@trelja said: "True transmission lines (TL) handily outperform both bass reflex / ported and sealed and aperiodic bass loading configurations."
I agree that good transmission lines sound wonderful, and have greater potential than either reflex or sealed or aperiodic boxes. As a manufacturer, with enclosure cost being the dominant cost, I believe that I can offer more bang for the buck with a good reflex box.
Going back to my DIY days, one issue that I ran into with transmission lines is the one-wavelength cancellation dip. At the frequency where the path length is equal to one wavelength, the backwave energy emerging from the open terminus is 180 degrees out-of-phase with the energy coming from the front of the cone. The two are physically close enough that significant cancellation occurs. I could hear the dip before I figured out what was causing it. You can see it in anechoic measurements of transmission line speakers:
The dip is probably not as bad in the power response (summed omnidirectional response) but it will still be present. Fortunately dips look much worse on paper than they actually sound - the ear is pretty good at ignoring them.
There are techniques for mitigating the dip (positioning the woofer partway down the line, using very dense stuffing, building a Helmholtz absorber into the line), but like practically everything else in speaker design, they involve tradeoffs.
From your experience, do you have any comments on the one-wavelength dip? Did Bud Fried do anything in particular to mitigate it?
Trelja again: "As great as TL is for bass, it’s all the more advantageous for midrange loading."
Again, I agree. I think the transmission line acts like a "trap" for the backwave midrange energy, so that very little of it reflects back into the cone.
I had an article published in SpeakerBuilder magazine back in 1986 that used transmission line loading for the two 7" midwoofers and for the 30" tall ribbon they were mated with. Over the years several people have told me they tried various enclosure designs for the same ribbon and my "W-shaped" transmission line geometry worked the best.
I still think about doing a transmission line "satellite" speaker that would be augmented south of 80 Hz by a distributed multisub system, since I believe the room effects are the biggest issue in the bottom two octaves. This would let me use a smaller transmission line box (less costly) and would take advantage of its superior midrange potential (which is imo its biggest benefit). Maybe one of these days.
What I have found with reflex boxes, and this is somewhat counter-intuitive, is that placing the ports right smack behind the woofer cone results in better midrange than from a sealed box. I think this is because the (flared) ports act like traps and remove at least some of the midrange energy that would otherwise reflect back into the cone. So this is perhaps a crude first approximation of one of the benefits of a good transmission line. On these speakers I include a reduced-level rear-firing tweeter to combine with the port-escape midrange energy, to correct the spectral balance of the reflections.