An allpass filter leaves the magnitude spectrum flat but delays different frequencies by different amounts (phase dispersion)

An allpass filter passes every frequency at equal amplitude — its steady-state magnitude response is flat across the whole audio band, which is why it is called ‘allpass’. What it changes is phase: it delays different frequency regions by different amounts, an effect called dispersion. So for a sustained steady tone an allpass is spectrally transparent, but for transients (sharp attacks and decays) the frequency-dependent delay smears the signal in time and gives it a distinct, recognisable colour. A common misconception is that ‘flat frequency response’ means ‘no audible effect’ — the allpass proves otherwise, since phase alone, in the short-term transient region, changes the sound. With a large delay an allpass generates a series of decaying echoes, which is why allpass units are stacked inside artificial reverberators to build up echo density without altering the overall spectral balance.

Examples

Feed a click through an allpass and it becomes a short ‘chirp’ as different frequencies emerge at different times, even though a sustained tone would pass unchanged. Schroeder reverberators chain several allpass stages after the comb filters to thicken the reverb tail.

Assessment

An allpass filter has a perfectly flat magnitude response. Explain why it can still audibly change a snare drum hit but leave a sustained organ note essentially unaltered.