###### Abstract
The feedback ANC system is also applied to some actual cases in which the primary noise cannot be detected conveniently and the reference signal is not obtained cost-effectively owing to the existence of many noise sources [19]. Additionally, a simplified feedback ANC system was developed in Reference [19]. The simplified feedback ANC system could decrease the computational load, but the numerical stability became weak in this system. The leaky FxLMS algorithm with a leakage factor was applied to strengthen the numerical stability in the simplified feedback ANC system [19]. Considering that the feedforward and feedback structures have their respective advantages, hybrid structures were developed to reduce correlated noise and eliminate uncorrelated narrowband noise [20; 21; 22; 23]. A decoupled hybrid algorithm was proposed by [PERSON] et al. to elevate the performance of the traditional hybrid control algorithm [15]. However, the above-mentioned hybrid control algorithm cannot carry out sound quality control. A traditional hybrid sound quality control algorithm was developed to control the sound quality of line-spectrum noise and reduce the broadband disturbance [10]. The traditional hybrid sound quality control algorithm includes a feedforward structure, a feedback structure, and a line-spectrum noise cancellation control structure. The reference signal in the feedback structure is synthesized by three different signals: the residual error signal, the output signal of the line-spectrum noise cancellation control filter, and the total output signal filtered by the estimated model of the secondary path [15]. It is obvious that the reference signal in the feedback structure depends on the output signals of the three control structures. The residual error signal is also related to the output signals of the three control structures. Therefore, among the three control filters, coupling actions exist that can deteriorate the performance of the whole control system, such as the convergence speed. To decrease the coupling effect of the whole control system and reduce the complexity of the control structure based on the traditional structure of a hybrid sound quality control system, two modified structures for hybrid sound quality control systems are constructed. The two modified structures can not only control the sound quality of the line-spectrum noise, but they can also cancel Gaussian random disturbance at the error sensors. The first one is a simplified structure that removes the output signal of the feedforward control filter from the combination of the reference signal in the feedback control structure [15]. The reference signal of the feedback control structure is replaced by the summation of the residual error signal and the output signal of the line-spectrum noise cancellation control filter. The second one is a modified structure that integrates the output signals of the feedback control filter and the line-spectrum noise cancellation control filter for the reference input signal in the feedback control structure. Unlike Reference [10], the coupling effects of the three controllers are researched, and the computational complexities of the three hybrid structures are presented. The two modified structures, as two alternate structures of the traditional hybrid structure, can alleviate the coupling effect and intuitively decrease the complexity of structure. Furthermore, in order to improve the performance of the leaky FxLMS algorithm in the simplified feedback ANC system, a simplified structure with a new leaky FxLMS algorithm is proposed to upgrade the noise reduction and elevate stability in the feedback control structure.