Found this on the net which might explain as I wasn't sure either.
Some speaker types are much more efficient than others: most notably, horn type systems are the most efficient. Some narrowband horn systems have efficiencies approaching 50% (this may still seem like a low number but for a speaker it is extremely high). The reason horn systems are so efficient is that the horn acts like an impedance matching device (to the air) ; essentially an acoustical transformer. Whenever you cup your hands around your mouth to increase the volume when shouting at someone, you are forming a (crude but effective) horn. Many high frequency and midrange speakers are of the horn variety. Due to physics, low frequency horn speakers are very large (if not huge); hence the reason for "horn woofers" being scarce. A number of low frequency horn systems are available, but they are usually of the folded horn design. Because a low frequency horn would be excessively large (a big consideration for portable sound systems), engineers came up with the folded horn design. Folded horns result in some losses (as compared to a "straight" horn), but the result is still a very efficient (again, relatively speaking) system.
Nice speakers though are you leaving them bare like in the pics or covering them?
They look an interesting project as much as I like to design speakers and just about understand it, I always end up just making the biggest box I can get away with and calling it a day.
I did once make a ported box for my old 15" sub in the back of my old Saxo (lolz, I knowz!) but I'm not really sure why I bothered - It was far too loud and peaky for what I wanted, so I ended up just blocking the ports with wadding.
Any reason you didn't use MDF to make them? Am I right in assuming the design is very sensitive to the parameters of the drivers? I've got a set of Richer Sounds era TDL floorstanders I'm looking towards re-casing into something a bit less tall, but I'll never be able to get the thiele-small data on the drivers.
Not 70's but for those interested in speakers, check this bad boy out...
We use these at work for acoustic testing. I have always wanted two of them in my living room. I am sure they would not be the best for music, but they can deal with 300w RMS and are designed to give totaly neutral and consistant output from 30Hz to 20KHz!