The rear-loaded horn is the most common cabinet design for the Lowther driver. Rear-horns are also used for other full range drivers, especially the Fostex Sigma series. Lowthers, Fostex Sigmas, and some other full-range drivers have low values for Qts (approximately 0.2 to 0.3) and have rising frequency response curves (rising SPL with increasing frequency) when mounted in open baffles or box cabinets. The solution is to attach a horn designed for amplifying bass wavelengths.
For full-range driver loudspeakers, this horn usually attaches to the rear of the cone, allowing the front of the cone to radiate the higher frequencies directly. The bass output is brought up to the same level as mids and highs. The mouth of the bass horn often faces forward, but can face to the rear. Because bass frequencies have a very long wavelength, the size of the horn needs to be large and long to cover low frequencies. A straight horn would be very long, so bass horns are usually folded to fit into a smaller space.
The ideal shape for a horn is a round cross section. The contour of the horn (the change in area as you travel down the horn) can be conical, exponential, hyperbolic, or tractrix expansion. Exponential and exponential/hyperbolic expansions work well for bass horns, while tractrix is ideal for the mid range. However, the tractrix contour has often been used with sucess for rear-loaded bass horns in Lowther speakers. Round cross section horns of large size (for bass frequencies) are difficult to build with conventional materials, so square or rectangular horns are often constructed. Square horns are actually quite difficult to build and so are usually only used in front loaded (front facing) applications. Rectangular horns are the most common. See my Horn Speaker page for more details on horns.
Most of the Lowther factory designs are rectangular cross section horns. These speakers have parallel walls, so the width of the rectangle is fixed. The height of the rectangle increases from small to large (from the throat to the mouth). Some people say that the parallel walls produce "colored" sound or ragged frequency response. Also most of these cabinets use flat wood to create the bends (curves) of the folded horn which might also color the sound. Multiple folds, particularly 180 degree bends can also be a problem. A few rear-horn speakers have only a couple of folds, and gentle curves.
Commercial Lowther Rear-Loaded Horns
- Lowther factory designs. Rectangular horns in the enclosures section of the excellent Lowther website maintained by Marc Wauters.
- Medallion speakers using Lowther drivers can be made from plans, a kit, or purchased from Lowther America.
- Medallion II Acoustas can be seen here.
- Beauhorn (Eric Thomas) build a nice rear-loaded Lowther horn with small front horn for midrange balance. They also make custom phase plugs for Lowther drivers. Check out the review at EnjoyTheMusic.com.
- A beautiful Hedlund horn built by Miroslav Ivanek using veneers is available commercially at Hornet Audio.
- Rethm loudspeakers are Lowther based and are built in India by Design Combine. Art Dudley gives a nice review in Listener magazine (Vol 7, No 1, Jan/Feb 2001).
- Moth Audio are building rear loaded horns for Lowther or Reps drivers using a hyperbolic design by Dr. Bruce Edgar. See them on their website under Products, and also under Parts.
- Carfrae (see London show report at TNT) build a large Lowther horn which has a rectangle increasing with no parallel sides and only one smooth curve bend. They also build the Little Big Horn which has parallel walls and one curving bend.
- David Clark builds a Lowther rear-horn which has smooth curves and no parallel sides, and is somewhat more manageable in size!
Other Commercial Rear-Loaded Horns
- RL Acoustique builds a rear- loaded horn using Reps R1 drivers and AER drivers.
- LothX is a company in Singapore producing nice rear loaded horns using their own drivers. A review of their Polaris speaker by Geoff Husband.
- Be Yamamura used to build single driver horns called the Dionisio 27 using Yamamura-designed drivers. This is a huge 27 Hz rear-loaded horn, 5 meters long and about 2 meters square area at the mouth. No information can be found about current availability.
- Building your own rear loaded horn — the Big Fun horn project by Ketil Parow. Excellent article with how-to and horn calculation information, using Lowther drivers.
- The All Fun horn project by Ketil Parow, a smaller brother to the Big Fun horn, VERY well documented, check it out. Also using Lowthers.
- The Hedlund horn has only one curve in its rear loaded rectangular cross section bass horn and is DIY construction. It apparently sounds very nice and uses Lowther drivers.
See projects under DIY > Rear Horn.
Mr Tetsuo Nagaoka from Japan devised many speakers for full range drivers. One of his most popular, the Swan D-101S (picture on right) won the readers choice in a Japanese book or magazine.
(From Stefan:) "From what I know, D-101s was designed for Fostex driver FE108super introducing a much stronger magnet than FE108sigma. Therefore this enclosure was also called "SUPERswan". It regards the super-driver's lower Qts ! Compatible for this enclosure are also 6N-FE108super and 6N-FE108ES. It seems also 6N-FE108ESII performs well even though again having a stronger magnet. Superswan's predecessor was Swan D-101a, designed for Fostex driver FE106sigma which is compatible with the recent FE108sigma. So if using the FE108sigma, one should prefer the D-101a !"
Description of the Swan D-101S in Japanese (large image).
Cutting diagram for wood for Swan D-101S (large image).
Jaime's website plans for the Swan.
Some Swan websites: