Back horns typically are fitted with full range drivers that have a rising response. The back horn cabinet amplifies lower mid and bass frequencies attempting to balance out the overall sound. The back horns I’ve built have not always been successful but I can hear and appreciate the sonic contribution these designs can accomplish. At worst, they can sound hollow and bloated. At best, they have amazing dynamics, speed and something called Jump Factor that really adds a sense of life and scale to the music. Back horns can sound very different from other designs and should be considered if you are curious to know what that sound is. Personally, I think they are worth the effort if your room can accommodate them. My favorite type of back horn is a BIB because the math and science to build these is well documented. Other designs are often touted anecdotally but once built their shortcomings are apparent. If you decide to build a back horn make sure to enjoy the process. It’s slow but potentially very rewarding.