This article was written by Johan Dreyer in 2006 posted at AA and the reposted recently at a Johan's local South African forum:
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:
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? All this as a semi joke. I can never tell you the sensation of having spent supercar prices on something that you have no idea how it is going to turn out. Then at 2am firing up that basshorn for the first time, run a freq sweep through it, run to the spectrum analyser and know that it is good. I fully understood why even God needed to rest on the 7th day!
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche