A sub woofer system is constituted of a real time digital signal processing part 5 that includes an A/D converter 5a, a low pass filter block 5b, a delay block 5c, and a D/A converter block 5d, an analog power amplifier 6 and a speaker 7. The length of delay time of the delay block 5c is set so that the length of group delay time of the digital signal processing part 5 may equal an integral multiple of the length of time corresponding to one wavelength of the crossover point frequency between the digital signal processing part 5 and a main speaker 4. The signal processing is performed under the conditions that have been set as such. In this case, a construction is made up wherein the length of a group delay time produced in the digital type real time digital signal processing part can be set to a given value. By this construction, it is possible to provide a sub woofer system which can improve the phase interference between the main speaker and the sub woofer system at the crossover point between the two.