So much has been written about speaker construction, that I will give just the bare facts. Driver = Eminence 151311, 15", 8 Ohms, 300W, sold by www.madisound.com for $79. Box = Non-ported, internal dimensions 17.5 x 9 x 16.5 inches. The design goal was a tight-sounding cab with more sensitivity than the internal speaker in my GK MB150E combo, for those 25% of gigs where the combo is underpowered. The 151311 driver modeled well in WinISD, the price was right, and Madisound is just up the street from me. This driver is only sold by Madisound, but the guy said it's a regular production item, not surplus. I have used this cab for one gig in a room with bad acoustics, and a rehearsal with a very loud big-band. To my ears, there was no trace of the boomy sound typically associated with 15" speakers -- it has pretty much the same tone as my GK. Given what is written about 15's for URB use, I would suggest that most of the problem is cabinet design. Typical 15" systems are designed into big boomy ported boxes because it can be done without sacrificing efficiency. The market does not seem to demand 15" closed box systems. But we DIY'ers are not so constrained. Anyway, this design seems like a way to get some decibels and decent tone without breaking the bank. I used some 3/4" birch plywood that was sitting in my garage.