An audio signal may be enhanced by adding enhancement signals, such as harmonics and/or sub-harmonics of the signal's bass components. To avoid any unwanted enhancement signals during soft passages or speech, it is proposed to monitor the audio signal for tonal signal components in a frequency range and to only add the enhancement signals if such tonal signals are detected. A tonal detector (3) may comprise sine and cosine generators whose frequency is preferably equal to a dominant frequency of the frequency range.