1. A method of producing a wide-band acoustic signal (a WB ) based on a narrow-band acoustic signal (a NB ), the spectrum (A WB ) of the wide-band acoustic signal (a WB ) having a larger bandwidth than the spectrum (A NB ) of the narrow-band acoustic signal (a NB ), the method involving extraction of at least one essential attribute (z NB (r, c), E NB ) from the narrow-band acoustic signal (a NB ), and estimation of a parameter describing aspects of wide-band frequency components outside the spectrum (A NB ) of the narrow-band acoustic signal (a NB ) based on at least one essential attribute (z NB (r, c), E NB ), characterised by allocating a parameter value to a particular wide-band frequency component based on a corresponding confidence level.