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deepthoughts

Horn length extension on Othorns?

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I was visiting one of the crazier bass enthusiasts out there this weekend, the infamous pennynike.  Anyone who's been in a few listening sessions with him know he has a serious appetite for bass.  He basically bought a home so he could add 4 of my dual 18", 11Hz tuned, Terraform D18s.
TF-D18-pair.jpg

He had a lust for more, and found Josh's Othorn design.  He grabbed 2 from Josh, and was loving the intensity they could add to the VLF extension the Terraforms provide in his basement bunker.  Of course if 2 are good, 8 should be even better... and some enabler built him up 6 more cabinets. 

That's how he got to this with a pair of SP2-1200 amps powering the 8 Othorns. 😲

attachment.php?attachmentid=2403272&d=15

He had everything set in place before he got around to covering the walls, where this picture shows a lot more of the craziness going on here:
20160913_191705_Richtone(HDR).jpg

 

I also want to point out the pink rigid foam wedged into the window well openings on the front wall...  Those were added after he literally BROKE the windows.

After visiting and seeing what was roughly going on, but not yet breaking out my microphone and just going on what he has measured, it would appear things would be much happier if we change the 2x2 stacks deep, to 3 units set on their side for a 6' tall stack, with the mouth against the front foundation wall.  As the front center location did measure rather well in his room, I'm thinking to put the 2 stacks of 3 firing at each other against the front wall.  This brings me to the question... for this thread.

I've done some creative horn extensions with some of Tom Danley's past designs, with very useful results.  With the 2 stacks firing at each other, lets assume for a moment we confined the height to the 6' tall stack.  This would provide a much tighter spacial load, along with extending the horn length by most of the 36" height of the cabinet.  We can vary the distance between the two stacks to adjust the area of the added segment, and I'm wondering if anyone has a model already in Hornresp who could check what range/type of effect an extension might have?

I'm thinking I would start with about 36" between the two, which gives an extension area for each of about 18" x the 24" height of each for about 36" figuring the 90 deg turn and mouth/end effects.  If anyone had time to model a 12" x 24" (1860 cm2), 18" x 24" (2790 cm2), and 24" x 24" (3720 cm2) extension that is 36" long, it would be appreciated and interesting.

-Mark

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Since Tapped Horns have their horn section defined by the location of the front and backs of their drivers, they do not entirely benefit from extensions. This is the reason that their response shape doesn't change when with multiples, unlike front loaded horns. This cannot be simulated with hornresp as the cabinet use all five available segments. Akabak would be the easiest way to simulate it that I know of, but I don't see an advantage. Don't mean to be the bearer of bad news.

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Those D18's have always been one of my favorite looking designs. And StainlessSteve seem to explain it best already from my understanding.

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EIGHT Othorns?!?!  I don't doubt he broke windows.  To be frank, if he is so concerned about getting the most performance possible from 8 Othorns, I suspect his health might legitimately be in danger, or else he just happens to like testing spaceship parts for launch worthiness in his spare time.

To address the question directly, I am far from being an expert in these things, but what @StainlessSteve says here sounds right to me.  The tapping effectively acts as a tune, where the horn between the driver and tap is 1/4 wavelength at the tuning frequency, below which native response drops with 24 dB/octave, plus the roll-off introduced by the requisite HPF to prevent unloading.  In other words, any boost in extension achieved by reconfiguring the cabinets will be mostly overwhelmed by the natural roll-off of the systems.  An optimized configuration could maybe help boost overall output though, and how the horns interact with the room boundaries is another variable.

I think getting the mouths closer to the front wall is the right move, but I'd be reluctant to fire them into the center of the room because this will result in symmetric side-wall reflections that combine to create a deeper null along the center line of the room.  Why not fire them into the side walls?  This will minimize distance between the mouths and the most rigid boundaries.  If smoother side-to-side positional response is desired, the horns could also be placed firing into the front wall in two-high stacks in a 4x1 configuration.

Of course if the real goal is to get more extension, maybe he should just *upgrade* to Skhorn's which can be tuned a bit lower at the expense of some output.  :)

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Without room gain eight of those cabinets would be able to hit 153dB at 40hz according to Ricci's measurements, with room-gain that would be even more. I would love to see some measurements! Lukeamdman hit 152dB at the listening position with eight ported Incriminator Audio Judge 21 and two Ipal 21" Othorns. And if anything else beats that (besides Tom's sonic boom generator) it might be popalock's 32 sealed 18"s and two sealed 24"s or Leons who built eight ported HS-24"s alongside his four LAB subs.

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I getting a serious lack of creativity in implementation here.:P  If you look at the front right corner, he is seriously limited in placement options there with the sump pump & well, so nothing can directly go in the front left corner.  What's also not obvious is that the right cinder block wall is a mid-wall of the basement with the same size space on the opposite side.  Complicating further, there is currently an opening the height of the floor joists at the top of the wall on the right running the full length of the room.  He is going to close off the ceiling in front first, and then the rest as that will go a long way to make things more symmetrical.  Yes, I already directed him to install high grade plywood so it might survive the high SPL.

I was hoping someone had a model they could quickly adjust to model the horn extension and boundary loading.  Fortunately Josh has the DXF files posted with the side profile.  Some quick measurements and re-approximating the path with just 1 segment from front to back of driver gave me the 4th segment to experiment with as the extension/loading.  The impedance and general shape are correct, with a little less energy modeled in the 40-60Hz range, but not much.

I do understand how the tapped horns function vs conventional horn, as I was working with Tom Danley when he did the first tapped horn design which was to solve an application I was looking to fill.  What started as a dual 12" with a high-flow port, quickly had the driver slide back in the port, and the port was lengthened into what we now know as a tapped horn.

Early on Tom measured a few of the tapped horns in singles vs stacks at Sound Physics Labs and then the early models from Danley Sound Labs which showed that the performance generally tracked the models where added spacial loading often damps the upper bandwidth harmonics.  Take most of the tapped horns and compare them with 4.0 Pi or 2.0 Pi loading vs say 0.50 Pi loading, and you will see the same trend.  Even before we create any sort of extended pathway in front of the horn, stacking the subs in a large block, or creating the same spacial load with boundaries confines the expansion past the mouth to a 90 deg opening that forms a radius out to the edge of the box face. 

Modeling this to an added length of 24-30" beyond the effective length of the single box with an effective area of 6000-10,000 cm2 shows very significant changes to the horn's response, especially in smoothness and broader output gain.  The effect is most beneficial at smaller spacial angles.

Looking at firing the sub into the other stack of subs or a corner boundary, we end up with a 24-36" extension, depending on how it's figured and sized.  Modeling that range with different exit angles shows you get a bit of a saddle in the response, but you can push the lowest excursion minimum down from ~28Hz to below 25Hz.  With 6 units, we're not too worried about the increased excursion around 30Hz, which I'm still seeing to be below ~15mm with 80V input (ouch that'll be loud).

From past experience with BDeaps, LAB Horns, and a few others in confined spaces, planning for room interaction becomes tricky, as the boxes become so big as to almost create new walls or boundaries that other interactions can get minimized or react a bit differently than expected.  For example, if we just stack the subs with the mouths to the center in a 2 wide x 3 tall stack, we now have a 6' square front baffle that is ~3' from the front wall.  Setting the stack of 3 subs with the mouths firing left or right along the front wall, we now have a 3' deep x 6' tall boundary and the rest of the box for the sound to wrap around.  Now it starts making sense to talk about just walling off the front of the room at 3' deep so the subs radiate from flush with an effective wall, but with box faces so large, how much difference will that actually make?  It will be different, but knowing how much gets much fuzzier.  In the end real world measurements and lots of experimentation tells all.

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1 hour ago, StainlessSteve said:

Oh, my apologies Mark. I didn't realize you were so knowledgeable in the area of the tapped horn.

Fun fact... Mark and Jeff from JTR both used to work with Tom D. back in the day. They've both been around the block so to speak. Definitely have high level horn design and all around audio kungfu. 

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On 5/16/2018 at 1:17 PM, deepthoughts said:

 

He had everything set in place before he got around to covering the walls, where this picture shows a lot more of the craziness going on here:
20160913_191705_Richtone(HDR).jpg

 

I also want to point out the pink rigid foam wedged into the window well openings on the front wall...  Those were added after he literally BROKE the windows.

After visiting and seeing what was roughly going on, but not yet breaking out my microphone and just going on what he has measured, it would appear things would be much happier if we change the 2x2 stacks deep, to 3 units set on their side for a 6' tall stack, with the mouth against the front foundation wall.  As the front center location did measure rather well in his room, I'm thinking to put the 2 stacks of 3 firing at each other against the front wall.  This brings me to the question... for this thread.

I've done some creative horn extensions with some of Tom Danley's past designs, with very useful results.  With the 2 stacks firing at each other, lets assume for a moment we confined the height to the 6' tall stack.  This would provide a much tighter spacial load, along with extending the horn length by most of the 36" height of the cabinet.  We can vary the distance between the two stacks to adjust the area of the added segment, and I'm wondering if anyone has a model already in Hornresp who could check what range/type of effect an extension might have?

I'm thinking I would start with about 36" between the two, which gives an extension area for each of about 18" x the 24" height of each for about 36" figuring the 90 deg turn and mouth/end effects.  If anyone had time to model a 12" x 24" (1860 cm2), 18" x 24" (2790 cm2), and 24" x 24" (3720 cm2) extension that is 36" long, it would be appreciated and interesting.

-Mark

 

This looks like a fairly small room, with cinder block walls and a cement floor? Yeesh that room probably has an extraordinary amount of gain to begin with. Blowing out windows??? No wonder. What's the dimensions on that space? I'd bet there is some weird effects going on due to that connection to the other space. It'd be a good data point to see the before and after effects of closing that connection off. 

Looks like if he stacks 6 of the Othorns on the 36x36 side he would have the same height overall but save 12" of depth out into the room. What he does with the other 2 Othorns I'm not sure...Good luck with the endeavor. I'll be interested to see what you find. The 36" extension sounds like it may work and he definitely has headroom to spare so a loss of a few dB midband shouldn't be noticed. 

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1 hour ago, deepthoughts said:

I getting a serious lack of creativity in implementation here.:P  If you look at the front right corner, he is seriously limited in placement options there with the sump pump & well, so nothing can directly go in the front left corner.  What's also not obvious is that the right cinder block wall is a mid-wall of the basement with the same size space on the opposite side.  Complicating further, there is currently an opening the height of the floor joists at the top of the wall on the right running the full length of the room.  He is going to close off the ceiling in front first, and then the rest as that will go a long way to make things more symmetrical.  Yes, I already directed him to install high grade plywood so it might survive the high SPL.

I see (now).  Some details in the photo are hard to make out, what with all those huge boxes in the way.  😉

2 hours ago, deepthoughts said:

...

From past experience with BDeaps, LAB Horns, and a few others in confined spaces, planning for room interaction becomes tricky, as the boxes become so big as to almost create new walls or boundaries that other interactions can get minimized or react a bit differently than expected.  For example, if we just stack the subs with the mouths to the center in a 2 wide x 3 tall stack, we now have a 6' square front baffle that is ~3' from the front wall.  Setting the stack of 3 subs with the mouths firing left or right along the front wall, we now have a 3' deep x 6' tall boundary and the rest of the box for the sound to wrap around.  Now it starts making sense to talk about just walling off the front of the room at 3' deep so the subs radiate from flush with an effective wall, but with box faces so large, how much difference will that actually make?  It will be different, but knowing how much gets much fuzzier.  In the end real world measurements and lots of experimentation tells all.

I think this is key.  Models in Hornresp and whatnot are helpful to understand what happens in an idealized 2pi space, but when you stuff 8 Othorns into a tiny cinder block room whose dimensions are smaller than most of the wavelengths of interest, much of that goes out the window.

How much footprint does that well and sump pump take up?  I don't know if there is space for this, but what happens if you arrange them into two groups of four with all units firing into the front wall?  Each group is two units wide and two units high and the width between the two groups is twice the width between each group and each side-wall.  I'd also invert the unit on top so the mouths are closer to the ceiling, and maybe put them up on platforms to get them closer to the ceiling for better symmetry.  Is there room enough in the left corner for that to work?  I'm guessing the room is maybe 12 feet wide?  So I guess that means 12" between the side wall and each group and 24" between the two stacks.  Maybe that's too close.  Or perhaps it works if there is enough distance to the front wall?

Another option may be to cluster all eight (or just six?) subs in the center, still firing everything toward the front wall but forcing all sound to exit along the side-walls.  In this configuration, the cabinets could be perhaps be angled to create expansions in order to reduce high order resonances and possibly boost output even more.

Obviously, both of these options will work much better if the ceiling and right wall gaps are closed.  Anyway, just throwing out more ideas here.  I know if I had that kind of appetite for bass and a dedicated room, I'd probably just build my system into the structure itself.

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2 hours ago, Ricci said:

This looks like a fairly small room, with cinder block walls and a cement floor? Yeesh that room probably has an extraordinary amount of gain to begin with. Blowing out windows??? No wonder. What's the dimensions on that space? I'd bet there is some weird effects going on due to that connection to the other space. It'd be a good data point to see the before and after effects of closing that connection off. 

Looks like if he stacks 6 of the Othorns on the 36x36 side he would have the same height overall but save 12" of depth out into the room. What he does with the other 2 Othorns I'm not sure...Good luck with the endeavor. I'll be interested to see what you find. The 36" extension sounds like it may work and he definitely has headroom to spare so a loss of a few dB midband shouldn't be noticed. 

It is rather narrow at just under 13.5' wide, with a small divider middle depth and a depth of I think >30 feet..  There is a lot of gain, but the hurdle is really getting the sound to the seats.  Just firing a 36" deep subwoofer with rear SpeakOn puts the front of the sub ~40" from the wall, which immediately sets up some potential issues around 85Hz.  He was also seeing some peculiar behavior when the subs were aimed at the right wall vs left wall I expect due to the big bass trap along the top of the right wall.   I'm hopeful that closing off the ceiling in the front of the room will dramatically improve that peculiarity. 

In my experience, getting the best coupling to the room and delivery of the frequency range to the listening position can easily make for more gains than 6 vs 8 boxes, and I think we can put the last pair to use at the rear of the room.

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13 hours ago, StainlessSteve said:

Without room gain eight of those cabinets would be able to hit 153dB at 40hz according to Ricci's measurements, with room-gain that would be even more. I would love to see some measurements! Lukeamdman hit 152dB at the listening position with eight ported Incriminator Audio Judge 21 and two Ipal 21" Othorns. And if anything else beats that (besides Tom's sonic boom generator) it might be popalock's 32 sealed 18"s and two sealed 24"s or Leons who built eight ported HS-24"s alongside his four LAB subs.

When pennynike1 had (2) Othorns + (4) Terraforms I measured 135db in his room, so +12db going from 2 Othorns to 8.  

I've hit 148db RMS and 154db peak in my room at the LP and I still haven't clipped the three SP2-12k amps.  

 

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