Isobaric Speakers - Design Principles

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 chamber of air in between. The volume of this "isobaric" chamber is largely immaterial, and is usually chosen to be fairly small for reasons of convenience. In a subwoofer, where the mid-range output is not needed, the optimum arrangement is front to front. 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 erroneous notion that the air pressure in the chamber between the loudspeakers is constant (the "isobaric" condition).

The two drivers operating in tandem exhibit exactly the same behavior as one loudspeaker in twice the cabinet. The cabinet is defined as the space behind the rear driver. The volume of air between the speakers has no acoustic function so that the saved space is less than 50%. All other aspects are unchanged like resonant frequency and maximum SPL. The new driver will have the same resonance frequency, Qts, distortion, excursion etc. as one driver with the same applied signal. Because the impedance is also halved, this performance is achieved with twice the power. The new efficiency is thus 3 dB lower than with one loudspeaker. 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.

The result is that the coupled driver pair (iso-group) can now produce the same frequency response in half the box volume that a single driver of the same type would require. For example, if a speaker is optimized for performance in a 40 liter enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 20 liter enclosure. The aforementioned volumes exclude the isobaric chamber. If the iso group is placed in the original 40 liter, the loading will be incorrect (if the 40 liter was a correct loading of the loudspeaker).

Of course if you double the moving mass of a single driver, halve its compliance and halve its impedance, you would attain identical results. Although this requires the ability to manufacture a custom driver, it has the advantage saving space and cost because only one driver is needed.