a multi - channel reverberation system has a substantially constant multi - channel power gain at audio frequencies and comprises multiple signal inputs , one for each input channel , a number of comb filter networks connected to each signal input , each comb filter network including a feed forward stage , a cross - coupling network cross - coupling the comb filters to increase the reverberation echo density , and multiple signal outputs , one for each output channel . preferably the feed forward stage of each comb filter provides a substantially constant multi - channel power gain at audio frequencies and the cross - coupling matrix is an orthogonal cross - coupling matrix cross - coupling a number of single channel allpass comb filters , positioned immediately before or after the delay lines , to create a multi - channel allpass comb filter with a unitary transfer function matrix at all frequencies .