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80Hz Tractrix Bass horn with K-31 12" woofers


jwc

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I am long overdue for DIY on the forum. Work has been a little too much for me this year. I have drawn up lots of stuff but haven’t been able to have enough time to follow through.

I posted some projects in the past and here are some of them if you are interested:

http://forums.klipsch.com/forums/t/84534.aspx

http://forums.klipsch.com/forums/t/77247.aspx

http://forums.klipsch.com/forums/t/58314.aspx

http://forums.klipsch.com/forums/t/62576.aspx

http://forums.klipsch.com/forums/t/71921.aspx

http://forums.klipsch.com/forums/t/56013.aspx

I came up with some bass horn designs to try and “match” the Jubilee. I think the La Scala bass bin fits the bill really but this was for the fun of it. Everything I drew up had some compromises or a few uncertainties.

So I built something for a test cabinet really. It is a 80Hz Tractrix Horn loaded with two K31’s (like the jubilee). The idea was to match the SPL of the Jub with the requirement of a sub….which some people are gonna do anyway with the jub.

I also wanted to compare to the La Scala II bass bin in “performance”. Mainly to compare SPL and “thwack”. Haven’t done this yet.

Tractrix bass horns don’t seem to be a favorite concept over the exponential. Others can jump on here to elaborate. But this horn wasn’t really intended to meet the demands of the real low frequencies.

This could also do as a midbass module on top of the MWM! It is about as tall as the module provided by Klipsch but has two 12’ woofers instead of one. Just slightly taller and narrower than the scala bass bin.

There are some other midbass woofers that I’m sure that would outperform the K31’s here…I have plans to try those Later. Right now…the back chamber is volume calculated for the dual K31’s. I have a list of other 12” woofer back chamber requirements that allow me to use this back chamber or easily change to another.

Right now I don’t have the open cell foam in the back chamber and I haven’t mounted the side braces to prevent resonances. Will be a little while for me to get to that.

I’m not gonna spit out too many details right now as far as physical measurements. I will provide that later for those interested.

jc

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This curve is from one meter from the exit of the horn. Mic on a pillow. The bass bin is about 4-5 feet out of the corner. Pink noise.

I’m pleased with what I’m seeing. I bet would be better after bracing.

jc

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Looks pretty good. Did you ever do any modelling in Hornresp? If not, I would be interested in plugging in some numbers just to see how things compare. The nice thing about straight horns is they should be really predictable and behave well.

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I posted some projects in the past and here are some of them if you are interested:

Oh, man; I really need to make friends with someone with your woodworking skills. I can crank out designs like doughnuts, but I can't saw a straight line with a laser beam.

Tractrix bass horns dont seem to be a favorite concept over the exponential. Others can jump on here to elaborate. But this horn wasnt really intended to meet the demands of the real low frequencies.

There may be a way around that problem. A couple of years ago I came up with a modification to the tractrix equations that led to what I call the "Elongated Tractrix" contour. It maintains the full 90° mouth of the tractrix, but the horn is longer, like an exponential, and in fact the overall contour is closer to a hyperbolic. I corresponded with Bruce Edgar (no relation!) about it, and he seemed interested at first. But then he simply disappeared, and I don't know whether it was because it was not a viable idea or he just got busy with other things.

I'm at work right now, and the derivation is on my computer at home, but if anyone is interested I can post it later.

Nice, nice work. I am envious.

Greg

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Mike, I have downloaded that program several times. I just never got into it. I some seem to go back to doing everything on paper. Helps me visualize things. Also somewhat "therapuetic" drawing the design.

The low end is what I would expect but I didn't think the K31's would go out that far. I'll get some horn numbers to you soon and you can try different drivers to simulate. Real easy to physically change drivers and potentially change the back volume if necessary.

jc

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Greg, I may take you up on that at some point. Right now I'm not interested in building a real low kicker. For one...they just get to big.

If I do another straight horn I will probably go for even a higher cutoff and make the horn wider. I'm interested in achieving the midbass "slap" and just more so interested in seeing a curve that follows along with what I planned. I might even design one that is only good down to 200Hz but go just beyond 2K Hz. Dual 10" woofers may be the way to go here.

BUT. If I make with the intentiaon to actually use the horn way out to 2K, I may put in a little more complex build to eliminate the "vertical non expansion". That may be the dip int the 200Hz range on the curve. Don't know.

jc

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I might even design one that is only good down to 200Hz but go just beyond 2K Hz.  Dual 10" woofers may be the way to go here.

Most woofers won't go to 2K. Using Keele's equations, I find that the best 12" are lucky to get to 500Hz before mass-rolloff, and the best 10" only go to ~800Hz. For 2kHz you might need to use a dedicated midrange driver or some EQ.

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I was really impressed with the Altec 816 clone that the late William Eckle built for a friend of his. Four of them, in fact. 3/4" mdf with 1/4" oak over that, for 1" thick walls. Two cabs on each side in an MTM arrangement, with an Altec 311-60 horn in the middle.

Bruce

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Just in case anyone's interested, here's an example of the "Elongated Tractrix" contour that I mentioned earlier. In this case I have set its length equal to that of an exponential horn with the same cutoff frequency. As you can see, the Elongated Tractrix acts like a hyperbolic horn over the majority of its length (good low frequency loading), and then expands like a Tractrix near the mouth. I have not done ANY analysis of this contour other than to modify the Tractrix equation to be able to generate it.

Greg

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This bass horn is 1/4 space. So would have a lower response up against one wall. I haven't run a curve on the wall.

Anyway. I can only do relative SPL with some kind of reliability. So what I did was run a curve with the mic exactly one meter from the middle of the mouth exit of the horn. Bass horn about 5 feet from any wall.

I also did this with the K69-510.....exactly one meter from the mouth.

Same input levels with pink noise.

We have seen the raw curve of the K69 with the 510 of Roy's. Mine looks similar. For interest sake I circled the two initial peaks on the 510 curve that are like the two peaks on Roys previous curves. They are at 111db and 108 db. So by looking at my comparison curves......you can see that my dual K31's horn loaded put out some high SPL.

jc

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Cinema specs K502 with K69

Am I Going overboard here with assumptions.

Hey jc very nice project!

You probably know this but the Frequency Response spec in the cinema brochure is taken behind a screen with horn eq applied.

jc as much as you like doing projects and trying to understand the results I believe you would really benefit and enjoy using programs with time based measurements to more completely investigate what and why you are getting the results from your projects. The ETF/R+D and programs like Dr Who has been using are very reasonable in cost and would give you very valuable results that just simply aren't available from a RTA program. It would also be a great learning experience as you get deeper into this hobby.

mike tn [:)]

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"...You probably know this but the Frequency Response spec in the cinema brochure is taken behind a screen with horn eq applied...."

Yep...knew that one..... The idea is to "grossly" image the bass horn putting out SPL at a level similar to the compression driver.

I know.....I could spend a little dough to get better equipment. I'm not ready to publish......all in fun.

jc

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