This article was written by Johan Dreyer in 2006 posted at AA and the reposted recently at a Johan's local South African forum:

http://vinylsa.co.za/forum/viewtopic.php?t=1016

I desided to give to the Johan’ thoughts some exposure at the horn-citric forum and it might provide some inspiration for someone. I might not necessary agree with everything that Johan express in this writing but I like very much the structure in which he put his thoughts. So, everything below is by Johan Dreyer:

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Hope it is not this years Christmas.

Step 1. Decide:Basshorn ( 20 - 200Hz- this one) or subwoofer (to 60-80Hz)? If you want a sub only there is much more leeway. You can fold it, only approximate the curves and make it looong. If you want a basshorn to work to say 2ooHz: keep it straight, short and as precise to the curve as you can.

Step 2. Sit down and write on apiece of paper:"No compromise bass horn "Now scratch out the "no". Set some immovable parameters. These you must stick to. Everything else will be a juggle.

2.1 Footprint is likely to be fixed. Do not fix cutt-off. The difference in size between a 25Hz and 20Hz horn is huge and of little sonic import(21Hz here). If your cuttoff is above 30Hz,you are waisting too much money and should try another alternative.

2.2 Stick to the hyperbolic exponential family.Co efficient 0.5 to 0.8. Choose as high as possible(0.7 here). This will keep your horn shorter and therefore lessen time delay.Please do not choose tractrix! It does not work for bass.It will not reach its cutoff and sounds open but flat with no drive. Believe me-It was avery expensive experiment!

2.3 Choose drivers and design the horn for them. Here Altec 515-16G were chosen. Remember you must reach the low midrange so heavy "sub"drivers will not work. Use multiple drivers in parallel-5 per channel in this case. This will keep your horn shorter (3m here) yet maintain full horn loading to 21Hz. Again minimise time delays.Do not use an anterior compression chamber. It will increase efficiency but also distortion and the thin paper cones in these drivers do not like it. With 5 drivers you are already at 110db/W with no anterior chamber. Design a rear chamber for the drivers using reactance annulling.That way youl'll get aflat reponse almost right to your horn cutt-off.Buy an RTA and mic. Use McBean's horn response program to evaluate your design. It came uncannily close to the final measured result.

2.3 Order the drivers now! You'll wait several months for them. If you are outside the US prepare for US customs confiscating them and demanding proof they are not weapons of mass destruction (honest - no joke!)

2.4 Draw a template for the horn. While drawing the curves remember to keep the top and bottom surfaces not parallel. 5 degrees(6 degrees here) flare here seems to be OK. Cut out the template ife size in mdf and hand it to the builder. Do not expect your architect or structural engineer to do this. They'll come up with an elegant snail or something. Cut the template precisely.If you start of slightly wrong by completion of built it will be very wrong. Do not design the structural changes yourself unless you are an architect or civil engineer. Get a master builder and tell him 0.5cm tolerances. He'll end up with 1cm but that's ok.

2.5 Wait. Building permission, 3 months, Casting the floor and curing the 75 cm slab 1 month. Building the brick walls and casting the top slab (1m thick at the back) and curing-2 months. Wait, wait, wait. Finally get the weapons of destruction from US customs. Whole project start to finish 1 year.

Step 3. Change the rest of the room. You likely lost a whole wall that now has no windows.Enlarge other windows.Use soundproof glass.It not only holds sound in, it cuts sound from outside-Important considering horns go as quiet as they go loud. This glass is quad laminated and threfore ads a degree of damping.Use special frames to prevent rattle.Get acoustic curtains and blinds.Be prepared to fight standing waves you never knew existed ( See Cristopher Wittmers post below).It took me 7 months to get rid of a nasty 80 hz wave.Pre-built we tested that room with multiple stacks of subs to evaluate for standing waves. The basshorn showed it up in 10 seconds.

Step 4. On completion prepare to write a cheque that will buy a pair of AG Trios with multiple basshorns, or a Wilson X2

Step 5. Now address the rest of the range abve 200Hz. The biggest battle is an appropriate transducer and horn to cover 200Hz to 1KHz.Then crossovers and amplifiers.The basshorn has a below 3 ohm impedance-Big Macs work well here electrically and soundwise (MC 352 here)

Step6 If you are lucky, on completion prepare for sound that will be beyond anything you ever heard-I promise. Unless you heard one of these bass "houses"nothing can prepare one for it. If you are unlucky- I don't know. Fortunately I was lucky.

Like I said -What Chritmas?

In the linked there above “Barn Conversion - James' Project” I have expressed my attitude toward low frequency bass. I certainly not “against” bass horn it but have “concerns”. The “open bottom” concept that I invented for myself is always make me to ask a question: how a badly implemented bass horn (and they are unfortunately are always compromised) would perform against the properly implemented infinite baffle (oversized sealed) enclosure.

I certainly would not argue with Johan experiences of building the bass horn, I did not nave them, but I would like for the sake of bass horn education to point out some moment that I found “slippery.”

Johan Dreyer wrote:

… Choose drivers and design the horn for them. Here Altec 515-16G were chosen. Remember you must reach the low midrange so heavy "sub" drivers will not work. Use multiple drivers in parallel-5 per channel in this case. This will keep your horn shorter (3m here) yet maintain full horn loading to 21Hz. Again minimise time delays. Do not use an anterior compression chamber. It will increase efficiency but also distortion and the thin paper cones in these drivers do not like it. With 5 drivers you are already at 110db/W with no anterior chamber. Design a rear chamber for the drivers using reactance annulling.That way youl'll get aflat reponse almost right to your horn cutt-off. Buy an RTA and mic. Use McBean's horn response program to evaluate your design. It came uncannily close to the final measured result …

The link to Johan Dreyer’s installation: http://cgi.audioasylum.com/systems/8608.html

The Altec 515-16G are 37Hz primary air resonance according to the paper and much higher in the reality. I have 4 of 515-16G, 2 brand ne never use and 2 used. My new drivers had 48Hz primary air resonance, I did not measure the used. So, how the 5 Altec 515 drivers can be loaded into a horn and to get 21Hz I have no idea. Even if one would do not use a back chamber and let them to run with open back then I feel a horn can hardly EQ them lower then ~50Hz. Anechoicly. I have no reasons do not believe to Johan that his horn does 21Hz but it is all room gains and not “maintaining the full horn loading”. BTW, nothing wrong with use of room gain, it is very good in fact but let to be accurate to understand what we deal with. Then, the 5 of 515-16G drivers is a very large mouth. I understand that it keep the horn shorted but it also very much reduces the EQ of the horn, making the 515 drivers to ask at the bottom as direct radiators with excessive exertion.

So, looking at all slippery moments I am asking myself: how the approach with above-mentioned Johan‘s horns armed with 515G drivers would compete with a pair or line-arrays in sealed 40-50 cu feet armed with 515-B/E/LFE drivers?

I have no answer to this question but architecturally I prefer the sealed solution direction. I can see 7 reasons why:

1)  “Open Bottom” concept is preserved
2)   Ability to move the LF sources and position it “strategically”
3)   No time-delay problems
4)   Encouraging to use a dedicated upper bass channels
5)   Cylindrical wave its all benefits.
6)   Since it is an “Open Bottom” solution the “open EQ” is available
7)   Ability to mitigate amount if drivers used.

It is what I have atop on my mind. Using horn I defiantly would like to talk about the good bass horns.  Johan Dreyer did not just talk but actually built a pair. Still, if I have a room for the project that I would think hard if I go for bass horns or for the sealed arrays, perhaps with 18 drivers. I, looking at my “7 reasons” do have a preference to sealed boxes. I do not believe in accuracy of bass horns, particularly in context of system integration.

Romy the Cat wrote:

I, looking at my “7 reasons” do have a preference to sealed boxes. I do not believe in accuracy of bass horns, particularly in context of system integration.

Markus wrote:

Romy the Cat wrote: I, looking at my “7 reasons” do have a preference to sealed boxes. I do not believe in accuracy of bass horns, particularly in context of system integration. What made you change your mind?

Markus wrote:

Re-reading, I realise you stated your preference for a sealed bass array only in relation to Dreyer's bass horn, not to bass horns in general.

http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=7213

There is a difference however between lower bass and midbass horns. The notion is that a horn must not be the lower channel in installation. The lowest channel has to be, in my view, implemented by a topology that supports the “open bottom” operation.