a cabinet;

a parting board for parting an interior space of the cabinet into a first chamber and a second chamber;

a speaker unit provided in the first chamber of the cabinet such that a front face of the speaker unit faces an exterior space exterior to the cabinet, the speaker unit being operable to vibrate in accordance with an input electrical signal; and

a negative stiffness generation mechanism provided to the parting board, the negative stiffness generation mechanism being operable to reduce an acoustic stiffness of the second chamber, wherein the acoustic stiffness suppresses vibration of the speaker unit, the negative stiffness generation mechanism including:

a diaphragm provided at a border between the first chamber and the second chamber, the diaphragm being operable to vibrate due to a drive force generated by the vibration of the speaker unit propagated through the first chamber;

at least one suspension for supporting the diaphragm against the parting board; and

a repulsive force generation section for, when the diaphragm vibrates and moves away from an equilibrium position in a vibration direction of the diaphragm toward the first chamber, applying a repulsive force to a front face and a back face of the diaphragm in a direction in which the diaphragm moves away from the equilibrium position toward the first chamber, the repulsive force being different from the drive force, and for, when the diaphragm vibrates and moves away from an equilibrium position in a vibration direction of the diaphragm toward the second chamber, applying a repulsive force to the front face and the back face of the diaphragm in a direction in which the diaphragm moves away from the equilibrium position toward the second chamber, the repulsive force being different from the drive force.