Noise estimate expressions. Three main different approaches can be used to estimate the standard deviation of the noise based on the noised signal. In the first approach the noise added to the speech signal has a unit variance (σ˜ = 1). This is a common procedure. In the second approach, Xxxxxx and Xxxxxxxxx (Xxxxxx and Xxxxxxxxx, 1994) proposed the estimation of the white Gaussian noise standard deviation using the median absolute deviation (MAD) calculated on the first analysis level of the wavelet transform of the noised signal. with MAD (X1) = median (|X1|) σ˜ = MAD(X1) 0.6745
