PURPOSE: To maintain a linear face characteristic and optimize composite sound pressure at a long distance by branching an input signal into two by a network for speaker connection, and processing each signal by specified method to add to a woofer and a tweeter.

CONSTITUTION: The input signal is branched into two by the network for connecting a multi-speaker, and the signal of one side is added to the woofer 10 through a constant-delay type LPF14. The signal of another side is added to a subtracter 16 through a constant-delay type all pass filter 18, and the signals to be added to the speaker 10 is substracted to add to a tweeter 12. Equations are made to satisfy respective relations where n is the degree of the LPF14 and filter 18, bi, βi are respective coefficients, P is a scale complex frequency, and HL(n)(P)AL(n)(P) is transfer function. Thus, linear face characteristic is maintained to optimize composite sound pressure at a long distance.