A decoder for generating an audio output signal comprising one or more audio output channels from a downmix signal comprising a plurality of time-domain downmix samples is provided. The downmix signal encodes two or more audio object signals. The decoder comprises a window-sequence generator (134) for determining a plurality of analysis windows, wherein each of the analysis windows comprises a plurality of time-domain downmix samples of the downmix signal. Each analysis window of the plurality of analysis windows has a window length indicating the number of the time-domain downmix samples of said analysis window. The window-sequence generator (134) is configured to determine the plurality of analysis windows so that the window length of each of the analysis windows depends on a signal property of at least one of the two or more audio object signals. Moreover, the decoder comprises a t/f-analysis module (135) for transforming the plurality of time-domain downmix samples of each analysis window of the plurality of analysis windows from a time-domain to a time-frequency domain depending on the window length of said analysis window, to obtain a transformed downmix. Furthermore, the decoder comprises an un-mixing unit (136) for un-mixing the transformed downmix based on parametric side information on the two or more audio object signals to obtain the audio output signal. Moreover, an encoder is provided.