Audio decoder for decoding a bitstream so as to produce therefrom an audio output signal, the bitstream comprising at least an active phase followed by at least an inactive phase, wherein the bitstream has encoded therein at least a silence insertion descriptor frame which describes a spectrum of a background noise, the audio decoder comprising: a silence insertion descriptor decoder configured to decode the silence insertion descriptor frame so as to reconstruct the spectrum of the background noise; a decoding device configured to reconstruct the audio output signal from the bitstream during the active phase; a spectral converter configured to determine a spectrum of the audio output signal; a noise estimator device configured to determine a first spectrum of the noise of the audio output signal based on the spectrum of the audio output signal provided by the spectral converter, wherein the first spectrum of the noise of the audio output signal has a higher spectral resolution than the spectrum of the background noise; a resolution converter configured to establish a second spectrum of the noise of the audio output signal based on the first spectrum of the noise of the audio output signal, wherein the second spectrum of the noise of the audio output signal has a same spectral resolution as the spectrum of the background noise; a comfort noise spectrum estimation device having a scaling factor computing device configured to compute scaling factors for a spectrum for a comfort noise based on the spectrum of the background noise as provided by the silence insertion descriptor decoder and based on the second spectrum of the noise of the audio output signal as provided by the resolution converter and having a comfort noise spectrum generator configured to compute the spectrum for a comfort noise based on the scaling factors; and a comfort noise generator configured to produce the comfort noise during the inactive phase based on the spectrum for the comfort noise. a silence insertion descriptor decoder configured to decode the silence insertion descriptor frame so as to reconstruct the spectrum of the background noise; a decoding device configured to reconstruct the audio output signal from the bitstream during the active phase; a spectral converter configured to determine a spectrum of the audio output signal; a noise estimator device configured to determine a first spectrum of the noise of the audio output signal based on the spectrum of the audio output signal provided by the spectral converter, wherein the first spectrum of the noise of the audio output signal has a higher spectral resolution than the spectrum of the background noise; a resolution converter configured to establish a second spectrum of the noise of the audio output signal based on the first spectrum of the noise of the audio output signal, wherein the second spectrum of the noise of the audio output signal has a same spectral resolution as the spectrum of the background noise; a comfort noise spectrum estimation device having a scaling factor computing device configured to compute scaling factors for a spectrum for a comfort noise based on the spectrum of the background noise as provided by the silence insertion descriptor decoder and based on the second spectrum of the noise of the audio output signal as provided by the resolution converter and having a comfort noise spectrum generator configured to compute the spectrum for a comfort noise based on the scaling factors; and a comfort noise generator configured to produce the comfort noise during the inactive phase based on the spectrum for the comfort noise.