The invention describes a method of deriving a set of features (S) of an audio input signal (M), which method comprises identifying a number of first-order features (f1, f2, . . . , ff) of the audio input signal (M), generating a number of correlation values (ρ1, ρ2, . . . , ρI) from at least part of the first-order features (f1, f2, . . . , ff), and compiling the set of features (S) for the audio input signal (M) using the correlation values (ρ1, ρ2, . . . , ρI). The invention further describes a method of classifying an audio input signal (M) into a group, and a method of comparing audio input signals (M, M′) to determine a degree of similarity between the audio input signals (M, M′). The invention also describes a system (1) for deriving a set of features (S) of an audio input signal (M), a classifying system (4) for classifying an audio input signal (M) into a group, and a comparison system (5) for comparing audio input signals (M, M′) to determine a degree of similarity between the audio input signals (M, M′).