A very loud speaker
Everything that needs to be said about the "Blue Thunder" loudspeakers has been said–oh well, written–back in 1994 in "Sound Practices". Use the link at the bottom to download a PDF of said article.
What hasn't been said is sad
The good news is that the "Blue Thunder" still is one heck of a speaker. The sad news is that Focal has stopped production of the Audiom 15VX2 driver. The good news is that there are quite a few very good sounding 15" drivers available. And the sad news is that their T/S parameters all differ from the 15VX2. The good news is that you'd only have to recalculate the dimensions of the "Blue Thunder" cabinet to suit your 15"er of choice. The sad news again is that I will not do the math for you…
Download: blue thunder_sound practices.pdf