1. An acoustic signal enhancement method comprising the steps of: applying a spectral transformation on a frame derived from an input acoustic signal to generate a spectral representation of the frame; estimating an a posteriori signal-to-noise ratio (SNR) and an a priori SNR of the frame; determining an a priori SNR limit for the frame; limiting the a priori SNR with the a priori SNR limit to generate a final a priori SNR for the frame; determining a spectral gain for the frame according to the a posteriori SNR and the final a priori SNR; and applying the spectral gain on the spectral representation of the frame so as to generate an enhanced spectral representation of the frame; wherein the a priori SNR limit is a function of frequency.