Ricci's dual B&C 21SW152 build thread. - Page 4 - AVS Forum
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post #91 of 518 Old 04-16-2010, 06:57 AM - Thread Starter
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Quote:
Originally Posted by Funky Waves View Post

Only thing I would do is maybe increase the flare out a little, and connect the three circles with an angled "pillar", that would help the rear panel. "Pillar" braces are geat, but wherever possible I find it works even better when integrated into a circular cutout brace as well to spread the support over the panel. I would have done it more like that on your big XXX enclosure but then it would have been impossible to line/stuff.

I see. I could use a diagonal crossmember to connect the front wall to the middle of the back and use the larger flare out.

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Originally Posted by soho54 View Post

Having the driver right at the mouth like in LTD02s example, is not a problem as long as you design the horn that way.

I can think of a of commercial tapped horn with the driver right in the mouth. It hits 20Hz pretty easily, and its driver sits around 30cm in.

Right. The designs I tend to like in simulation have the driver further back in to extend the lowend and bring up the notch preceding the first sharp resonance though.

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Originally Posted by LTD02 View Post

isn't there a more elegant way to brace a cab though? just figured i'd kick the question out. i'm just wondering about those nasty looking entrails...not wondering about the performance, just if there would be a more elegant bracing solution.

I'm sure there is but I'm not sure whether it's worth the extra effort.

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Originally Posted by LTD02 View Post

"Having the driver right at the mouth like in LTD02s example, is not a problem as long as you design the horn that way."

soho, glad you chimed in...can you elaborate a little bit more on your comment? specifically, what do you mean by "...as long as you design the horn that way."?

The relationship of the driver to the horn mouth has a large effect on the response in the middle of a TH's passband. Depending on your configuration it may be good to place the driver at the mouth, or not.

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Originally Posted by LTD02 View Post

also, soho, a concern that i had with the tapped horn idea was at what point the driver becomes 'large' relative to the travel of the sound pressure, or if there is such a point, or if the sound pressure will simply find its way around the driver with little change from model. is it possible for the driver to get in the way and jam up the horn in some way?

I've been wondering about this too. There doesn't seem to be anything concrete on this. I'm sort of figuring that keeping it below 1/3rd of the total area would be best. Just a guess. Perhaps this obstruction in the horn path should be modeled as a restriction. I calculate the B&C at around 725cm.
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post #92 of 518 Old 04-16-2010, 07:16 AM - Thread Starter
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Originally Posted by vasyachkin View Post

read my response to funkywaves. the point is to reduce turbulence. all that making the port longer accomplishes is reduce air speed in order to reduce turbulence. but shoving it up the back wall only increases turbulence so you are accomplishing the opposite of what you intend this way..

I understand that. Previously the port was venting directly into the back corner at about one port width away. All that I did really was to add a large corner flare and a flare to the top termination. It does seem to add a turn in the port but I'd have to think this is better than terminating straight into a corner. Shortening the port raises the tuning which I do not want to do. Decreasing the port area to shorten it increases airspeed. Increasing cabinet volume changes the response making the tuning get peaky and ballooning the total size. Increasing the depth is an option. I can probably cut the front footprint from 26w x 30 h to 24w x 29h.



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Originally Posted by vasyachkin View Post

enlighten me - what is a power port ? .

That's what is used on the XXX. Patented by Polk Audio in the 90's. They gave out a excel calculator for them to a few DIYer's in the mid 90's.


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Originally Posted by vasyachkin View Post

don't bother. you have to do a hyperbolic port, not flared one. hyperbolic one will be easier in your case if its a slot rather than round. .

Easier? How so? EDIT: Nevermind. I see that you meant easier to make a hyperbolic slot port than a round one. Yes.





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Originally Posted by vasyachkin View Post

LINK ! i want to read it too !.

I thought that you were the one who posted it originally! I'm certain it was JBL and it was a study on port flaring performance.




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Originally Posted by vasyachkin View Post

that's your mistake. minimum width of the port can be made as low as you want. because the turbulence takes place at the ends of the port it is only the width at the ends that counts. that's the entire point of hyperbolic vent - it decouples the width at the center from the width at the ends. .

Would not the minimum area of the port still define the limits of the core speed and laminar air flow?
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post #93 of 518 Old 04-16-2010, 10:00 AM
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"Would not the minimum area of the port still define the limits of the core speed and laminar air flow?"

i think you are right ricci. i haven't studied fluid dynamics, at least not formally, but i know the auto industry has invested enormous dollars in order to try to answer precisely the question of whether or not port shapes, configurations, and/or curves can increase air flow through a tube. nobody has been able to improve on the simple flared tube. laminar flow is a little different idea...important for golf balls and base balls (and really important for swimmers and submarines), but not so much for air travelling in a tube. i haven't seen a high performance automotive motor with anything other than simple straight or curved intake and exhaust tubes (or minor variations there on). ferrari and bmw have invested big, big bucks into optimizing air flow through tubes. the best that they have come up with is a simple, small flared, port.

here are the intake tubes on the m3. i don't know how many euros bmw spent trying to wring every last ounce of hp out of this motor over the past several decades, but it is probably on the order of 100 million.
http://www.m3addict.com/e46-bmw-m3/S...nifold-002.jpg

if there was any practical disadvantage to using curved tubes, ferrari would not use curved intake and exhaust tubes.
http://www.mnpctech.3dpixelnet.com/p...ferrari599.jpg

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post #94 of 518 Old 04-16-2010, 10:11 AM
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Quote:
Originally Posted by Ricci View Post

....
I've been wondering about this too. There doesn't seem to be anything concrete on this. I'm sort of figuring that keeping it below 1/3rd of the total area would be best. Just a guess. Perhaps this obstruction in the horn path should be modeled as a restriction.

My Shiva X2 Mark I Tapped Horn has a S3 of ~650 sq cm. The Shiva X2 is a big driver. I made the baffle as narrow as possible, the driver frame is so close to the sides I had to cut the PL bead out to install the driver. I recessed the frame of the driver 1/4 inch into the baffle, but the magnet is still within an inch of the access cover. While it measures pretty well, it chuffs at high excursions. I'm of the opinion that the driver has sufficiently choked the horn's path to cause audible chuffing.

The Shiva has a cross-sectional area of approximately 400 sq. cm, which is ~62% of the area at S3, which is WAY too much, hence the Mark II redesign that is currently underway, where it will be zero.
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post #95 of 518 Old 04-16-2010, 10:13 AM
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btw, the hyperbolic structure proposed by the troll comes from the profile of nuclear reactor chimneys.

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post #96 of 518 Old 04-16-2010, 10:16 AM
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Originally Posted by lilmike View Post

My Shiva X2 Mark I Tapped Horn has a S3 of ~650 sq cm. The Shiva X2 is a big driver. I made the baffle as narrow as possible, the driver frame is so close to the sides I had to cut the PL bead out to install the driver. I recessed the frame of the driver 1/4 inch into the baffle, but the magnet is still within an inch of the access cover. While it measures pretty well, it chuffs at high excursions. I'm of the opinion that the driver has sufficiently choked the horn's path to cause audible chuffing.

The Shiva has a cross-sectional area of approximately 400 sq. cm, which is ~62% of the area at S3, which is WAY too much, hence the Mark II redesign that is currently underway, where it will be zero.

great info lilmike. much thanks. based on your experience, any "gut instincts" for how much is too much when it comes to drivers clogging up horns? won't hold you to them, just curious, what your intuition is with respect to this.

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post #97 of 518 Old 04-16-2010, 10:32 AM
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I wish I had enough time and $$$ to actually build and test all the ideas rattling around in my head.

We did not notice the chuffing at all with the Anarchy TH at high drive levels, which are about 50% obstructed, but I am pretty sure that is as much due to the reduction in driver displacement as it is the reduction in horn occlusion. I would definitely like to see less than 33% blocked for a high excursion / high output sub, lower is definitely better, hence me rethinking the way I fold things.
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post #98 of 518 Old 04-16-2010, 10:36 AM - Thread Starter
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post #99 of 518 Old 04-16-2010, 10:59 AM
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what a read ricci...

the port flare in the jbl k2 and the revel salon2 probably share some cues as to what the harman guys are thinking is state of the art. interestingly, i see that revel claims an assymetric, hyperbolic, port in their salon2.

"The Salon2's woofers use aluminum cones rather than the Salon1's mica/carbon-filled copolymer cones, and are reflex-aligned with a hyperbolic, downward-firing, 16" by 4" port with an asymmetrical flare rate, to eliminate "chuffing" noise when the speaker is driven at high levels. The tunnel's tapered shape "behaves as if it is longer than a straight ducted port," according to Revel."

http://www.stereophile.com/floorloudspeakers/608revel/

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post #100 of 518 Old 04-16-2010, 11:02 AM
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i'll go ask for a pic on the revel/harman speakers forum and report back...

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post #101 of 518 Old 04-16-2010, 11:03 AM
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Nice paper. Didn't B&W at one point dimple their (sub?) ports? Not sure how much of that was marketing...
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post #102 of 518 Old 04-16-2010, 11:25 AM
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tlag, i call marketing b.s. on that one. laminar flow around a golfball or baseball moving trough the air is much more of a resistance point than air moving through a tube. those dimples create little turbulences that cause the air not to "stick" to the ball. air sticking to a tube in a ported design would be the least of my concerns.

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post #103 of 518 Old 04-16-2010, 11:29 AM - Thread Starter
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Quote:
Originally Posted by tlag View Post

Nice paper. Didn't B&W at one point dimple their (sub?) ports? Not sure how much of that was marketing...

It's covered in the Harman paper I linked. Effectively they summarized it as having no discernable performance advantage and a few detrimental effects.
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post #104 of 518 Old 04-16-2010, 11:32 AM
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Originally Posted by Ricci View Post

It's covered in the Harman paper I linked. Effectively they summarized it as having no discernable performance advantage and a few detrimental effects.

Why, yes it is! Hadn't gotten there yet...
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post #105 of 518 Old 04-16-2010, 02:03 PM
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A few posts up I was asked a few questions, so I'm gonna back things up a little...

Quote:


is it possible for the driver to get in the way and jam up the horn in some way?

Yes it is, but I think it takes more than most would think. You will get chuffing from the air being cut by the frame spokes (as lilmike said,) before you will do much from the reduction in area at the low pressure end of the horn alone. Obstructions in the throat area should be marginally worse on FR, but it should start in the upper region.

It might be worth sweet talking lilmike into running some sweeps with some simulated blockages to tell the tell.

In some AkAbak sims I tried before I found a 75% blockage was needed at the driver to really do much of anything FR wise. The pressures in the horn varied a little, but not much at all. I wouldn't trust this without a real world test to back it up though. I have about 75% faith in it.

Quote:


specifically, what do you mean by "...as long as you design the horn that way."?

I find there are four ways a drivers parameters fall in a TH.
1- No good at all.
2- Good with the driver away from mouth
3- Good with the driver close to the mouth
4- A general driver that can swing 2 or 3

I was just stating that there is nothing wrong with the driver being at the mouth, if it gives you the response you are after.

Quote:


The designs I tend to like in simulation have the driver further back in to extend the lowend and bring up the notch preceding the first sharp resonance though

You can get to the same low corner with the right driver either way. You can't shrink it as much though.

I am not sure what you are referring to at the end there? Which resonance are you talking about? (In HR set the Power to 0 and count the peaks low to high as odd numbers). Are you talking about the dip before the always nasty 5th? If so, you just had the enclosure to small, or you need to use another flair profile.
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post #106 of 518 Old 04-16-2010, 07:30 PM
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thanks soho....so, if i read you right, you are saying with respect to driver located near the mouth of a horn, that as long as it models okay, it should build okay?

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post #107 of 518 Old 04-16-2010, 08:42 PM
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post #108 of 518 Old 04-16-2010, 09:08 PM
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Originally Posted by soho54 View Post

LTD02,

Yes...

don't be a smart ass !

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post #109 of 518 Old 04-16-2010, 10:15 PM
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post #110 of 518 Old 04-16-2010, 10:32 PM
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Originally Posted by Ricci View Post

Found it.

http://www.harman.com/EN-US/OurCompa...ions/11094.pdf

Ricci you're the man ! I just read all of it. This is one of the best papers i have read on audio.

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Shortening the port raises the tuning which I do not want to do. Decreasing the port area to shorten it increases airspeed.

that's like saying if i take the steak knife out of my head it will make me 2 inches shorter and women like taller men.

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Originally Posted by Ricci View Post

That's what is used on the XXX. Patented by Polk Audio in the 90's. They gave out a excel calculator for them to a few DIYer's in the mid 90's.

well that is certainly what i would be using if i was shoving a round port against the wall.

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Easier? How so? EDIT: Nevermind. I see that you meant easier to make a hyperbolic slot port than a round one. Yes.

at this point i want to take back what i said about not bothering with a round port. after reading the Harman paper now i have to suggest you use a single round port. basically i underestimated the magnitude of boundary effects.

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Originally Posted by Ricci View Post

I thought that you were the one who posted it originally!

no i posed a Revel brochure in this article:

http://www.diy-av.net/forum/viewtopic.php?f=51&t=436

and the brochure has a paragraph about the port, but no analysis. the paper you linked i never seen before.

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Originally Posted by Ricci View Post

Would not the minimum area of the port still define the limits of the core speed and laminar air flow?

well after reading the article i see that actually we were both right and we were both wrong. namely:

at low power levels turbulence begins to appear at the ends of the port while the center of the port is laminar. in this scenario as i have suggested the center is irrelevant and it is the flaring that counts.

however as the power keeps increasing eventually the entire port becomes turbulent and at this point, as you suggested, the limiting factor is now actually the cross section at the center of the port.

so if you were making a home hi-fi speaker ( that mostly operates in laminar range ) you would use maximum flare as both Revel Salon and JBL Everest speakers do. however if you're building a PA speaker which always operates at the limit then it isn't clear whether any flaring at all should be used, just a bit edge rounding. and actually on JBL PA speakers i don't think they use any flaring?

the article does seem to suggest that you should use a round or an elliptical port, rather than a slot port. my JBL monitors on my desk use elliptical ports. Revel and Everest both use round ones.

I can only conclude that Genelec used slot vents on many of their speakers for the sake of simplicity. They are definitely cheap bastards - or they would use passive radiators like Mackie does.
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post #111 of 518 Old 04-16-2010, 10:43 PM
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If i were you i would probably make the driver side-fire, make the cabinet narrow and deep like a Bose acoustimass and put a long round port in there with harman style flare on the outer side and a combination of harman style and polk style end on the inner side. Even with the polk design though i would still make sure it isn't close to the back wall.

instead of doing it as in the image below:

http://www.tnt-audio.com/jpg/rt600ibotb.jpg

i would double the spacing and add significant flare at the end of the port.

so the overall box should look similar to this:

http://i.testfreaks.com/images/produ...-5.3106462.jpg

only the port would run all the way to the back and a single driver would side fire.

But we can get more creative if you want !

We can make the box HORIZONTALLY FLAT ( just as it is shown in that bose image ) and have the driver DOWN FIRE. it would have maybe 6" tall feet to provide some clearance for the driver. This would protect the driver as well as allow you to fit a long straight port ...

OR ! in the same horizontal flat box you can make a curved port that would turn 270 degrees around the driver continuously - similar to Genelec spiral laminar enclosure. i really like THAT idea ! this would really be your highest performance option ...

i have already developed a stand design for you that will allow you to quickly raise that sub 6 inches when in operation and collapse it flat when in transportation. basically you just build a telescoping base with removable "stoppers". you could even adjust the height this way. and when the stand is collapsed in transportation it would completely seal the driver off so it would be 100% protected.

come on man ! build it ! why build something boring when you can build something unique ?



if the sub is 1 foot tall with 6" stand then it will be just perfect height to sit on.
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post #112 of 518 Old 04-17-2010, 02:22 PM
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thanks soho. i didn't read your comment as being anything more than an honest answer. vas' comments are best when ignored.

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post #113 of 518 Old 04-19-2010, 09:30 AM - Thread Starter
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Vas,

One thing about the Polk port is that the flow guide and the spacing on both ends are integral to the operation and tuning of it. The port in the XXX cab would've needed to be 54" long if an equivalent straight 9.4"(equivalent area left after the center flow guide) vent was used. The power port allows you to reduce the length considerably. The straight section ended up at 36.5" instead. Supposedly it also allows higher vent velocities before turbulence and large compression effects but I've not seen hard data on that.
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post #114 of 518 Old 04-19-2010, 02:42 PM
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Quote:
Originally Posted by Ricci View Post

Vas,

One thing about the Polk port is that the flow guide and the spacing on both ends are integral to the operation and tuning of it. The port in the XXX cab would've needed to be 54" long if an equivalent straight 9.4"(equivalent area left after the center flow guide) vent was used. The power port allows you to reduce the length considerably. The straight section ended up at 36.5" instead. Supposedly it also allows higher vent velocities before turbulence and large compression effects but I've not seen hard data on that.

it affects both tuning and turbulence - that is obvious. the question is which one should be given priority ?

all i am saying is that i would sacrifice some effective length for an improvement in smoothness of airflow.
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post #115 of 518 Old 04-19-2010, 04:00 PM
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i would also consider raising the tuning to 30 - 35 hz. JBL don't go lower than that in their pro subs i think.

all these simulations you do in your software are useless. because they don't simulate what actually happens when the port goes turbulent.

i don't even have time to think about what actually happens right now, but i would rather trust JBL's experience than some freeware box modeling software

by raising port tuning from 25 hz to say 32 hz you will double the airflow capacity of the port. it will double not just because of greater area but because the center of the pipe will be further away form the sides so less coupled by viscous forces to them.

i think it's in your best interests to stave off the point where the port goes resistive ( turbulent ) as much as possible. because a 35 hz resonance is better than a 25 hz resistive leak.
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post #116 of 518 Old 04-20-2010, 07:23 AM - Thread Starter
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Quote:
Originally Posted by Ricci View Post

The power port allows you to reduce the length considerably. Supposedly it also allows higher vent velocities before turbulence and large compression effects but I've not seen hard data on that.

Let me re-phrase this. It shortens the length of the straight section, but you are effectively maintaining the length through the inner and outer bends throught the baffle to disc spacing. It appears that the real improvement is against turbulence and compression. This is accomplished because the mouths of the vent expand greatly to encompass a huge area. Also there is effectively no core to the port since it is taken up by the flow guide. I'm not sure how this would affect the ability to remain linear. Seems as if you'd still be limited by the smallest area of the vent.
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post #117 of 518 Old 04-20-2010, 11:42 AM
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Quote:
Originally Posted by Ricci View Post

Let me re-phrase this. It shortens the length of the straight section, but you are effectively maintaining the length through the inner and outer bends throught the baffle to disc spacing.

good to know you think i am a ****** and don't understand obvious things.

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Seems as if you'd still be limited by the smallest area of the vent.

limited ? maybe. but the speaker isn't just about its limit - its about how it gets there as well.

bottom line is if theres a wall the air will have to turn one way or the other.

it will turn better with a guide than without.

however the smart thing to do is to let it slow down before turning.

power port more or less does that, but if it was combined with some flare it would let the air slow down even more before turning.

in a folded port there is no slowing down - the turn is taken at full speed - that's the consideration here.
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post #118 of 518 Old 04-20-2010, 11:56 AM - Thread Starter
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Quote:
Originally Posted by vasyachkin View Post

good to know you think i am a ****** and don't understand obvious things.
.

Wasn't specifically addressing you. More like putting that out there for others who may not. About a 9 on the tension scale there Reub...
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post #119 of 518 Old 04-20-2010, 11:32 PM
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you've probably seen this, but since you mentioned data:
http://www.subwoofer-builder.com/flare-testing.htm

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post #120 of 518 Old 04-21-2010, 12:03 AM
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Quote:
Originally Posted by Ricci View Post

Wasn't specifically addressing you. More like putting that out there for others who may not.

another thing i thought about when reading that Harman paper which i don't remember if i already mentioned is this:

the paper explains how turbulence consists of eddies and it explains how eddies seed each other in a chain reaction sort of way.

however these eddies fist begin to appear on the very outer edge of the port.

so when the air is blowing OUT of the port all of these eddies will get essentially flushed out of the port and they will not be able to seed any new eddies. the paper mentions that optimum port EXIT geometry is STRAIGHT and i am pretty sure this is the reason.

that is because in a straight port the eddies on the exit will form just OUTSIDE the port. while in a heavily flared port the eddies will form in the FLARE region which is still inside the port.

the bottom line here is that u want to keep eddies OUT of the port as much as possible so they can't seed new eddies thereby making the port go turbulent.

now on the port ENTRY the eddies will tend to form on the inside of the port edge rather than outside so in this case flare is needed. and again its needed for the same reason - to avoid creating eddies that will ultimately get sucked into the port where they will start a chain reaction.

whats my point ?

my point is that any port ELBOW creates eddies right in the MIDDLE of the port with NO chance for them to be flushed out of the port by airflow. this i think is the main reason to avoid elbows.
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