an audio signal is divided among exponentially related subband filters . the spectral flatness measure in each subband signal is determined and the measures are weighted and combined . the sum is compared with a threshold to determine the presence of music or noise . if music is detected , the noise estimation process in the noise reduction circuitry is turned off to avoid distorting the signal . if music is detected , residual echo suppression circuitry is also turned off to avoid inserting comfort noise .