Regarding the damping of the TLS. You get maximal damping at velocity maxima. So for the fundamental you have a pressure maxima and velocity minima at the closed end and the velocity maximum at the open end of the pipe. So having as little damping as possible at the last half of the pipe is a good thing. What you want to reduce is the uneven harmonics that has velocity maxima at 1/3 and 2/3 of the pipe for the 3rd harmonic at 1/5 2/5 3/5 and 4/5 for the 5th harmonic and so on. Having damping material so close to the open end will reduce the output of the fundamental. I would try denser stuffing in the first section of the pipe if there is to much harmonics slipping out.
An update to my Alpair TL project. I made a pair of stands to raise drivers to about 900mm from the floor. Top and base plates are made from the same birch plywood as cabinets. Each column consists of four extruded aluminum tubes. 80mm bolts are anchored in these tubes with epoxy poured into both ends of every tube to the depth of about 60mm. Top plates are bolted to cabinets using threaded inserts. The whole assembly turned out very sturdy.
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