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In the Forum: Horn-Loaded Speakers
In the Thread: Jessie Dazzle Project
Post Subject: Details : Mid & upper bass hornsPosted by jessie.dazzle on: 7/17/2008
Romy's quotes in blue:
Daniel J. Plach's quote in green:
"...The back chamber mechanism is good. Are you planning to do the same in the midbass horn?..."
Yes, it will be similar, but more robust; I have an idea for a locking-type seal, which I would probably double. If that doesn't do the trick, I'll make a permanent plug corresponding to the correct volume, once that has been established.
"...Be advised that the driver will generate a lot of pressure in there and the sipping of air will reduce the effectiveness of the back chamber..."
The original idea was to use the adjustable chambers to find the correct volume, then replace the sealing mechanism with one that would not permit sliding the piston, but would seal absolutely air-tight... As it turns out, the adjustable seals are already very air-tight; this however may change with time due to seal compression... It's something I will have to monitor and decide about in time, but it does not present any great obstacle to me.
"...Also, do not forget that most likely you will end up with the ultra low size of back chamber when the vertical back wall sitting to the driver magnet will form already too large chamber. What you might end up doing is to glue to the back of the slideable wall a space-killing indentation that would have a contra-template of the Fane driver. This would effectively to push out the air from the driver sided..."
Yes, this is exactly the plan, once ready to integrate the mid-bass horns. I currently have the horns tuned for 115Hz, which puts the piston somewhere in the middle of its stroke (This was not completely the result of luck; I did some calculations to get a feel for the volume before deciding on the dimensions of this chamber; I did not at the time trust my understanding of the formula).
"...I would put even some filing into the driver frame’ holes…"
Once I have the mid-bass horns, my thoughts are to start rolling off the lower end of the upper-bass horns somewhere around 200-240Hz, meaning I would probably tune the upper-bass horns for the same 200-240Hz. At that frequency you may be right; a really small back chamber volume may be necessary.
"...Also, turned John Hasqiun to your, Jessie, project and he pointed a very valuable point that you might overlooked. He suggested that this design shell have a way to vent the back chamber as it being adjusted, otherwise he will over or under pressurize the woofer cone and damage it..."
No, I did not overlook this detail... If you go back to the photos of the piston, you will see a little brass bleeder valve (read the captions under the photos for more explanations).
"...Going for exponential midbass is probably good idea; most likely I would do the same if I go for 50Hz horn..."
Well I have been considering Hyperbolic/Exponential because... Mainly because I seem to remember it is somewhat shorter than pure exponential, while offering other advantages, which have slipped my mind (it's been about a year and a half since doing research for the conception of this horn). Hang on a minute...
Daniel J. Plach (AES E-Library) wrote:
"...The use of hyperbolic-exponential horns makes possible practically any desired resistance or reactance characteristic near cutoff. The hyperbolic-exponential horn allows much more uniform transmission down nearer to cutoff than is possible with the exponential type..."
???
"...You might consider to make the double-action midbass horns. What I mean is that pretend that you horn is 8 feet. So, you make the first 5 feet from the throat is firm and solid and the last , the most larger 3 feet, detachable and perhaps with odd edge profile. The next time you move and realize that your horn would block your entrance in bathroom, or the corner of the horn blocks a half of your window and look to prominent on the room then you juts change the termination part of the horn…"
Yes this is probably a good idea, but I may have a difficult time with it knowing that 1/3 of the horn is missing. Also, I am free to base future real estate decisions on the dimensions of the finished system. One very real constraint is making sure the parts will all pass through the door of the place where the horns will be built.
Construction plan :
In order to make the walls of the horns rigid enough, without going into the kind of weight that only cranes can move, I have decided to make the flare (everything down stream of the driver mounting plane) like a surf board; meaning from a composite sandwich consisting of a double layer of glass fiber, then 2" of rigid urethane foam, then another two layers of glass fiber. The result should be a fairly light-weight non-resonant structure. The back chamber (meaning everything up stream of the driver mounting plane) will be made using the same materials that went into the other horns (hemp-reinforced refractory plaster, in this case, having a wall thickness of around 3").
"...It is very interesting to see what you do Jessie, in contrary to many audio people I know who just endlessly run their mouth on audio subjects you actually do the things..."
Well I will say that it does entail sacrifice, but what is being sacrificed in my case is really no big deal. One of the things I have been telling myself all throughout the course of this project was that (contrary to some of the other ball-and-chain projects with which I have burdened myself) this one will be a really nice thing to have should I manage to live to retire and grow gracefully old. Though I don't have a turn table, when you wrote about your "End of Life Phono Corrector", I understood exactly what the title meant.
jd*
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