1. A speech recognition system comprising: an audio input unit arranged in a terminal; a speech recognizer; an adjustable speech recognition enhancer, separate from and arranged between the audio input unit and the speech recognizer, comprising: a parametrizable pre-filtering unit, a parametrizable dynamic voice level control unit, a parametrizable noise reduction unit, a parametrizable voice level control unit, and a parameter setting unit; wherein the parameters of the parametrizable pre-filtering unit, the parametrizable dynamic voice level control unit, the parametrizable noise reduction unit and the parametrizable voice level control unit are adjusted by the parameter setting unit to the characteristics of the specific audio input unit and/or the characteristics of the specific speech recognizer of the speech recognition system for adapting the audio input unit to the speech recognizer, wherein parameters set by the parameter setting unit are stored in a data storage, and wherein the adjustable speech recognition enhancer is adapted to process speech wave forms only in the time domain.