1. A method for tissue differentiation comprising: (a) obtaining M acoustic signals s i (t), i=1 to M, from M locations on a body surface; (b) for each of N frequency bands, and for each of the signals s i (t), i from 1 to M, subjecting the signal s i (t) to band pass filtering using N band-pass filters, so as to generate NXM signals sij(t), i=1 to M, j=1 to N; (c) generating K images I 1 to I K , where K≦N, using the signals sij(t), i=1 to M, j=1 to N; (d) dividing pixels into a predetermined number L of categories C l , l from 1 to L, using the images I 1 to I K ; and (e) for each category C l , l from 1 to L, and for each pixel p(x,y), calculating a probability p l of assigning the pixel p(x,y) to the category C l .