When an amplitude of vibration of a first diaphragm of a first speaker unit vibrated by applying a first and a correction AC signals having a frequency in a first frequency band to the first speaker unit is equalized to that of a second diaphragm of a second speaker unit vibrated by applying a second AC signal being the same as the first AC signal to the second speaker unit being the same as the first speaker unit attached to a closed type cabinet having the same internal volume as a speaker cabinet, a sound pressure reproduced by the first diaphragm is equal to or lower than that reproduced by the second diaphragm when a third AC signal having a frequency of a minimum resonance frequency or higher of the closed type cabinet determined by the internal volume thereof and the caliber of the second speaker unit is applied to the second speaker unit.
