I have choosen the throat diameters but which compression Driver?

A compression driver has a exit angle, sometime we talk about rapid flare vs slow flare it’s the exit angle of the device in degrees, we talk in semi-angle, for example an TD 4002/4003 have a semi-angle of 4/4.5°, so the “full” coverage is 8/9°.

More the flare is important less the horn is deep because you we have to smoothly bring the flare to reach the coverage angle of the horn, semi-angle 45° for example, so coverage 90°. As more and more compression drivers are designed for Line Array the flare with an important semi-angle, it’s rapid flare, that is not an advantage for our usage.

In an OS horn where we can choose the throat angle of the horn, it’s not a problem, but for HCD and bi-radial we prefer a slow flare, aka an angle as close as possible to 0°, the JBL 2450SL is 0°, it’s a no-flare device, it’s why I advice it for 1.5" usage generally, it’s also find-able used at an acceptable price.

Note: about diaphragm materials, Truextends Be is embossed Be, not vaporized Be like TAD does (more expensive), so even if it’s Be it’s far that TAD does.

It’s hard to set precisely a new diaphragm in a compression driver, even if modern driver facilitated it. Diaphragm characteristic is taken into account in the full device, sometimes changing by another doesn’t bring a lot of improvement (and sometimes it’s worse), so be careful with this.