A method and a system for defining a threshold value (O.sub.max, O.sub.min, O.sub.TR) serving to limit the output signal of a processing unit which is fed an input signal. An input-signal level is determined and the threshold value (O.sub.max, O.sub.min, O.sub.TR) is set as a function of that input-signal level. By virtue of the fact that the threshold value is set as a function of the input-signal level, i.e. in adaptive fashion, it is also possible to limit transient noise whose level is well below the maximum value of the threshold value. As a result, when the method or system is applied in a hearing aid, the hearing comfort of the wearer of the hearing aid can be significantly enhanced.