Using frequency characteristics determined for individual ones of a plurality of analyzed bands of a predetermined audio frequency range with frequency resolution that becomes finer in order of lowering frequencies of the analyzed bands, a synthesized band is set for each one or for each plurality of the analyzed bands, and then a time-axial response waveform is determined for each of the synthesized bands. The response waveforms of the synthesized bands are then added together to thereby provide a response waveform for the whole of the audio frequency range.