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Chris A

A K-402-Based Full-Range Multiple-Entry Horn

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How did you decide where to drill the throats for the woofers?  Why 2 holes per woofer?  How did you determine the size of the holes?

 

Well, the side entry ports on the conical section of the horn determine the low pass point of the woofers. I chose 475 Hz and used Hornresp to determine where that point within the horn is. I know that was a good crossover point from the last 7 years of using Jubs crossed about at that point to the bass bins.  I wanted to get as far away from the throat of the horn as I could to avoid diffraction issues with the holes because I knew that they needed to be big holes--due to using 15" woofers. 

 

I surveyed all the Unity and Synergy designs and scaled the through hole sizes for each model.  I knew that the holes needed to be in the "ditch" between the side and top/bottom walls based on this survey.  I used the maximum diameter that I dared, and elongated the holes toward the mouth along the "ditches" until I got about a 10:1 compression ratio on the 15" woofer piston area vs. the hole slot area [which is a rule-of-thumb maximum for the compression ratio used in practice]. 

 

No one puts a hole in the middle of a rectangular conical horn wall, I noticed.  I followed suit (I had no reason not to). All the designs have two holes per woofer--except the SH96, which is a special case having one very elongated hole per each of its four 15" woofers--two per side.

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Mark, I believe all your questions are answered above except for the one about "blasting a 15" woofer against a plastic horn". I'm not sure that I've got an answer for that one.  I do have an answer for you on stiffening the horn: it's already stiff in the horizontal walls and mouth areas.  I plan to stiffen the top and bottom side walls.  I have a better long-term solution in mind, but I'm not ready to share that one yet.

 

Chris

Would attaching some Dynamat to the exterior of the horn help stop possible vibration from the woofers?

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I believe that rigidity is preferred to something that damps vibrations when it comes to the horn itself.  Once you're well out of the first 1/4 wavelength zone, then you can damp vibrations.  Unfortunately, for this horn, the entire structure is in the 1/4 wavelength zone.  Rigidity is key.

 

JMTC.

 

Chris

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Question: What would you think about using the JBL 2360A horn (much less expensive to ruin) as a base, and adding a pair of 15" woofers?

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What would you think about using the JBL 2360A horn
I don't think the sides are flat enough in the correct part for them to work, since it has the long diffraction slot. The geometry would, to me, not be close enough.

 

Now that Chris has done this to a K402, I think a lot of folks are slapping their heads and wondering why they didn't see it.

 

Bruce

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Yeah, but trying to find just the horn is going to be pretty pricey...if you can even just buy the horn.

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Ellis63 asked about the trade-offs.
Another unmentioned trade-off is the voltage sensitivity penalty in the treble range for having the high frequency source located at the throat.

 

This isn't so much a player at home playback levels, but is a limiting factor when attempting to scale the design for pro-sound usage. $$$$

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Another unmentioned trade-off is the voltage sensitivity penalty in the treble range for having the high frequency source located at the throat. This isn't so much a player at home playback levels, but is a limiting factor when attempting to scale the design for pro-sound usage. $$$$

 

I believe that Danley solved that issue with his "Paraline" horn folding/manifolding technique, which is of course way off topic for home hi-fi needs.  That invention really nails the coffin on line array technologies, and really sets Danley out in front of the pack.  Kudos to him for that. 

 

Chris

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What would you think about using the JBL 2360A horn
I don't think the sides are flat enough in the correct part for them to work, since it has the long diffraction slot. The geometry would, to me, not be close enough...

 

Bruce

Yes, you need a large horn with a conical throat area...no slots.  The K-402 fits that bill exactly.  See the figure below for how the conical geometry supports Unity horn HF and LF drivers on the same horn:

 

post-26262-0-98420000-1452584762_thumb.g

 

This answers Mark's question above for how do you decide where to put the holes/drivers in the conical horn sidewalls.

 

Chris

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It is possible to build your own dual-flare conical horn of the right dimensions very inexpensively.  Here are some notable DIY projects, and Bill Waslo's Synergy horn spreadsheet can do the heavy lifting in terms of designing the cut sheets.

 

http://www.diysoundgroup.com/forum/index.php?topic=23.0

 

http://www.libinst.com/

 

I recommend a horn size like that of the K-402 (about 35" x 21" mouth, 17" depth).  The coverage polars will be extremely good if you put a secondary flare toward the mouth.

 

Chris

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Chris,

 

I looked at my 402s last night and the approximate area you drilled the holes and mounted the woofers.  It appears the flat surface is only about 10 inches meaning the woofer is bigger than the mounting surface.........discharging into the box over the exterior top and bottom of the horn.  Is this an issue?

 

Does Danley seal his drivers to their mounting surface so they discharge only into the holes?

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It appears the flat surface is only about 10 inches meaning the woofer is bigger than the mounting surface.........discharging into the box over the exterior top and bottom of the horn. Is this an issue?

 

As an outsider and just guessing, I would think he mounted plywood (larger in diameter than the driver) to the horn and then mounted the driver to the plywood.

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Chris,

 

I looked at my 402s last night and the approximate area you drilled the holes and mounted the woofers.  It appears the flat surface is only about 10 inches meaning the woofer is bigger than the mounting surface.........discharging into the box over the exterior top and bottom of the horn.  Is this an issue?

 

Does Danley seal his drivers to their mounting surface so they discharge only into the holes?

 

As mentioned above, I simply sanded down the little stiffening rib that extend down from the throat mounting plate on the side of the horn - about 6.5 inches worth, so that the mounting pad for the woofer fits snugly up against the throat (compression driver) mounting plate at the throat of the horn.  The entire surface of that side of the horn has a very slight rise in the middle, along where Klipsch sanded down the flash material on the mid-line.  Sanding each side of the horn flat took about 5-7 minutes.  The mounting pads only require the smallest amount of modeling clay around the through-holes to the horn to form an air-tight seal.  No biggie.

 

Once I get everything ironed out, I'll probably just epoxy the woofer mounting pads down to the horn sides to ensure air-tightness, since I don't believe that I'm even going try to return this KPT-305 assembly back to its regular configuration.  This New Center loudspeaker configuration is just too good to go backwards again.

 

Mark, when you think about the multiple-entry horn design, it's exactly the same design as one side of a Jubilee bass bin (i.e., there is no bifurcation of the horn into two mouths).  There's a sealed back chamber, a cone woofer mounted to a port in the horn, and the expansion of the horn to the mouth. The only real difference is that the port in this case is not on the apex of the horn, but located about 6.5 inches away on the side wall.  The Danley plot above of the flare rate change vs. axial distance from the throat shows you everything that you need to know about how it works out.

 

Chris

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It appears the flat surface is only about 10 inches meaning the woofer is bigger than the mounting surface.........discharging into the box over the exterior top and bottom of the horn. Is this an issue?

 

As an outsider and just guessing, I would think he mounted plywood (larger in diameter than the driver) to the horn and then mounted the driver to the plywood.

 

Yes Carl--except I used MDF that I had on hand because it's a bit denser and less prone to resonances than plywood.  This is one of two high SPL portions of the horn and it needs to be rigid and free from resonances.  MDF works well there.

 

Chris

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Does Danley seal his drivers to their mounting surface so they discharge only into the holes?

 

Yes, and so do I.  It's just like a Jubilee bass bin: sealed back chamber (air suspension) into a horn through a "slot" (port) sized for the desired compression ratio.

 

Chris

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OK, so the woofer face is sealed to the horn.  That's what I was wondering.

 

The plot above indicates distance from the throat for the holes based on low frequency cutoff.  It seems you used crossover frequency of 475 instead of low frequency cutoff of the woofers (like 30-40 HZ)............which would have placed the holes much further out toward the mouth (as Danley suggests he does).  What am I missing here?

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OK, so the woofer face is sealed to the horn.  That's what I was wondering.

 

The plot above indicates distance from the throat for the holes based on low frequency cutoff.  It seems you used crossover frequency of 475 instead of low frequency cutoff of the woofers (like 30-40 HZ)............which would have placed the holes much further out toward the mouth (as Danley suggests he does).  What am I missing here?

 

The location of the holes relative to the throat determine the highest effective frequencies that can pass through the bass ports, i.e., the acoustic low pass frequency of the bass ports.  This is unique to multiple-entry horns: they have an acoustic low pass frequency for the ports not on the apex of the horn, like an analog low pass frequency in the electrical domain for RLC networks. 

 

The other frequency that you refer to is a typical exponential horn cutoff frequency.  Conical and conical-tractrix horns (like the K-402) have a cutoff frequency, but it doesn't control the true cutoff of the horn like it does for exponential horns. 

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OK.  As I said before I'm impressed.  Took some balls to chop up that horn.

 

I still think it could be restored for tweaking or removal easily by plugging those holes with resin, epoxy, silicone, or whatever pretty easily.

 

Cool project.

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I still think it could be restored for tweaking or removal easily by plugging those holes with resin, epoxy, silicone, or whatever pretty easily.

 

Yes, but who would want to? 

 

I feel the price is cheap for the result--both in terms of the loudspeaker performance and the knowledge gained.  Hopefully some of that knowledge can also be spread about to turn into loudspeakers whose home hi-fi performance you currently can't buy.

 

Chris

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Man, I need to get up there and have a listen.  This stuff fascinates me.

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