Abstract: In some application scenarios of feedforward active noise control, the output of the secondary source is transmitted to the reference sensor. This contaminates the quality of reference signal and deteriorates the stability and performance of the active control system. Based on the concept of equivalent secondary path, this paper uses the phase deviation between the equivalent secondary path and the real secondary path to analyse the convergence behavior of the system with acoustic feedback. If the phase deviation exceeds 90 degrees at some frequencies, the control filter is difficult to converge around those frequencies. The noise reduction performance decreases, and the system may become unstable. Simulations and experiments are carried out to evaluate the performances of 4 approaches for solving the acoustic feedback induced problems, which includes the method of using a unidirectional microphone, the method of using the Filtered-u LMS (FuLMS) algorithm, the feedback neutralization method and the online modelling method. Both experiment and simulation results show that all 4 approaches can improve the noise reduction performance and are effective when there is acoustic feedback in the feedforward active control systems, but each approach has its own advantages and disadvantages. The unidirectional method is the most convenient method, but the directivity is poor in low frequency range. The FuLMS algorithm has low computational load; however the convergence is hard to be guaranteed. The feedback neutralization method has the best performance but has weak robustness when the system is time variant. The online modelling method doesn't require an extra filter, but its performance is slightly worse than other solutions due to complicated adjustment parameters.