduced after control. In addition, the disadvantages of the exploitation of the parametric loudspeaker as the control source are discussed as well.