1. A method of classifying a spectro-temporal interval of an input audio signal (x(t)) comprising: first modelling ( 62. .. 71 ) said spectro-temporal interval of said input audio signal according to a perceptual model to provide a first representation (Rep 1 ); second modelling ( 62. .. 71 ) said spectro-temporal interval using a modified noise substituted input signal according to said perceptual model to provide a second representation (Rep 2 ); and classifying ( 52 ) said spectro-temporal interval of said audio signal as being noise or not based on a comparison of said first and second representations.