An audio system comprises a post-processor (12) arranged to alter successive fragments of a decoded audio signal (14) to provide successive fragments of post-processed audio signal (16). A masking threshold generator (20) provides an estimate of a masking threshold () for successive fragments of post-processed audio signal (16). A noise level generator (17) provides an estimate of a noise level () for successive fragments of the post-processed audio signal (16). A distortion generator (17) determines a degree (D) to which the noise level exceeds the masking threshold for successive fragments of the post-processed audio signal (16). A regulator (18) controls the post-processor according to the degree to which the noise levels exceed the masking threshold.
