Method for reducing noise in speech signal and method for detecting noise domain.

A noise reducing method for speech signals is provided in which the probability of speech occurring is calculated by spectral subtraction of subtracting the estimated noise spectrum from the spectrum of the input signal, and the maximum likelihood filter is adaptively controlled based upon the calculated speech occurrence probability. Adjustment to an optimum suppression factor may be achieved depending on the SNR of the input speech signal, so that is it unnecessary for the user to effect adjustment prior to practical application. In addition, a method for detecting the noise domain is provided in which the value th employed for finding the threshold value Th1 for noise domain discrimination is calculated using the RMS value of the current frame or the value th of the previous frame multiplied by the coefficient α, whichever is smaller, and the coefficient α is changed over depending on the RMS value of the current frame. Noise domain discrimination by an optimum threshold value responsive to the input signal may be achieved without producing mistaken judgment even on the occasion of noise level fluctuations.