A passive subwoofer is a type of speaker designed to reproduce low-frequency sounds, such as bass. Unlike active subwoofers, passive subwoofers do not have a built-in amplifier. Instead, they require an external amplifier to power them, which can be connected to a home theater system or audio setup. These subwoofers are often used in conjunction with other speakers, like satellite speakers, to create a full-range sound experience. They are popular in home audio systems and professional sound setups because they can be customized with different amplifiers to suit specific audio needs.