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.

A local dealer who builds custom horn systems has a 70hz horn with a square opening and a 90 degree bend in the horn throat.  Even with that bend, the horn is well north of six feet long so it is very hard to work all of this into a single cabinet.  He has the drivers meant to work with the horn and they have a practical operating range from 60hz to 1,000 hz.  This is something wildly impractical to work with.  To me, the best sounding nearly practical horn he has used was a Japanese wooden replica of a Western Electric 22A horn, which all who have heard it agree that it sounds better than an original Western Electric 22A horn.  The wooden 22A horn system I liked had the horn suspended above a cabinet that held the woofers (twin 18" woofers) and horn tweeters.  THIS is what horn systems should ideally sound like: huge and enveloping soundstage (large scale to the presentation), macro and micro dynamics with a sense of ease and a relaxed sound, sense of power and weight even when playing at low volume.

By horn system standards, mine is a mini, with twin 12" woofers in a "compact" Jensen/Onken bass reflex cabinet.  That cabinet is 24" wide, by about 20" deep and about 30" tall.  On top rests the tweeter (bullet tweeter) and a Western Electric 12025 multicellular horn (24" wide) driven by Western Electric 713b drivers.  This is, to me, one of the best sounding compression drivers ever made.   My system delivers extremely good dynamics, particularly at low volume, and very little of the peaky or rough sound one hears with most horn systems.  But, it cannot deliver the sense of scale of the big boy horn systems.