Thanks John, although useful at times, however FDW is not really want I was after. I was more interested in applying variable smoothing when exporting in order to use the exported IRs in an FIR filter design tool. I tried applying FDW and variable smoothing on a measurement butcould not get the frequency responses to be closely similary at mid to high frequencies, regardless of the FDW values chosen.Not really, fractional octave smoothing is a post processing action on the frequency response, not a characteristic that is embedded in the IR. In principle one could carry out an inverse FFT on a set of smoothed data and export that IR, which would then provide the same frequency response without any smoothing. Alternatively exporting an IR with an FDW applied has a smoothing effect.