To form an audio signal (SAS), frequency components (NAS) of the audio signal which are allotted to a first subband are formed by means of a subband decoder (LBD) using supplied fundamental period values (lamda LTP) which respectively indicate a fundamental period for the audio signal. Frequency components (HAS) of the audio signal which are allotted to a second subband are formed by exciting an audio synthesis filter (ASYN) using an excitation signal (u(k)) which is specific to the second subband. To produce this excitation signal (u(k)), an excitation signal generator (HBG) derives a fundamental period parameter (lamda p) from the fundamental period values (lamda LTP).; The fundamental period parameter (lamda p) is used by the excitation signal generator (HBG) to form pulses with a pulse shape which is dependent on the fundamental period parameter (lamda p) at an interval of time which is determined by the fundamental period parameter (lamda p) and to mix them with a noise signal.