Isobaric loudspeaker
configuration for bass speakers in a sound system
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.
Design principles
Two identical loudspeakers are coupled to work together as one unit: they are mounted one behind the other in a casing to define a sealed chamber of air in between them. The volume of this "isobaric" chamber is usually chosen to be small for reasons of convenience and to better couple the drivers. In a subwoofer, where the mid-range output is not needed, the optimum arrangement is front to front, i.e. the outer cone faces another outer cone and the drivers are wired out of phase. In isobaric designs, the two drivers are placed either "cone to magnet" and wired in phase with one another or "cone to cone" or "magnet to magnet" and wired out of phase with one another so that their cones move together when driven with an audio signal. The term “isobaric” points to the somewhat erroneous notion that the air pressure in the sealed chamber between the loudspeaker is constant (the "isobaric" condition), when in fact there will be small changes due to the differences in the drivers technical parameters and the air that each is pressurising.
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