Recently, 5-degrees-of-freedom(DOF) reverberation model was proposed as a method of representing subjective perception of reverberation as objective measures. This model approximates sound energy decay curve by five objective measures widely used in the field of concert hall acoustics, which are reverberation time, early decay time, initial time delay gap, clarity and R/D ratio. It can be used helpfully to reflect subjective perception on the design of a reverberation filter. But a selected 5-DOF reverberation model cannot determine uniquely an impulse response. Because it cannot tell us anything about the frequency characteristics of a filter and the change of slope in it means the change of pulse density or the change of pulse decay rate of an impulse response. Therefore, besides 5-DOF reverberation model, additional constraints are needed for the design of a filter to make natural reverberation. To find the constraints, listening test is necessary because naturalness of reverberation must be judged by human's auditory perception. For listening tests, absolute ranking method and relative comparison method were used and for statistical analysis, analysis of variance and paired comparison method were used. It is suggested that the constraints for a reverberation filter are irregular frequency response in frequency domain, and pulse density increased with time square and sufficient late pulse density of 1000 pulses per second in time domain. The design method of a reverberation filter is also suggested by listening tests, including these three constraints. At first, we design the FIR filter from 5-DOF reverberation model and three additional constraints, and then move each impulse of the filter randomly within certain range to satisfy irregular frequency response.
