That is because it is not a tapped horn.
You might remember that you, me and Bill Fitz discussed these issues in the past. And I still disagree with you guys. If you recall, you guys insisted that a horn needed a certain amount of efficiency to be called a horn. But tapped horns are not horns, in fact the vast majority of front loaded horns are not horns either, depending on your definition of horn. A "true" horn will have a mouth circumference equal to it's tuning wavelength. And there is NO accepted definition of horn that I know of that includes a certain amount of efficiency to qualify as a horn. This thing might be less hornlike than your average tapped horn, but no tapped horn is a "true" horn anyway. So semantics don't really matter.
The only way to increase efficiency is to decrease the usable bandwidth. The more optimised a enclosure is from the efficiency standpoint the smaller the usable bandwidth.
Not true, or misleading at best. As I stated in the past, efficiency is all about size. I can show you 30 db of gain over at least 3 octaves from almost any driver if there's no size limit imposed (in a front loaded horn). And bandwidth is all about how many impedance peaks you have and how you space them out.
Many people are creating designs that have the elements of a tapped horn without the efficiency gains associated with an optimised design.
"Optimized" has no meaning without a definition of the desired characteristics and design goals. If small size is a goal (and it most often is), then a design is most certainly optimized even if it doesn't have high efficiency.
A true taped horn optimised for at least 7 db gain over an equivalent sized conventional vented box will have a wavy output. Plus or minus 3db at least. And the electrical impedance will show the points where the maximum efficiency has been gained. There should be three of them.
A properly designed tapped horn is a sports car. There are serious trade offs between a sports car versus a sedan. Many bumps on the road.
Not a nice flat line.
Again not true. Here's a high efficiency, wide bandwidth tapped horn with a whole lot more than 7db of gain. It's +/- less than 2 db, no big bumps on this road. It's about 8x larger than the average design for this driver. For a given tuning efficiency is based on size, not good design or some type of magic. And there's no definition that states a tapped horn has to have a wildly peaky response.
[IMGDEAD]http://i58.tinypic.com/jjtpvs.png[/IMGDEAD]
Getting all caught up in definitions and semantics is a huge problem. It forces certain ideas that cause you to miss the forest for the trees.
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