Bass horns are big. Big in the mouth, and very deep.
Folded horns are less deep (same mouth) but the folds reduce the bandwidth.
Corner horns look small because the end of the horn is really the corner of the room. This gives a very substantial reduction in the mouth-area you must build, but the room corner is not the perfect shape, and you have to keep the area around corner fairly clear of absorption and doors. And unless you drill-out the corner and extend the horn beyond it, a corner horn must be severely folded.
It is unusual for any folded horn to be "flat" over 10:1 of bandwidth, say 40Hz-400Hz. And most have peaks and deep dips at the extremes, even throughout their nominal bandwidth.
The classic example is the Altec A-7. Even though it is not folded and not really small, it is very un-flat. And I know some models show flat response on test: it isn't so in real rooms. One recording operation got known for "rich bass".... they were not mixing for full bass, they were being fooled by the A-7's real-world gutlessness. When mixed to sound full on an A-7, the recording was bass-heavy on a truly flat-bass speaker like the AR-3.
Every speaker has an efficiency. Horns really only work well at "high" efficiency, 25% to 50%. Or rather, you can load a horn to lower efficiency, but below 5% you could build a simple box speaker in a LOT less space. In fact the total volume of a horn and a box for a given efficiency and lower cut-off are the same, except a box scales cleanly to low efficency and a horn pretty much has to be high-efficiency size.
And for all the sins of cones and boxes, listening through a long tube also has problems. If you actually build it long enough to not have wavelength ripple, and straight enough not to have internal reflections, you get severe directionality effects: wide at the low end of the band and very beamy at the top of the band.
In one way the beamy is good. Horns like to work at high efficiency but you can't match a heavy voice coil to thin air at high frequencies. Horn efficiency droops. Letting a horn go beamy keeps the on-axis response fairly flat. This is fine for one listener in an anechoic space. But it sounds different everywhere in the room, and it is reverberant in the bass and very dry in the top.
Good horns have been made. Community Light and Sound's Leviathan was about as close to a perfect 40Hz horn as would fit in a large truck. It would fill the average monitoring room, and fill it with bass, but the 800Hz output would only cover half the width of the room. They made some good midranges, though missed the CD boat and hung onto multi-cell. A decade later they faced the impossible problem of something between a 15" paper cone and a 4" alloy dome, and made a massive midrange driver and a CD horn to go with it. I think it killed the company. (The name lives but it can't be the same people; or else they had a mid-life crisis and decided to build what sells.)
Horns are of course unbeatable when you need large output on small electric power and can accept less-smooth response. After decades of working with and on horns, I think the "clarity" of horns is bogus. It is just different types of flaws than we hear everyday in home and car speakers. I think an ideal horn is the best way to make sound, but there is no ideal horn. I think direct drivers do pretty good and a LOT smaller. And amplifier power is a secondary issue today.
There is an in-between. Large (8 foot) arrays of cones have some of the properties of horns. If you stack sixteen 15" cone subwoofers (4x4 array) you will get about 20% efficiency up to 200Hz, as good as any practical horn. Some of the boxy response peaks go away with such large arrays. Some of the FM distortion of hard-driven cones is reduced by good large-area loading. If you scale this array and a similar-mouth horn, both to nominal 40Hz cutoff, the horn will cut-off and blatt quite abruptly (requiring a high-pass for high-power use) while the cones will fade away smoothly with usable output at 30Hz.
For rooms less than a cow-palace, a vertical line array of 4" drivers, in the corners, as tall as the room (about 22 drivers) will really couple air. It even defies the 6dB/double-distance rule, throwing sound far onto the room. The directivity is constant and room-wide up to around 4KHz. And line-arrays are back in fashion even for cow-palaces.
Richard Burwen built his home around a couple bass horns. You sat in a small chamber with two "walls" that were really horns.
I think I've wandered off the topic. I got 87 network locations hooked-up today and I need my nap.
