An acoustic reproduction range expanding apparatus includes a closed or bass-reflex speaker system which has a low-frequency drive speaker and can reproduce a sound to a predetermined lowest frequeny. A port portion adapter consists of a mounting portion attached to an openng formed in a closed cabinet or a bass-reflex port of a bass-reflex cabinet, and a port portion. The port portion constitutes a Helmholtz's resonator together with the cabinet when the port portion is attached to the cabinet, and has a port shape so that a resonance frequency becomes lower than the lowest frequency. A drive portion adapter consists of an amplifier for driving the low-frequency drive speaker, and eliminates or invalidates an internal impedance inherent to the low-frequency drive speaker. The amplifier generates a negative impedance component in an output impedance.