The isobaric loudspeaker configuration was first introduced by Harry F. Olson in the early 1950s, and refers to systems in which two or more identical woofers (bass drivers) operate simultaneously, with a common body of enclosed air adjoining one side of each diaphragm. In practical applications, they are most often used to improve low-end frequency response without increasing cabinet size, though at the expense of cost and weight.
The name is derived from the term isobar ("equal pressure"), which comes from the Greek word "isobares", meaning "of equal weight".[1] As the word implies, the enclosed air does indeed experience roughly equal pressures from each diaphragm it contacts, but those forces are actually parallel, rather than opposing, so the air is forced to move.