In this study, we suggest filter design criteria which make natural sounding artificial reverberation. The design methods for the reverberator are also proposed. We select the objective measures of reverberation for the primary criteria and investigate their meanings. They have been widely studied in architectural acoustics and many objective measures have been proposed. We consider the measures such as reverberation time, early decay time, initial time-delay gap, objective clarity, and R/D ratio, and interpret that they are parameters determining the sound energy decay curve. Introducing 'degree of freedom', we determine the number of independent objective measures which completely determine the curve. In addition, we try to approach the naturalness of reverberation. We define the unnaturalness of reverberation as being of ringing metallic sound. We perform jury tests to find out the conditions of natural reverberation. As a result, we obtain natural reverberation by introducing randomness to the impulse response of reverberator. Moreover, we propose the standard deviation of the impulse response's power spectrum for a measure of unnaturalness of reverberation and find out that there is some relationship between them. We propose design methods for reverberator with given criteria and perform the implementation by FIR filter.
