PURPOSE: To prevent an output from being changed even when the impedance is switched by connecting two units having different minimum resonance frequency in series or in parallel and selecting the unit with the higher minimum resonance frequency as a passive radiator.

CONSTITUTION: The full range units or the woofer units 13a, 13b different in minimum resonance frequency are connected electrically in series or in parallel and they are selected by changeover switches 16a, 16b so that they are activated at the same time or the unit having the lower minimum resonance frequency is applied with an electric input. Assuming the impedance of the woofers 13a, 13b as 8Ω, when the switches 16a, 16b are thrown to the position 1, the two speakers are connected in parallel, and the impedance is 4Ω. In throwing the switches 16a, 16b to the position 2, only the woofer 13a having the low minimum resonance frequency is driven and the woofer 13b acts like a passive radiator.