Isobaric loudspeaker

They are most often used to improve low-end frequency response without increasing cabinet size, though at the expense of cost and weight.

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.

With optimal out of phase designs, distortion is slightly reduced due to the cancellation of suspension and other driver non-linearities.

The reason for the unchanged resonance frequency is simple: the new combined loudspeaker has twice the moving mass compared to the single driver but also half the compliance because of the doubled suspension.

Any non-linear behavior of the speakers affects sound pressure within the chamber, and could give rise to distortion components.

These may also occur when the speaker driven to high levels for an extended period of time and the voice coils of the two drivers dissipate vibration and heat at different levels because of differing air circulation (one driver is exposed to the outside air, and one is fully enclosed in a chamber).