Aperiodic predates me by quite a bit. The 1st published paper that i am aware of is Ted Jordan's article (that was related to the release of the Goodmans ARU -- acoustic resistance unit)
http://p10hifi.net/planet10/TLS/downloads/TedJordanAperiodic.pdf
The most widely spread aperiodic enclosures were the Dynaco A10 & A25, The A35 & A50 were also an aperiodic variation. The Fried R & Q are also examples as is one of the more recent NAIMs was configured similar to the A35/A50
GM is pretty bang on the money ... he should be, i he was one of the guys i learned from
MJK would sim aperiodic enclosures is Martin had calibrated data for the resistance units. As GM points out 1 lb/ft^3 can get some boxes pretty close (a classic TL -- of which there are few true examples) is aperiodic.
One can think of aperiodic as in between BR & sealed, with a continuum of near aperiodic going in either direction. Different drivers will work best at different places on the continuum. (for the rest of this ramble i'm just goig to use aperiodic to mean the space between BR & sealed). Aperiodic has the advantage over a BR in that it is more tolerent to the changes in T/S parameters that occur as you turn the volume up or as the weather changes. It has the advantage over sealed in that it is lower pressure or can get you a lower Q out of a smaller box.
One can use standard simulation tools to get a starting point, but after that it is seat-of-the-pants. A impedance measure tool, GM's clik test, and lots of listening help fine tune.
I approach designing an aperiodic box one of 2 ways.
1/ If the driver works in a sealed box (ie FR125, SDX7), i'll design for a Q higher than i'd like (a bit over 1 is my usual target, but i have one in the works where i'll start with Q=0,7) and then add resistive ports as in the holes + foam behind (borrowed from PEARL/Bill Perkins), or a slot with squished fiberglass (as in Dyna A25 -- the SS variovent is similar, but the long narrow slot is easier to tune)
2/ If the driver really needs the support on the bottom that a BR gives (ie all the Fonkens), i design a BR with a very specific shape roll-off and then make the ports long and with a high aspect ratio to add restance to the port. If this is not enuff to achieve the goal i'll add a specific open cel foam to the port. (this last is the same idea as stuffing a sock in the port)
some graphic support:
impedance of a push-push SDX7 aperiodic TL i am working on
impedance of a Fonken (you can see that this is way closer to the BR end of the spectrum)
and the before & after of a miniOnken with stuffed ports
PEARL PR-2
dave