The cancellation plenum is the horn's front chamber. It acts as a low pass filter and is one of the parameters used when modeling a horn. The magnet of the rear-facing driver protrudes, which tends to make the front chamber volume larger. But there is no reason why you can build up the internal area around the cones to displace volume if the horn design requires a small front chamber.![](http://www.pispeakers.com/12Pi_small_front_chamber.gif)
The black area in the front chamber shows how space can be displaced to make the volume smaller, if required. These can be placed behind, above and below the driver. There is a lot of space surrounding the drivers that can be easily filled with the use of filled blocks. It is not a large extra step to add these, so if the front chamber must be made small, that's one way to do it.