Originally Posted by rdilliker
I was auditioning speakers this weekend and the B&W 704s are a 2 1/2 way speaker. I notice in various threads some people seem to confuse the number of drivers with the number of "ways" so this may help others clarify some misunderstandings too. With the 2 1/2 way speaker what is the "1/2 way"?
A 2.5 way is just a 2 way speaker with an extra driver that rolls off starting low - typically around 200hz - at a 6db slope. The extra woofer can be called the .5 woofer and handles the BSC, Baffle Step Compensation. Baffle Step losses are theoretically (but often not in practice) 6db/octave, so instead of of compensating for those losses with response shaping of a woofer alone like would be done in a 2 way, the 2.5 way has a dedicated driver whose job is to reproduce these losses. Both woofer btw have no high-pass filter, and reproduce the full range of bass.
The two woofers in a 2.5 way can be anything, they don't have to be the same size, they can be completely different or exactly the same. There's nothing in the design that dictates what the woofers must be.
The primary advantage of a 2.5 way is increased sensitivity. As far as a corporation is concerned the advantage of a 2.5 way is they're much easier to market and sell than a 2 way, simply because they're more impressive to look at. As a designer though, 2.5 ways suffer from unacceptable performance issues - the same drivers put into a 2 way MT or MTM will almost always sound better.