Famous DIY designs: lines, horns, dipoles and proven two-ways

DIY loudspeakers are not only about bass reflex. Over a hundred years many designs have appeared that solve the same problem, what to do with the sound from the back of the cone, each in its own way. Let's go through the most interesting ones, with examples that have become legends.

Quarter-wave lines: Voigt, TQWT and ML-TL

The back wave does not go into a closed volume but into a long line. At the frequency where the line is a quarter of the wavelength, the line's outlet plays in phase with the cone and adds bass.

The Voigt pipe is the oldest form, patented by Paul Voigt in 1934. The line widens towards the outlet, which sits near the top or bottom of the column.
The TQWT is its modern form, with an outlet smaller than the line's cross-section. The best-known example is the Needle with a Dayton RS100 full-range driver. In the German-speaking world the Mindstorm (CT 213), referred to as the original TQWT from HiFi-Selbstbau, has a similar reputation.
The ML-TL (mass-loaded transmission line) is a straight or slightly tapered line with a port at the end. The port adds mass, so the line can be shorter. Martin J. King worked this type out in detail.
The transmission line in the narrower sense is a long, heavily damped line whose aim is rather to absorb the back wave and support the bass gently.

Lines suit full-range drivers with a low Qts and slim columns, where height comes for free. An archive of older Voigt pipe pages is collected at Real World Audio.

In Loudsmith: TQWT, ML-TL and transmission line are among the cabinet types. The ML-TL is calculated as a straight line with a port at the end: the line is the inside of the column, its length is the inner height and the driver position comes from the drawing. The port is picked from tubes you can buy so that the air in it stays under 17 m/s. In the project library you will find the Needle and Martin J. King's ML-TL for the Lowther DX.

Back-loaded horns: the Frugel-Horn and Nagaoka

A back-loaded horn guides the back wave through a folded, gradually widening horn. The horn raises bass efficiency, so a full-range driver plays louder and deeper than its size would suggest.

The best-known small example is the Frugel-Horn, an open corner-horn design for a 4" full-range driver. According to its authors, more than a thousand of the Mk3 version have been built. It is meant for a room corner, which acts as an extension of the mouth. Critics on the forums point out that a true horn for deep bass needs an enormous mouth, so small horns work largely thanks to the corner. In Japan there is a tradition of Nagaoka designs with double-mouth horns.

In Loudsmith: we do not simulate horns yet. The best program for them is Hornresp, see Loudspeaker design software.

Open baffles and dipoles: Linkwitz ORION and LX521

The most radical answer to the back wave: let it radiate freely. A driver on a bare baffle plays forwards and backwards in opposite phase, so it radiates in a figure-of-eight and almost nothing to the sides. The room is excited less by side-wall reflections and the sound comes across as spacious and natural.

The price: below a frequency set by the baffle width the front and back waves cancel and the bass falls by 6 dB per octave. It has to be equalised and needs long-throw woofers. Siegfried Linkwitz called the open baffle the most demanding environment for a long-throw woofer. His ORION uses two 10" woofers and an active crossover with equalisation. The four-way active dipole LX521 from 2012 is still regarded by many as the peak of this approach (review).

In Loudsmith: the open baffle is one of the eight cabinet types. The model is based on Linkwitz's dipole models. The library contains Linkwitz's ORION-3, THOR subwoofer and PLUTO-2.1, and several H-frame open baffles by Martin J. King.

Passive radiator

A cone without a magnet instead of a port. It does the same as a bass reflex, but without flow noise or pipe resonance, and it suits small boxes in particular, where a long port would not fit. Why I chose one for my own speaker is worked out in Speaker box calculations step by step.

Proven two-ways

For a first build the forums give the same advice: build a proven design. It has been measured, tuned and built by hundreds of people. Three designers are mentioned most often:

John Krutke, Zaph Audio. Dozens of thoroughly measured designs, such as the ZA5 range, the ZD5 or the ZA-SR71. His crossovers tend to be as simple as they can be. The archive is no longer updated but is still online.
Paul Carmody. Popular designs for beginners and advanced builders, and a getting-started guide explaining why to start with a proven design.
Troels Gravesen. A Danish designer with dozens of thoroughly documented projects, such as the CNO-25. Jantzen Audio sells kits based on his designs.

Other sources of proven designs are Humble Homemade Hifi (free for private use) and, in the German-speaking world, Visaton with its HiFi Bauvorschläge annuals, which include the VIb 2000 GF column, for example.

In Loudsmith: the project library holds 66 designs: 29 from Zaph Audio (ZA5.2, ZD5, ZA-SR71, the ZRT Revelator and more), 12 by Martin J. King, 3 by Linkwitz and the Nova Andromeda from Humble Homemade Hifi. You open each one with a single click and see its response, crossover and drawing.

At a glance

Design	Back wave	Strengths	Weaknesses	In Loudsmith
Sealed box	absorbed	tight bass, small box	lower bass sensitivity	yes
Bass reflex	port	deeper bass from the same volume	port noise, pipe resonance	yes
Passive radiator	cone	like bass reflex without the noise	cost, steeper roll-off	yes
TQWT, Voigt, ML-TL	line	bass from a slim column	tall cabinet, tuning the damping	yes (TQWT, ML-TL, TL)
Back-loaded horn	horn	high efficiency	large, complex cabinet, colouration	no
Open baffle	free	spacious sound, fewer reflections	bass needs equalising	yes

Want to try one? Open the app, pick a design in the Project library and look at its calculation. Or enter your own drivers and compare all the cabinet types at once.