Still have to take Baffle diffraction into account with a TLine?

I'm thinking of making a T-Line for my front room so I was thinking can I mount a tweeter in the top of it as long as I section it off? And do I still need to take baffle diffraction into account?

I'm thinking of using a 6" driver (not decided yet) with a quarter wave design?

What do ya think?

Yes, you still need to take baffle diffraction into account. The problem is that if you move the tweeter far enough away to minimize the diffraction, you'll run into problems with lobing in the crossover region.

I highly recommend using good 6" woofers in t-lines. I've been doing this for years with great results. I've had expecially good luck with using very high crossover frequencies (4-5khz), which minimizes diffraction problems and, assuming you have a woofer that works well in the upper mids, gives a really nice midrange.

I've tuned my cabinet for about a 42hz quarter-wave length using a tapered line design with the opening having the same area as the cone of the woofer. I'm using about a pound of polyfill for the woofer-side half of the line. The result is some of the best bass I've ever heard, liquid, fast and really deep where needed. Transient response is excellent as well. My woofers are the late, lamented Vifas. These days I'm not sure what I would use.

Good luck and have fun. T-lines are a lot of fun to build.