Isobarik Boxes

Isobarik loading has become pretty popular for car audio use in the last few years. Again this is not a new concept, having been originally introduced by Harry Olson in the early 1950's. Technically, "isobarik" is not really an enclosure type; it is a loading method. This loading method involves the coupling of two woofers to work together as one unit. This is typically accomplished either by placing two woofers face to face or by coupling two woofers with a small chamber. The result of coupling the two speakers is that the coupled pair (iso-group) can now produce the same frequency response in half the box volume as a single speaker of the same type would require. For example, if a speaker is optimized for performance in a 1 cu.ft. sealed enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 0.5 cu.ft. sealed enclosure.
There is, of course, a penalty involved. Whenever you use isobarik loading, you are sacrificing 3dB of efficiency compared to a single driver in twice the air space. In practical terms, this is not usually a big deal since the powerhandling is doubled (two speakers instead of one) and the impedance is typically cut in half if we parallel the two speakers (twice the power, assuming the amplifier can deliver the necessary current.) The end result is about the same output as the single driver in the bigger box but at twice the amplifier power (and twice the speaker cost.)

Isobarik loading can be used within any enclosure type, including bandpass designs. The ported and bandpass isobarik designs can be difficult to design and build due to very small enclosures with large port requirements. Isobarik bandpass designs, in particular, can be literally impossible to build with certain speakers. There are some things to look out for with each type of isobarik design, such as mechanical noise and uneven heat dissipation which can present potential sound quality and reliability problems. All the methods which involve opposite cone motion require that the speakers be wired in reverse polarity relative to each other. These designs also provide a performance advantage in that their opposed cone motion averages out suspension non-linearities (differences in inward and outward suspension control,) which reduces distortion.

If you are strapped for space and can afford the extra speakers and more complex enclosure, the ability to have a compact subwoofer system with no real sacrifice in performance is well worth the extra effort and expense. On the other hand, if you have a lot of space and are looking to get the maximum amount of output without sacrificing sound quality, using multiple iso-groups can give you the best cone area/box volume ratio while still retaining good fidelity.

More on isobariks
If you're interested in using an iso-loaded subwoofer system in your vehicle, you should be aware of the benefits and detractions of the different isobarik configuration options available to you. Please consult the Isobariks tutorial for more information.