An isobaric loudspeaker is a loudspeaker in which two or more identical woofers (bass drivers) operate simultaneously, with a common body of enclosed air adjoining one side of each diaphragm. They are most often used to improve low-end frequency response without increasing cabinet size, though at the expense of cost and weight. Isobaric loudspeakers were first introduced by Harry F. Olson in the early 1950s.
As the name implies, the speaker operates via an isobaric process in which the enclosed air remains at roughly constant pressure, but is forced to move by the paired forces of the two speakers.