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Ooh looky - a TH that does from ~42Hz-500Hz +/-3dB
Ooh looky - a TH that does from ~42Hz-500Hz +/-3dB
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Old 15th September 2016, 03:59 AM   #1
Brian Steele is offline Brian Steele  Grenada
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Default Ooh looky - a TH that does from ~42Hz-500Hz +/-3dB

Stumbled across this while trying to come up with a new TH alignment (POC6) that focuses on minimizing the out of band peaks rather than just concentrating on passband flatness. The driver in question is the cheap Parts Express PA310 12" driver (measured parameters).

The "trick" here seems to make S1-S2 a bit longer than usual, and then stuff it. The peaks can then be brought down substantially in level with only a very minor impact on low frequency performance.

Folding it might be a challenge though. Neither the THAM fold or SS fold seem to be good fits...
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Old 15th September 2016, 06:24 AM   #2
zettairyouiki is offline zettairyouiki  United States
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I think the problem you'll run into is that the closer you get to the top of the passband, the smaller the wavelengths are going to be compared to the horn size. At those higher frequencies the waves will stop oscillating down the horn and start bouncing around within the horn, creating all kinds of interesting interference. A straight FLH can avoid this, but a TH by nature will require at least one fold.

That's just my guess as to what will happen, anyways. I'd still be interested to see what would happen if it were built.
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Old 15th September 2016, 10:14 AM   #3
snappyturtle is offline snappyturtle  United States
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Quote:
Originally Posted by zettairyouiki View Post
I think the problem you'll run into is that the closer you get to the top of the passband, the smaller the wavelengths are going to be compared to the horn size. At those higher frequencies the waves will stop oscillating down the horn and start bouncing around within the horn, creating all kinds of interesting interference. A straight FLH can avoid this, but a TH by nature will require at least one fold.

That's just my guess as to what will happen, anyways. I'd still be interested to see what would happen if it were built.
Similar to what I've read 're: folded midrange horns. I'd suspect a low pass at 200 - 250hz would be used. That being said, I'd be willing to build a prototype as I have a bunch of sheets of ply just hanging out in the garage.
If that dayton woofer could get repurposed into a two way with a seos waveguide I would consider it.
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Old 15th September 2016, 11:37 AM   #4
USRFobiwan is offline USRFobiwan
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Ooh looky - a TH that does from ~42Hz-500Hz +/-3dB
What if you make the folds based on the target upperband frequenty?
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Old 16th September 2016, 01:23 AM   #5
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by zettairyouiki View Post
I think the problem you'll run into is that the closer you get to the top of the passband, the smaller the wavelengths are going to be compared to the horn size. At those higher frequencies the waves will stop oscillating down the horn and start bouncing around within the horn, creating all kinds of interesting interference. A straight FLH can avoid this, but a TH by nature will require at least one fold.
Oh, I have no doubt that the output of this TH won't be so nice and smooth all the way up to 500 Hz when folded up in a box . Based on my POC3 design, I suspect that deviations from the sim will start around 250 Hz, if not a bit earlier, and will manifest as "notches" in the measured response above that frequency. Still, to get a fairly flat passband from just over 40 Hz to just over 200 Hz isn't bad. Perhaps with a better driver and using a similar approach (damping S1-S2 to reduce the out of band peaks), it might even be possible to extend the response at bit lower.

The trick is to come up with a particular fold for this TH. a dual-expansion THAM type fold might work if it can be rejigged to have the drive mounted facing the back rather than the bottom. However that design has a big fold in the S3-S5 section and that will likely result in response aberrations appearing a bit earlier in frequency. A reworked "Keystone" type fold perhaps? There is a bit of unused volume in that fold that can be used for a longer S1-S2. However it's a single-expansion design, so a totally different TH alignment will have to be worked out in HornResp to fit.
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Old 16th September 2016, 04:08 PM   #6
weltersys is offline weltersys  United States
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Quote:
Originally Posted by Brian Steele View Post
1)Oh, I have no doubt that the output of this TH won't be so nice and smooth all the way up to 500 Hz when folded up in a box .
2)A reworked "Keystone" type fold perhaps? There is a bit of unused volume in that fold that can be used for a longer S1-S2. However it's a single-expansion design, so a totally different TH alignment will have to be worked out in HornResp to fit.
Brian,

1) One of the smoother horns from 350 to 6kHz I have owned or measured is the Electro-Voice "Boat Horn" (I unfortunately can't recall the model number), a folded "re-entrant" horn with two (basically) 180 degree turns. Careful attention to folds is needed if smooth response above the transition from laminar flow to ray-tracing frequency is desired. Note the "Boat Horn's" "ripple" does not exceed +/- 1.5 dB above 500 Hz, very simple EQ corrections can make their response virtually flat.

2) The "Keystone fold" is no different than the type of folds used in horns for centuries. My only "claim to fame" (if that ever happens ) is the combination of an easy-to-build fold with an exit shape that smooths out the ragged upper response typical of a "tapped" horn, regardless of fold pattern. Note that the "Keystone Correction" works it's "magic" in the low frequency laminar flow region, fancy corner rounds don't do anything other than reduce valuable cabinet volume in the usual "sub-woofer" range below 160 Hz or so.

The EV/University "Boat Horn" is normally white in color, the pair below have one edge of the horn sawed off for better upper polar response and a smaller package (under one cubic foot) used for yelling at powerboats leaving large wakes in no-wake zones, which flops a sailboat around like a fish out of water. Painted them black when I started using them in parades, they get louder than a dozen bagpipers with only 10 watts :^) .
Using a 12 dB per octave passive crossover, the pair can do over 130 dB at one meter at around 100 watts, well within the safe range of these little aquatic beasties.

I will be adding a small woofer and tweeter to the pair after several other projects are complete, six "B-Low" Keystone subs in the works.

Art ("On the Waterfront") Welter
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Last edited by weltersys; 16th September 2016 at 04:28 PM.
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Old 23rd September 2016, 01:30 AM   #7
Brian Steele is offline Brian Steele  Grenada
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Originally Posted by weltersys View Post
2) The "Keystone fold" is no different than the type of folds used in horns for centuries.
Hmm... it looks like I might have referred to the wrong fold. I thought the KS fold had a feature slightly different to the other popular folds - a bit of unused volume at S1 which could be put to the use I'm considering for this TH design. However I went and reviewed your design and that's not the case at all.

Now I'm wondering whose design I was thinking of....

Here's another example of a "smooth" TH - this time using the Eminence Kappa 3012LF driver, which on paper is a bit more capable than the Dayton PA310. The design process is basically the same - extend S1-S2 a bit, then stuff it, and that starts flattening the response above about 120 Hz or so, with little impact at lower frequencies. Then mod S4-S5 and overall horn length to achieve an acceptable bass response curve. In this case the net size works out to around 210 L, which is a bit big for a TH based on a 12" driver. Or is it?
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Old 24th September 2016, 03:09 PM   #8
weltersys is offline weltersys  United States
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Quote:
Originally Posted by Brian Steele View Post
Hmm... it looks like I might have referred to the wrong fold. I thought the KS fold had a feature slightly different to the other popular folds - a bit of unused volume at S1 which could be put to the use I'm considering for this TH design. However I went and reviewed your design and that's not the case at all.

Now I'm wondering whose design I was thinking of....

In this case the net size works out to around 210 L, which is a bit big for a TH based on a 12" driver. Or is it?
Brian,

210 L is not all that big for a "horn" cabinet, but "big" is a relative term, all depends where you plan to stick it

You may have been thinking of the "stub" used at the beginning of the horn path featured in many DSL TH designs. I have described the "stub" (Tom's term, a reference to antenna design) with examples in several posts, including those in XOC1's DSL TH-118 clone thread.

I had experimented with a "stub" of sorts in the Keystone, but as it did not improve performance in any respect, was not incorporated in the final design.

Art

Last edited by weltersys; 24th September 2016 at 03:12 PM.
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Old 24th September 2016, 03:33 PM   #9
Brian Steele is offline Brian Steele  Grenada
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Originally Posted by weltersys View Post
I had experimented with a "stub" of sorts in the Keystone, but as it did not improve performance in any respect, was not incorporated in the final design.
In the sims it doesn't do anything at all - until you start stuffing it :-). Once it starts to get stuffed, the out of band peaks and dips diminish significantly. The end result is a smoother out of band response with little or no impact on in-band performance. Of the course the question would be if it's out of band, why bother? Well, it could mean that simpler and perhaps higher x-overs can be used, and distortion caused by in-band frequencies would be not be amplified by those out of band peaks...

Anyway, it wasn't the DSL designs I was thinking of. This was definitely a fold more like the one in the KS, but with a portion before S1 blocked off and unused. But who knows - a similar technique may have deployed in the DSL designs too. I've never got a chance to look inside one to confirm.
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Old 24th September 2016, 04:04 PM   #10
tb46 is offline tb46  United States
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Hi Brian,

Were you thinking of Don Snyder's LAB12 TH fold?

Lab12 - Tapped Horn -

And for the THAM fold, you can add another section at the beginning like Forsman did, but turn it around to give you the elongated L12 you are looking for.

Thorn F1 - a learning experiance with Tapped Horn

I'd draw one up, but I'm still not finished w/ my bathroom. :-)

Regards,
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Last edited by tb46; 24th September 2016 at 04:20 PM.
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