I. Introduction
Acoustic noise problems are becoming increasingly ubiquitous in industry and daily life. The methods to acoustic noise control can be broadly grouped into two categories: passive methods and active noise control (ANC). The former applies the acoustic insulation materials to produce a modest attenuation over a broadband frequency range [1], [2]. However, since the low-frequency noise possesses strong penetrating capability, the acoustic insulation is relatively ineffective and costly. By contrast, ANC attempts to introduce the anti-noise wave through an appropriate array of secondary sources to reduce the primary (unwanted) noise levels, based on the principle of superposition [3]–[5]. The ANC systems have been used in many different applications, where the noise fields are dominated by the low frequencies, such as engine noise in automobile and mechanical vibration in manufacturing.