The automatic switching of acoustic systems and especially of hearing aids into different processing programs is to be improved. To this end there is provision for the signal processing device of the acoustic system to feature a high-frequency detector for analysis of an input signal, with the output signal of said device being able to be used for processing one or more input signals by the signal processing unit. Specifically the presence of an active mobile telephone which emits electromagnetic radiation in the high-frequency range can be established in this way. This enables the system to be switched into telephone made with a high degree of certainty to suit a particular situation.