Differences Between Folded Horn Speakers and Transmission Line Speakers

Quarter wave back loaded horns have a constantly expanding cross section to the folded horn.  That expansion prevents standing waves from developing in the horn.  A transmission line has a more constant cross section and has to have a lot of damping material in it to reduce standing waves.  This damping greatly reduces the amount of acoustic energy that back wave can contribute to the sound, hence the much lower efficiency of transmission lines.  But, it is a much harder trick to make that back horn have the properly expanding size and the opening has to be quite large.

The transmission line speakers I heard had very tight bass and sounded quite good.  They were also reasonably compact and did not require a lot of depth.  The various folded horns, like the Tannoy speakers, sound really good, although the bass did not go extremely deep in frequency, and most were quite lively sounding.  In recent years, I’ve heard single- driver backloaded horns systems, like those from Charney Audio, that blew me away because they sounded so lively, rich, and smooth—so much more complete and free from peakiness than most other single driver systems.  The Charney speakers were compact in width and height, but were quite deep to allow for the back horn.  A big plus with backloaded horns is that the high efficiency allows for use of low-powered tube amps thst are my favorite amps.

With horn compression drivers, typically used for midrange and high frequency drivers, air is confined in a chamber and this does effectively increase impedance to allow for a better coupling of the driver to the air.  This certainly does improve efficiency greatly.  With woofers with a back-loaded horn, there may be some such impedance improvement, but, the biggest improvement comes from not trying to dissipate this back wave, rather, to lengthen its path so that the backwave, which starts out inverted in phase with the front wave, is now in phase and augments the front going wave.  Transmission lines are more in the business of dissipating the back wave energy at higher frequencies through use of damping material lining the tunnel.

Ported speakers tend to require more experimentation to get setup optimized than sealed speakers.  The payoff, if you can find a good placemeny, is higher efficiency.  This IS a big deal to those who like low powered amps (like me).  I have a local dealer that makes sealed box high efficiency speakers with compression driver midranges.  But, the speakers are quite large in size, and even though their custom woofers are 18” in diameter, they don’t go extremely low (tradeoffs are inescapable).

Are building the kit or building the cabinets from scratch?   That looks like quite a project from scratch.  These speakers sound very lively and nimble and deliver much more bass than one can normally expect from a single, small, driver.  Bigger versions with 8”-10” fullrange drivers can be so good that they can compete with any design.

Good luck on your project, it looks like fun.