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Old 15th October 2016, 02:58 AM   #11
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Quote:
Originally Posted by Entropy455 View Post
Just-a-Guy, I’m a bit confused. Are you suggesting that a Lab-12 driver is better suited for a full-size strait 20 Hz exponential horn, than a driver with a lower MMS, lower Qts, and higher BL? Which driver would you specifically recommend for my application?
No, not at all. I'm suggesting that the Lab 12 was designed specifically for the purpose of being an "ideal" driver for the Labhorn. (Ideal is in quotes because it was changed a bit from the mathematical ideal for a few reasons.)

The Lab 12 doesn't have particularly low qts, low mms or high bl. So how can it be ideal? We'll get to that...

Quote:
Originally Posted by Entropy455 View Post
From the engineering papers I’ve read regarding horn design, the throat of a horn is as close to a purely resistive load on the driver, as one could ever hope to obtain. In an equivalent spring-mass system, the horn’s throat impedance is simply a damper – in which case you need raw & blunt applied-force to overcome the damper (i.e. a high BL). You specifically do not need extra mass (MMS), nor do you need additional spring-K (high Vas), to overcome the damper. The ideal horn driver has a low Qts, mainly due to the high BL, and low MMS. Or am I completely out-to-lunch on my understanding of horn driver selection???
A well designed ideal full size horn is mostly resistive, most horns are far from ideal or full sized. Your project has a chance to be both.

Leach's math from the "On the specification of moving coil drivers for low frequency horn loudspeakers" paper is considered one of the best horn models available for full size ideal reactance annulled horns. And as the name implies, this paper allows you to specify the ideal parameters for both the driver and the horn. This paper is the basis for the Hornresp System Design tool, both the "with driver" and the "from specifications" parts of the tool.

If you run the System Design - From Specifications tool you will find that it doesn't routinely spit out low qts, low mms, high bl specs all the time. It will give radically different ideal driver specs based on the goals (bandwidth, number of drivers, etc).

Danley used Leach's math to design the Labhorn and the Lab 12 driver. You could use it to find the ideal driver parameters and horn dimensions for your project. Or you could use wildly inappropriate drivers instead, it's up to you. You can get good results even with bad driver choices.
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Old 15th October 2016, 03:12 AM   #12
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As far as a recommendation for an ideal driver - I'm not going to make one. A single 12 inch driver with 8 mm xmax coupled to a $20000 concrete horn the size of a small house is not my idea of a good design. I would design it much differently, but then again, that's what makes us all individuals.

Your concrete horn is going to crack (probably in the first year) and even if it didn't, it can be easily beaten in every performance metric by a system costing several times less and a fraction of the size.

Because I like horns this is interesting to talk about but I'd seriously recommend not going forward with this as planned. There are much cheaper and better ways to get a system that will perform much better.
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Old 15th October 2016, 04:36 AM   #13
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Just-a-guy - this is the second time you’ve implied the B&C 12NW76 driver as being wildly inappropriate. Can you please explain why? I’ve been tinkering with the design tool as you’ve suggested, and the driver does not seem wildly inappropriate.

Regarding your hesitation to build horns from concrete - I can summarize ACI-318 for you in one simple blurb: “Concrete is strong in compression and weak in tension, thus install your rebar wherever the concrete is in tension.” Note: this is why 10 story concrete parking garages remain standing safely after magnitude 7.0 earthquakes. . . I can assure you that my horns will not catastrophically fail (or degrade) from cracking.

Concrete is about 100 bucks a yard. I figure I’ll need roughly 30 yards of 3000 psi mix, about 1.5 tons of Grade-40 Number 4 rebar – give or take – depending on the final size of the mouth. About 300 bucks for a pumper-truck rental, and another 1500 in plywood, form ties, and 2x6 boards – making this roughly a 6000 dollar project (less the cost of the construction permit, which I’m sure the county will require). I already own a dozer, excavator, and a forklift – thus my dirt-work costs are practically non-existent.

Nonetheless, I am always open to new ideas. Which type of system would you recommend that I construct – that will outperform a pair of 20-Hz straight exponential horns, in every performance metric - costing several times less, and at a fraction of the size?
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Old 15th October 2016, 05:04 AM   #14
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Has anyone actually heard (with their own ears) a pair of outdoor full-size 20-Hz straight exponential horns? Or even seen a picture of one? I personally have not – in either case.

The only person I’ve ever talked with who claims to have heard a set, is a design engineer at Radian. He said that when you remove the ceiling & walls from the equation, the nodes completely go away, and then the magic happens. You must stand back from the horn mouth at least 1.5 wavelengths, so that the horns may properly load the air. The sound is full & intense, and disappears into the abyss just as quickly & completely as it was formed. There’s no need to construct tube-traps, or Helmholtz resonators - to absorb standing waves & residual energy. No room rattles, no loud spots, no nulls - it’s just pure sound.

That conversation was my inspiration for this project. . . . I intend to use Radian drivers for the highs.
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Old 15th October 2016, 06:07 AM   #15
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Originally Posted by Entropy455 View Post
Are the colored graphs and off-axis plots you posted from Hornresp, or are they from another software?
All the charts shown in bjorno's post are from Hornresp.
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Old 15th October 2016, 08:55 AM   #16
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Originally Posted by Entropy455 View Post
...when you remove the ceiling & walls from the equation, the nodes completely go away, and then the magic happens. .....
That conversation was my inspiration for this project. . . . I intend to use Radian drivers for the highs.
I often felt that the sound of a Klipsch bass folded corner horn filled a room with clean wonderful unboxy sound like nothing else (and made an oddly excellent match to clean ESL speakers).

Granted, it is wonderful to be able to play notes an octave lower than any feasible horn can go. Feels terrific. But the plain truth is that ordinary music rarely goes below maybe 35 Hz, no kidding, and no big loss if your sub can get no lower. Now-a-days, easy to add a sub to handle music south of 35 Hz to supplement the horn, as a buddy of mine was doing in 1967; concept may sound peculiar to you, but two Klipschorns and two sub-subs sure sounded great (and he built his house around his speakers).

How do you time-align a 15 foot long horn with the rest of your speakers?

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Last edited by bentoronto; 15th October 2016 at 08:58 AM.
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Old 15th October 2016, 09:22 AM   #17
AndrewT is offline AndrewT  Scotland
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Originally Posted by Entropy455 View Post
Has anyone actually heard (with their own ears) a pair of outdoor full-size 20-Hz straight exponential horns? Or even seen a picture of one? I personally have not – in either case................
There's a Scandinavian Guy that built a wood outdoor horn. Maybe 8 to 10 tears ago.
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Old 15th October 2016, 01:35 PM   #18
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Originally Posted by Entropy455 View Post
Just-a-guy - this is the second time you’ve implied the B&C 12NW76 driver as being wildly inappropriate.
I never said the driver you chose was inappropriate. I implied it might be inappropriate based on horn math.

Quote:
Regarding your hesitation to build horns from concrete - I can summarize ACI-318 for you in one simple blurb: “Concrete is strong in compression and weak in tension, thus install your rebar wherever the concrete is in tension.” Note: this is why 10 story concrete parking garages remain standing safely after magnitude 7.0 earthquakes. . . I can assure you that my horns will not catastrophically fail (or degrade) from cracking.
Parking garages have cracked concrete. If you want to spend your time patching concrete that's up to you. This is not how I would do it.

Quote:
Concrete is about 100 bucks a yard. I figure I’ll need roughly 30 yards of 3000 psi mix, about 1.5 tons of Grade-40 Number 4 rebar – give or take – depending on the final size of the mouth. About 300 bucks for a pumper-truck rental, and another 1500 in plywood, form ties, and 2x6 boards – making this roughly a 6000 dollar project (less the cost of the construction permit, which I’m sure the county will require). I already own a dozer, excavator, and a forklift – thus my dirt-work costs are practically non-existent.
Baltic birch is about $55 a sheet. Glue is about $20 a gallon. Paint, duratex or truck bedliner isn't much either, depending on which product you use. You could build this out of wood for a couple thousand dollars max. And it would be a lot less work and take a lot less time.

Quote:
Nonetheless, I am always open to new ideas. Which type of system would you recommend that I construct – that will outperform a pair of 20-Hz straight exponential horns, in every performance metric - costing several times less, and at a fraction of the size?
First, straight vs folded has no practical significant at subwoofer frequencies.

Second, the horn itself isn't a huge problem, I see more problems with your choice of a single 12 inch driver. For a 20 hz full size horn I would likely not use less than eight 18s. That balances the driver cost and the horn cost and allows for orders of magnitude better performance.

Third, let's just run the sim, as I've recommended to you twice already and see what the sim says. This is a 30 second attempt so it's not optimized in any way. I chose a low compression ratio and a relatively small rear chamber to keep excursion in check, 2 pi space and Sd = eight 18 inch drivers and a 20 - 100 hz passband. I let the simulator fill in all the blanks for horn and driver specs.

Click the image to open in full size.

I didn't run your sim but I'm pretty sure this horn is a lot smaller than yours and will go a lot louder with a lot less distortion. And it's just a 30 second first pass, it could be better in a lot of ways.

For construction, I'd build it as 8 modular baltic birch folded horns.

And it would beat your design in every conceivable performance metric.

Quote:
Originally Posted by Entropy455 View Post
Has anyone actually heard (with their own ears) a pair of outdoor full-size 20-Hz straight exponential horns? Or even seen a picture of one? I personally have not – in either case.

The only person I’ve ever talked with who claims to have heard a set, is a design engineer at Radian. He said that when you remove the ceiling & walls from the equation, the nodes completely go away, and then the magic happens. You must stand back from the horn mouth at least 1.5 wavelengths, so that the horns may properly load the air. The sound is full & intense, and disappears into the abyss just as quickly & completely as it was formed. There’s no need to construct tube-traps, or Helmholtz resonators - to absorb standing waves & residual energy. No room rattles, no loud spots, no nulls - it’s just pure sound.

That conversation was my inspiration for this project. . . . I intend to use Radian drivers for the highs.
Again, straight vs folded horns have no practical distinction at subwoofer frequencies.

And the audible characteristics of the outside environment applies to any system. The only thing horns will do better than a Bose wave radio is efficiency. You make it sound like you want to build this massive thing outside instead of properly treating a room - I think that's a dubious motivation. If this is all about the audible experience it's better to do it properly indoors. Then you can experience it whenever you want. How often are you going to be sitting outside for critical listening sessions?

Besides, listening outside isn't all that great. Rooms create a spaciousness effect that you can't get outside (unless you are setting up a 7.1 system outside, which at this point wouldn't surprise me).
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Old 15th October 2016, 04:10 PM   #19
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You suggested that I construct a different system that will outperform a pair of 20-Hz straight exponential horns, in every performance metric - costing several times less, and at a fraction of the size. And your recommendation was that I build a pair of 20-Hz straight exponential horns. . . I love it! You’ve recommended (qty 8) 18s in the throat to bat – I love that too! As the whole point of this thread (original post) was how to properly incorporate multiple drivers into a bass-horn design. . . .

I have a few conceptual questions though. Background: I’ve built dozens of sound systems over the past 25 years – primarily bass-reflex enclosures. For right or wrong, here’s a few of my observations:

18s sound like 18s. Even with a mega-motor structure, the higher MMS degrades the transient response, and fidelity suffers. And 18s with a lighter cone sound better (tighter bass – primarily due to the faster transient response) – however these drivers are often Fs-limited in how low you can tune a bass-reflex enclosure - which kinda defeats the purpose when you’re trying to fill the bottom end. I’ve played around with the 15” long-throw woofers – big rubber surrounds, low FS, workable VAS, with a giant Xmas -and these speakers sound like mud also. Don’t get me wrong – they can reproduce bass, but they simply don’t sound good. They sound rubbery compared to an accordion-surround driver. And yes, I was feeding them plenty of clean power (MT-5000, active crossover, heavy-gauge leads).

21 inch drivers – I’ll never buy one. Practically every review says they sound like mud (for the reasons stated above).

12 inch drivers can chew the low-end out with clean authority. However you need a wall of them to fill a large room – which adds an additional set of problems in itself. Actual VAS values can vary quite a bit - even with speakers from the same assembly line, manufactured on the same day. When you have a wall of slightly out-of-alignment speakers, you can hear it. And sizing each enclosure for the specific driver (while pure in the realm of high-fidelity) is a cumbersome work practice in my book. It also causes problems if you ever need to replace a driver. Additionally, it has been my experience that the really good sounding 12” drivers have a higher Fs anyway (primarily due to the high BL & lower MMS), and cannot tune very low anyway.

This is what’s attracting me to horns – the idea that a speakers Fs is not a major design obstacle. The resistive load of the throat provides significant damping at FS, essentially minimizing the effect. The drivers that are suggested for bass horn applications (high BL, low MMS, low Qts), are good mid-bass drivers at best – in a bass reflex application.

Here's my question just-a-guy, will 18s still sound like 18s, in a horn application? Your simulation recommends an MMD of 323 grams. That’s nearing 21” driver territory. . . .
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Old 15th October 2016, 04:16 PM   #20
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Here’s a much more significant design question issue. It’s the concept of half-space, vs full space. I understand the basic idea – in that if you put a horn along the ground (half-space), that the ground itself acts as an extension of the horn – where you can theoretically half the size of the horn mouth, and still achieve a reasonable SPL. Problem – one of the engineering design papers I read was able to mathematically show that the ideal horn mouth circumference – for both full-space, and for half-space loading - is equal to approximately the lower cutoff frequency wavelength of the horn – which is the full-space horn mouth area in both regards. This engineering reference clearly shows that using half-space to reduce the size of the horn mouth, while effective in its ability to reproduce the desired SPL, will cause appreciable increases in horn harmonic distortion.

Question: how significant is this introduced harmonic distortion phenomena (to what the ear hears?)

I’m trying to picture myself cutting the mouth of a trombone to half its original size, resting it up against the wall, then playing it and expecting it to sound just as good. . . . Will it really?

Last edited by Entropy455; 15th October 2016 at 04:40 PM.
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