The flagship of the a-Class subwoofer family, the formidable as-418 installs four custom-engineered, 18-inch drivers into a compact enclosure comparable in size to most dual-18-inch subwoofer designs. Drivers are configured into a pair of front-to-front, isobaric (push-pull) configurations. The direct radiation from each front driver is combined with the output of the companion rear driver in a tuned vented enclosure. This Compound Force design enables more output and less distortion, while at the same time delivering four 18’s in an enclosure no larger than a typical dual 18-inch subwoofer.
This is not a new concept, having been originally introduced by Harry Olson in the early 1950’s. Technically, “isobaric” 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 that 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.
Isobaric loading can be used within any enclosure type, including bandpass designs. The ported and bandpass isobaric designs can be difficult to design and build due to very small enclosures with large port requirements. Isobaric 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 isobaric 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 because their opposed cone motion averages out suspension non-linearities (differences in inward and outward suspension control), which reduces distortion.
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.
The face-to-face or “clamshell” configuration as it is commonly called is the most compact and, therefore, the most practical isobaric loading method to use, considering the tight confines of the average automobile. This configuration also provides the beneficial side effect of canceling driver non-linearities.
If there is one recurring theme in engineering, it’s that Mother Nature is lazy. She has made it a law that anything at rest wants to stay at rest, and, similarly, anything in motion would much rather stay in motion in a straight line. Such is life in general, and a speaker’s dynamics are no exception. It’s called the law of inertia, and there is no escape.
When a subwoofer does its job, it is called upon to compress and rarefy the air in the listening environment many times per second, and more often than not it is required to do so over great distances. This places a great strain on the cone itself as it fights to retain its shape in the face of intense acceleration and deceleration. Ideally, a speaker’s cone would be infinitely rigid and wouldn’t deform under any circumstances, but this is not a perfect world a perfect world, and we have to deal with the consequences of fighting Mother Nature.
As the cone pushes outward, it is somewhat flattened out as it attempts to kick-start the air in front of it into motion. Likewise, the cone is deformed the other way when the cone returns and attempts to compress the air in the subwoofer enclosure. The extent of this deformation is a function of the cone’s geometry, construction and the amount of power with which the subwoofer is driven. A good engineer will design his cones so that this effect is minimized, but there is only so much engineers can do if he wants to keep moving mass and/or costs low.