You need to decide what length of the horn you can afford. When I mean “afford” I ment the position of the horn in context of the rest of your system. You know the lowest cut off - 40Hz. So, the most suitable for you length and the mouth size will give you the throat diameter. If you have unlimited space in your room then be advised that starting form some point the increase of length and minimization of the throat will increase loading but also will introduces some unwanted colorations. You see, the longer horn is them more “parallel” it is. A horn that we use for audio have very slow opening at beginning and in very long horn the “beginning of the horn” section is very long, it might be a foot or two of virtually parallel walls. For the frequencies that the 40Hz horn will be caring the “virtually parallel walls” are the essentially the parallel walls.  I have no idea when it becomes a factor but too long horns tend to be congested and colored.

So, you have a horn, you have a throat diameter, then you need to find a driver. To decide on a driver is very complicated ceremony and my absolute rule is following in it: “It is absolutely imposable to predict with a serenity how a driver will sound after it will be loaded in a given horn.” You might do for compression driver, or for conventional driver, in fact any driver that you will be using with back chamber will become a compression driver.   The section of driver is a big subject… Linearity is not something that I would even care; do not think about it as you have no control over it anyhow. I laugh when people talk about linearization of bass horn frequency response.

Where are you? A well-sounding 40Hz horn is very interning accomplishment to experience. If you are near US’ New England and if you listen serious music then I would like to be invited after you finish the projects.

"Meant to be?"

With 100.5 dB efficiency in quarter space I plan to use a dedicated tube amp.  

Yes, some people use high powered amps to see how loud they can get Def Leppard, but I don't see any reason to.  And some tapped horns are also used in PAs with PA amps. 
Just like front loaded horns have different applications, etc. 

I don't have GOTOs... (only in my dreams) just Radians for now, I'll baseline with them. 
I had thought of using a Lowther or pair of Fostex for mid-bass, actually using a pair in parallel. 
I have several options, driver sizes, and throat sizes to play with. 
Somewhere between 8" and 15", with corresponding throat size. 
Probably Fane 8, B & C 10s, or 18 Sound 15? 

It's down to build, listen, change, listen... 
How else can you know how it's going to sound in the horn???
Robert

Romy the Cat wrote:

You need to decide what length of the horn you can afford. When I mean “afford” I ment the position of the horn in context of the rest of your system. You know the lowest cut off - 40Hz. So, the most suitable for you length and the mouth size will give you the throat diameter. If you have unlimited space in your room then be advised that starting form some point the increase of length and minimization of the throat will increase loading but also will introduces some unwanted colorations. You see, the longer horn is them more “parallel” it is. A horn that we use for audio have very slow opening at beginning and in very long horn the “beginning of the horn” section is very long, it might be a foot or two of virtually parallel walls. For the frequencies that the 40Hz horn will be caring the “virtually parallel walls” are the essentially the parallel walls.  I have no idea when it becomes a factor but too long horns tend to be congested and colored. So, you have a horn, you have a throat diameter, then you need to find a driver. To decide on a driver is very complicated ceremony and my absolute rule is following in it: “It is absolutely imposable to predict with a serenity how a driver will sound after it will be loaded in a given horn.” You might do for compression driver, or for conventional driver, in fact any driver that you will be using with back chamber will become a compression driver.   The section of driver is a big subject… Linearity is not something that I would even care; do not think about it as you have no control over it anyhow. I laugh when people talk about linearization of bass horn frequency response. Where are you? A well-sounding 40Hz horn is very interning accomplishment to experience. If you are near US’ New England and if you listen serious music then I would like to be invited after you finish the projects. Rgs, the Cat

Romy the Cat wrote:

Yes, the Dresden’s talk about 40Hz horn and 90Hz horn as the similar horns, particularly if he does not want to "cheat", stroked me as odd too. 90Hz horn is a nice a manageable 40” piece of furniture. The 40Hz is the monster in a house and a good reason to divorce for some women. Also, it VERY hard to differentiate between the “accurate, smooth, full-bodied sound” at 40Hz and 90Hz as at those frequencies the location of the horns (The Macro-Imbedding) and the construction of the horn become too important. I ran 125Hz horn and am getting out it 95Hz in “hot spot” (not the best way to do the thing BTW), so God knows what Dresden will get with a hole 4 time larger. Still, unquestionably the 40Hz is WAY more powerful weapon to fight for good sound. I also do not at ease with notion that the low-passing of long horn is too bad. It is most likely Dresden’s will have a MF channel at 500-600Hz that will decay very fast. Still most of the bass drivers in a straight horn will shoot with excessive HF and the low-pass of a long horn will be good remedy against using a high order low-path crossover on bass channel. The Cat

serenechaos wrote:

There's a big difference in 40 - 400 and 90 - 400 Hz.  40 - 400 might also be stretching the bandwidth which will be horn loaded?  (FLHs by nature are narrow band, 40 - 400 is over three octaves...) And, besides the coloration Romy mentioned, a very long horn will act as an acoustic low-pass filter.  Integration.  I mean all drivers having the "same type voice."  I don't know what you plan below 40 or above 400, but they need to have the same general "Sound."  Or you can end up with something that sounds like so many good drivers playing, instead of one integrated "Sound."  Just a few of the problems I've been dealing with in building a 65 Hz mid bass horn.  Which is MUCH smaller than 40 Hz.  But is easy to integrate with a tapped horn from 25 Hz up to where it comes on.   Something that would hit 40Hz does sound like an interesting challange...  Robert

1)      If you decided to use the 12” JBL 2206H driver then I doubt  that you might talk about 4” throat. A 12” driver with wide outer suspension might go with 7”-8” throat. The 6” might do, though it might be a stretch. The 4” I think it too much.

2)      With 40Hz cutoff of non-rapid Tractrix opening (that most like you will use) and 4” throat you no way will end up with 7'6" horn’s length. I have no way now to look at the right number but how I “feel” is you shell be arriving under 8’ of length and 40Hz cutoff with 8” throat. With 4” throat you will be looking at 12” or so…

3)      If you planning to move next year then it does make send to stats he constriction at the new place. You will take used consideration the idiosyncrasies of the new real-estate, room layout, and the recourses of the local subcontractors. With properly-made mass-loaded 40Hz it might be important.

4)      So, the 3-Way system. 40-400, 500-10K and up most likely. It might do. I personally like the “fundamental” channel that allow MF driver to run free. If you have a good MF driver that cans confidants, linearly and with good quality to all the way down to 500Hz then it is very good.

5)      Your MF channel is very much directional and need be at conventional distance from floor.  Your 40Hz will have a mouth let say 6x4 feet that is very unconventional and will not be sitting next to the MF channels. So, you most likely will hang it above or place them on the sides. The 400Hz of first order it relatively directional signal, so be reaper that the 40Hz will suck imaging out MF channel. Pay attention what Macondo does. It has a small “local” 100Hz horn that cares the “location” of lower MF and ground it to the necessary spot. In this configuration the location of lower frequency channels become less important. Jessie for his Project used the similar configuration and pay attention how offensive he was planning to go – he plans to put his 40Hz horn in other side of listing position…. and who know – it might work and his upper bass horn hold the right location of  fairly directional lower mid range.  There are a lot of moments like this in horn practice.

6)      Rane active crossovers in my view are very sub acceptable, particularly in bass,  but it is me… what we are taking about bass then it is very difficult to go with active devise….

7)      If you have an idea how to make the 40Hz horn visually pleasing then it might be very interesting. I truly would like to see how it will turn out to be. I will do my 40Hz horn if I decide to move, so any ideas of make 40Hz with “humane face” sound very practical to me. Alternately you might go not for 40Hz but for 25Hz and instead of moving to North Carolina you could move into your new 25Hz horn. :-)

8)      If you planning to make the 40Hz horn commercially available than what you feel might be the conditions? Jeffery (from Indiana?) does 40Hz for GOTO driver and the last time he showed up at this site he was looking at $15K for the job. It sounds like here and there reasonable.  Is any specific price target that you have in your mind. I do not ask how much it will cost but rather what you feel is the prices that make you worth to do it. The reason I ask is that if I go for the similar project then I would like to know where the threshold is after (or before) which it makes sense do not do it myself but to outsource it.

Dresden wrote:

… One very important component I'm leaving out of the cost of the horn is amplification (the horns will be active, using class-D amps).  The reason for my choosing class-D amplification is that the modules and various components for building what I consider very capable amplifiers can be purchased at low cost and assembled by me (again, the mantra is to save money for the purpose of appropriating it toward the casting of the horn mouth, said cost as of this date undetermined).  I have found two manufacturers which offer class-D amplification at reasonable costs, said amplification capable of delivering the target power and sustainable performance (one is located in Germany, the other in England).

I see absolutely no rational to go for class-D amplification on bass horn. Leaving inside all alleged imperfections of class-D amplification’s Sound the only advantage that class-D amplification has is cost, size, heat generation and power. Yes, you can buy for $40 a module that will be cold-running and output 200-400W with 95% efficiency but the bigger question: do you need it?

The JBL driver that you selected is 95dB sensitivity (would like to see more but it is me). Using it as a compression driver will give you extra 2-3dB sensitivity; the horn’s equalization will give you 6-7dB. So, we are will be arriving to ~103dB of sensitivity. A proper horn positioning might give you an extra 6dB, let say 2db from the room’s gain to make the number even. So, we are talking about 105dB. Even if you have a large room of 1500 sq feet then with 10W of amplification you will be able to develop sound pressure enough to make you deaf. So, why would you need a powerful amp in there, particularly the class-D amp? As single ended SS or a chip amp for 10-20W will do fine and might cost very much the same or negligibly more. You might look at the many high-quality amp kits available from some high-end oriented companies; some of them might be good. I think that you shell be able to find a prebuilt kit for under $100-$150 (retail) that would have a good SS single-ended amp, two stages amp of 20W. it is what I would be looking if I go for an “active” bass horn.