Isobaric enclosure (Compound enclosure)

Isobaric enclosure (also known as compound enclosure) employs two woofers working synchronously with a small sealed air volume contained between their membranes. A pair of woofers in such a configuratuin may be mounted in another enclosure, for example in a closed box, a bass-refelx enclosure or a bandpass enclosure.

There are three isobaric configurations possible. The drivers may be installed face-to-face (ie. cone-to-cone, fig. A), back-to-back (magnet-to-magnet, fig. B) or front-to-back (cone-to-magnet, fig. C). In the cone to magnet configuration drivers are wired in phase, so that both cones move in sync in the same direction. In the other two configurations drivers are wired out of phase in order to obtain the same result. Regardless of the configuration, drivers are typically wired in parallel so they have half the impedance of a single driver and draw twice the current for a given input voltage.

Because the two drivers move in sync the pressure in the small sealed volume of air between them is supposed to be constant - this is the reason for isobaric (meaning constant pressure) name. It is also claimed that even order distortion is cancelled in cone-to-cone and magnet-to-magnet configurations, like in a push-pull configuration. In reality this is not the case. The pressure in the sealed chamber is changing in tact with the signal. While distortion and pressure are changed in comparison with the operation of a single driver, they are not cancelled, contrary to often encountered claims.

Reduction of enclosure size is the biggest benefit of isobaric/compound configuration. While the Fs and Qts of a compound pair remains the same as for a single driver, the Vas for a compound pair is two times smaller. This is the reason why a substantial reduction of volume is possible - the box size is half of that for a single driver. But the volume of the air in the chamber between the membranes of a compound pair drivers is not a part of the calculated reflex or closed enclosure, so it increases the overall size.

The other practical drawbacks of compund system are increased cabinet complexity and possible problem with fitting a suitable port in a bass-reflex enclosure. Of course there is also the cost of the additional driver.

Well known isobaric loudspeakers have been manufactured by Dynaudio, Linn (Isobarik), Totem and several other manufacturers.