Further Progress with Colorless Artificial Reverberation

Abstract

In a previous paper [J. Acoust. Soc. Am. 32, 1520 (1960)], artificial reverberators having flat amplitude-frequency responses were described. Such “flat” reverberators preserve the amplitude spectrum of the reverberated sound, thus avoiding any undesirable “coloration.” In the present paper, an alternative approach to colorless artificial reverberation is described in which several “combfilters” (delay lines with multiple reflections) connected in parallel are employed to create a subjectively colorless reverberation. Such reverberators do not have a flat frequency response in the physical sense, but are nevertheless subjectively indistinguishable from flat reverberators if the following (sufficient) conditions are met: (1) The number of combfilters must be at least three. (2) Their delays must be substantially different and incommensurate. (3) The average distance (Δf) between adjacent relative maxima of the frequency response of the reverberator must be smaller than about 5 cps. (This condition is equivalent to requiring that the sum total of the round-trip delays employed in the combfilters exceed 200 msec.) (4) For reverberation times T in excess of 2 sec, the product 〈Δf〉⋅T must be smaller than 10. To meet the additional requirement of a flutter-free transient response, the reverberator must furnish at least one echo per msec for times in excess of about 100 msec after the direct sound. Finally, for the realistic simulation of large halls, it is recommended that a delay of the order of 40 msec be introduced between the direct sound and the reverberation. The advantage of the present “comb-filter approach” to artificial reverberation is that it allows greater flexibility in mixing direct and reverberated sound. Also, the delay lines for the combfilters do not have to meet specifications as stringent as those employed in flat reverberators. A tape recording of artificially reverberated organ music will be played.