The piggy-back tunnel-loaded isobaric configuration is probably the second most popular isobaric arrangement (the first being the face-to-face or "clamshell" configuration). It is cosmetically easier to integrate into the vehicle (as it does not have any potentially ugly subwoofer baskets protruding into the vehicle), but, unfortunately, this aesthetic benefit is offset by several important disadvantages:
1. The coupled air between the two drivers adds to the moving mass of the system and thus results in a less than optimal coupling between the drivers. Remember that the idea is to get these two subwoofers to act as one driver, and by adding a springy mass between them, this ideal is somewhat compromised. Some might find that this leads to a beneficial lowering of the system Q (when the volume indicated in blue in the picture is sealed), but, more often than not, this effect is undesirable because it makes response predictions more difficult.
2. The coupling chamber negates one of the primary benefits of iso-loading (small enclosure size). By the time we account for the displacement of this coupling tunnel in determining the gross volume of the blue chamber, the enclosure starts to approach the volume required by a single conventionally mounted driver. This begs an obvious question: Why are we doing this?
3. Since the drivers are both firing in the same direction, we do not reap the benefit of cancelled driver non-linearities as we would with a design implementing a push-pull configuration. Why are we doing this again?
4. The driver whose magnet structure is housed in the coupling tunnel is in a highly unfavorable cooling environment and will be subject to power compression at lower levels. Basically, the drivers will be more or less equal performers at first, but as things start to heat up and the impedance of the front driver rises due to rising voice coil temperatures, the drivers start to fight each other to some degree rather than complement one another. This results in increasingly non-linear behavior with possible unpleasant audible side effects (e.g. sloppy transient behavior). Really: Why are we doing this?
In essence, the "piggy-back" configuration is more of a cosmetic "oh neat-o" design more than anything else, and we recommend that it not be used, especially for high-powered applications where the thermal power handling of the drivers would be called into question.
Learn about other Isobaric Enclosure Types:
|Back-to-back tunnel load|