DIY ?uestion

I have built a number of transmission lines. Generally, I’ve set the line length to be 1/4 of the wavelength of the lowest frequency but assume that the packing effectively reduces the speed of sound by 1/2 (most likely because of the packing (I use polypropylene.) The line tapers from 1.25 x the driver’s piston area to the piston area at the rear or bottom exit. Try to find an efficient, high power-handling driver to use. I like 8" drivers that can handle at least 100W continuous and have SPLs at least 92dB and around 0.3 Qts.
They are very smooth down to very low frequencies. My current LF speakers are rolled off below 25Hz because the go down much lower and that information is just noise, can't be heard, and only shakes stuff around.

@trelja In addition to big, very LF TLs, I built a series of small TLs using 4" LF drivers. The first iteration used a higher Q driver that refused to deliver the calculated 56kHz roll-off until a bit of putty was added to the driver dust cap. The weight of the putty was calculated to drop the Q to be ≤ the .3 . This worked and effectively supported the benefits of a low Qts. That said, I am currently running a pair of big TLs that work down to 17Hz using Focal kevlar drivers with Qs around .55 but these are bi-amped and DSP controlled and never worked properly using passive second order x-overs without bottom-end compensation.