Tapered Quarter Wave Tube and Transmission Lines
Two superb websites are:
The tapered quarter wave tube (pipe) (TQWT or TQWP) or Voigt pipe was designed by the main Lowther driver designer, Mr. P.G.A.H. Voigt. The quarter wave pipe is one of the simplest ways to effectively enhance bass output. The design is a cross between a transmission line and a horn. A description of the Voigt pipe is here at the Norway Lowther Club. The pipe is very tall and may appear imposing in some living situations. One way to make a more conventional appearing speaker is to fold the pipe in half. These designs are quite simple for a home builder, not requiring the complexity of a folded horn.
Filippo Punzo has been experimenting with using Helmholtz resonators inside a TQWP design with Supravox 215RTF monomembrane (you must add a supertweeter) and bicone. See this diagram of his TQWP. The frequency response is here.
Bert Doppenberg of the BD Design has a Fostex-based TQWT kit.
David Weems has a book called Designing and Building Your Own Speakers which describes TQWT projects using Radio Shack drivers. Chris Beck built some using the Radio Shack 40-1354 and was pleased with the sound (for a while, then decided he wanted more bass and more treble).
John Rutter has developed a TQWT spreadsheet for calculating enclosure size. You can download it here. (Copyright John Rutter).
Martin King has developed Mathcad simulations of quarter wave pipes. Learn much more at Quarter Wavelength Loudspeaker Design.
Herbert Jeschke has written of his Adventure with Voigt Pipes in the DIY section, using the Radio Shack 40-1354 driver. This is also featured in the NY Blast single driver shootout.
Nocturne's TQWT with Fostex Sigma FE208 drivers. Nice write up.
Roiene TQWT available commercially in Germany. Also see Roiene in the Drivers section.
TQWT using Audax HT210A2 by Derek Walton.
Martin King's TQWT modeled in Mathcad with Martin's Transmission Line model then built by Martin.
One idea (not mine unfortunately) is to combine two TQWTs with somewhat different length/resonance frequencies. The problem with the TQWT is that the frequency response in the bass is somewhat "lumpy", due to a kind of "comb filter" Effect. If you took two drivers (say the 12 Ohm Triangles) and tune one TQWT to (say) 40 Hz and the other higher (best tuned by trial and error), one could achieve a design where the two different "combs" interlock, making for a substantially flatter response.
This design was described (and might have originated) in the article Build the ACE: Details of a new Quarter-wave Design by Edward Michelsen in Hi-Fi News, October 1970. A diagram of the ACE is here. Essentially the design uses two dissimilar folded quarter wave tubes sharing a single closed inner section, the outer sections vent to the floor, the inner section is stuffed. The driver (Lowther) faces up and the rear wave radiates directly down the closed inner section while the front wave reflects off a large stabilizer plug and exits horizontally from the top of the design.
To make matters more interesting, one could mount one driver front-facing and the other facing upwards. This would give something akin to the "Ceiling Boundary Ambiance Enhancement" used by Dick Olsher in his Samadi Speakers. The UK Company Castle uses a similar approach but with conventional drivers.
A transmission line cabinet essentially has a long tube or port leading from the rear of the speaker cone. Check out the Transmission Line Speaker Page for much more information.
Lynn Olsen's Ariel Speakers are a transmission line design but using 2-way drivers (mid/bass and a tweeter) rather than full range drivers.