B&C 21" Tapped horn idea - Page 4 - AVS Forum
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post #91 of 109 Old 09-01-2013, 06:15 PM
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how about something like this?

1w1m 2pi space:


hornresp:


inside (excluding bracing):


max spl at 2000 watts and 15mm excursion:


acoustic impedance:


edit: in the hornresp model lrc is not accurate, but it doesn't change the model results at all.

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post #92 of 109 Old 09-01-2013, 06:30 PM
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and maybe throw some corners in there to reduce turbulence around the corners.


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post #93 of 109 Old 09-02-2013, 06:05 AM
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and maybe throw some corners in there to reduce turbulence around the corners.

There isn't any 'turbulence' in a bass horn. However, where the distance from bend to bend approaches 1/4 wavelength portions of the wave can reflect back, resulting in cancellations. The main purpose of reflectors is to eliminate parallel faces at either end of each segment that can lead to those cancellations. When the distance from bend to bend is more than 1/4 wavelength cancellations won't occur, and the wave passes intact, because the principles of diffraction apply, and with wavelengths in excess of 14 feet the bends are acoustically invisible.
'Turbulence' will occur with folded horns that operate into the midrange, where the shorter wavelengths act more like individual particles than as a single cohesive entity the way long wavelengths do. That makes flat reflectors relatively ineffective above 800Hz or so, but rounded reflectors will minimize cancellations an octave and more above that. But by the same token rounded reflectors don't work any better than flat reflectors below 400Hz or so, as the symptoms they address don't occur that low.

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post #94 of 109 Old 09-02-2013, 06:40 AM
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"There isn't any 'turbulence' in a bass horn."

that is what you have said in the past and I disagree. there is non-linear air flow around corners. if turbulence is not the correct technical term, fine, but the air flow directors in the corners help smooth the air flow. they may have other advantages as well, but smoothing air flow is certainly one of them.

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post #95 of 109 Old 09-02-2013, 08:05 AM
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they may have other advantages as well, but smoothing air flow is certainly one of them.
Not IME. Corners, bends, braces and anything else in the horn that can cause all sorts of problems with short wavelengths are no more an obstacle to very long wavelengths than is a house of cards to a bulldozer.

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post #96 of 109 Old 09-02-2013, 08:10 AM
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"Not IME. Corners, bends, braces and anything else in the horn that can cause all sorts of problems with short wavelengths are no more an obstacle to very long wavelengths than is a house of cards to a bulldozer."

even with a constant air flow, which would correlate to 0hz, sharp bends give rise to turbulence which disrupts and impedes the air flow.

we just disagree on this one. no problem.

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post #97 of 109 Old 09-02-2013, 10:52 AM
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"Not IME. Corners, bends, braces and anything else in the horn that can cause all sorts of problems with short wavelengths are no more an obstacle to very long wavelengths than is a house of cards to a bulldozer."

even with a constant air flow, which would correlate to 0hz, sharp bends give rise to turbulence which disrupts and impedes the air flow.

we just disagree on this one. no problem.
As always the proof lies in the testing. I've made more than a few prototype cabs both with and without reflectors, as well as reflectors of different shapes. With respect to sub horns I found no advantage to using reflectors unless it was to be expected that the user might run them high enough for a 1/4 wavelength bend to bend distance to be realized.

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post #98 of 109 Old 09-02-2013, 01:07 PM - Thread Starter
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As always the proof lies in the testing.

And tested it shall be. Since this one won't be built in place, I plan to assemble it at a friend's car audio shop, where there will be plenty of room. Once finished, there is a large empty lot well away from anyone that might be disturbed or excessive traffic noise, and we can find out what it will really do.

On my list of "to do's" is to build a cardboard mock-up and see if a 4' x 5' X 2' structure (guess calling something that size a structure instead of a box is semi-appropriate) will fit down the stairwell. Otherwise it's all a moot point.

Am I inferring correctly that while the need for reflectors might merit debate , in the worst case they would not be harmful, certainly be easy enough to do, contribute little in the way of weight & might even stiffen things up a bit?
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post #99 of 109 Old 09-02-2013, 02:25 PM
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not sure if I linked it up before, but here is one that is that size:

http://www.avsforum.com/t/1420594/hornsub-with-lms-r-how-to-construct/30#post_22333009

not too bad.

the reflectors won't hurt at all unless they are way too big and choke off the flow around the corner.

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post #100 of 109 Old 09-02-2013, 04:40 PM
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the reflectors won't hurt at all unless they are way too big and choke off the flow around the corner.
Google 'Huygens wavefront construction'. You should be able to find his take on reflectors, though it applies only to the midbass. At any rate, he does point out, correctly, that a constriction of the path through the bend caused by a flat reflector has little, if any, effect. That's because the restriction works according to Venturi's Principle. They'd have to be grossly over sized to create a problem. But, to be on the safe side, I size reflectors on the conservative side, just large enough to present an oblique angle to most, but not all, of the oncoming wave front.

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post #101 of 109 Old 09-13-2013, 07:19 PM - Thread Starter
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Okay, good news! The mock-up fits down the stairwell. Now, what do I do about turning this picture into some measurements? I know a little basic algebra and geometry, but need a few points to work from ... I'd like to keep things as precise as possible and not just "Kentucky windage" it.

What I am a bit chagrined to admit is that I am still a very vague as to where the lengths of the segments are measured, and just where the cross-sectional areas are measured. Any help would be welcome indeed. The lumber is just gathering dust, in the way, and I am sooooo looking forward to hearing what difference another 15dB at 22Hz makes in the soundtracks, kick drums at 125dB at my MLP, etc. biggrin.gif Just need to figure out how to cut my lumber up and where to tack it together.

I appreciate your patience and all the input!
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post #102 of 109 Old 09-13-2013, 07:55 PM
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LOL! but can it be moved safely???

"I know a little basic algebra and geometry, but need a few points to work from ... I'd like to keep things as precise as possible and not just "Kentucky windage" it."

no worries. i'll take a look at the file again and see where it stands. most of the dimensions need only be close, but some of them are important and we'll get those precise.

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post #103 of 109 Old 09-13-2013, 09:28 PM - Thread Starter
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LOL! but can it be moved safely???

The drivetrain and Caterpillar treads aren't a problem, as I'm scavenging them from a skid-steer and they should mate up fine ... haven't quite figured out how to route the exhaust but that's NBD wink.gif

But all kidding aside, I AM going to have to rig a block and tackle from the stairwell ceiling to heist it over the stairwell dividing wall. Small effort for the payoff, heh heh
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post #104 of 109 Old 09-13-2013, 10:46 PM
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these should be pretty close if i haven't made any gross errors. we should double check in the morning...

the green, red, and blue sections correspond to the paths in hornresp.

i removed the flow directors and have not shown any bracing on the measurements in order to try to keep that as clear as possible.

lots of good bracing will be required.

the first couple measurements in the horn path in front of the driver are fairly important, try to get them exact. i have marked them with an = sign. the rest are marked with a tilde ~ which means get them close, but if they are off by a bit, it won't matter.

of course, performance will change a little bit if next to a wall, aimed into a corner, or other placement, so some experimentation will be required for optimal performance. i think aiming it into the corner will shift the whole response down a bit, but i'm not 100% on that.

the measurements are "inside panel to panel" so don't count the thickness of the wood. the measurements are the air space only.

attaching some small 1"x2" cleats next to where the panel go might help make assembly a little easier.

at least a second order high pass filter at 20hz as a starting point for protection would be advised.











frequency response, rescaled, 94 volts (1100 watts into nominal 8 ohm load), 2pi space.

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post #105 of 109 Old 09-14-2013, 09:26 PM - Thread Starter
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That is enormously helpful, my thanks.

I'm anticipating that the cross sectional area at S-2 will need to account for the area across the baffle cutout and the thickness of the wood ... pretty straightforward arithmetic, but if that adjustment isn't made, it seems the compression ratio would be slightly low?

If the baffle is 1.9cm thick, and the cutout is ~~ 50cm in diameter, that's a cross-section of 95.3 square centimeters. With S-2 being 600 cm square, or the 3.88cm X 60.96cm width without allowing for the area of the cutout ... okay, if I subtract the cross of the cut out from the total desired cross of S-2, and divide the result by the width of the throat, I come up with 600 minus ~95 = 505, then divide that by the width of the throat, (600 - 95)/60.96 = an actual distance adjusted from 9.85 cm ( 3.88") to 8.28cm (3.26") between the baffle and the other face, figures slightly rounded off.

Does that make sense? A little out of my depth as usual.
This really is going to happen, and I'm determined it will be before the next ice age!

"frequency response, rescaled, 94 volts (1100 watts into nominal 8 ohm load), 2pi space" Looks pretty strong to me :^)
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post #106 of 109 Old 09-14-2013, 09:47 PM
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"I'm anticipating that the cross sectional area at S-2 will need to account for the area across the baffle cutout and the thickness of the wood ... pretty straightforward arithmetic, but if that adjustment isn't made, it seems the compression ratio would be slightly low?"

that would be considered in vtc (volume throat chamber) and atc (area throat chamber). since it is the area of the driver, enter 1680 for atc and since it is 2 cm or so high, enter 3360 for vtc. it doesn't make any material difference in the response, which is why i just left them zeroed out.

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post #107 of 109 Old 09-14-2013, 09:51 PM
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"Looks pretty strong to me..."

i agree. after several design iterations, it seems like it really maximizes what that driver has to offer. should be very nice.

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post #108 of 109 Old 09-16-2013, 08:05 PM - Thread Starter
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it doesn't make any material difference in the response, which is why i just left them zeroed out.

Great, and one less thing to worry about.
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post #109 of 109 Old 09-16-2013, 08:42 PM
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619 thread starts. 916 build starts. weird. :-)

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