1. A method for tissue differentiation, comprising: (a) using M sound transducers to obtain 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 s ij (t), i=1 to M, j=1 to N, wherein N is an integer that is at least 2; (c) generating K images I 1 to I k ; where K≦N, using the signals S ij (t), i=1 to M, j=1 to N, wherein K is an integer that is at least 2; (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 , wherein each category corresponds to a different tissue type; 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 .