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Old 06-08-2016, 03:36 AM
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Default Port area!!!!

Quote:
Originally Posted by williamhung View Post
Hey Fade quick question, How do you determine proper port area by using displacement and xmax numbers? always wondered about this and nobody seems to be on the same page about it.
Minimum vent diameter is determined by displacement (Sd * Xmax) and tuning frequency (Fb).

There are lots of handy dandy calculators online, but honestly... they're grossly unrealistic. If you plug in an 18" with 30mm of Xmax and a tuning of 30hz... it'll spit out a minimum port diameter of 22". I don't have to tell you that in an average size box for an 18", a 22" diameter port would be like 15 f'n feet long.

Links...
http://www.ajdesigner.com/phpvent/su...m_diameter.php

http://www.diysubwoofers.org/misc/portcal.htm

(This one is a bit more forgiving) http://www.carstereo.com/help/Articles.cfm?id=31


But here's the thing... vent mach (how fast the air in the port is moving) is highest at tuning (actually, just below). But you know what is lowest at tuning? Excursion.

Here's an Ethos 18" tuned at 30hz on 2,000w. Remember, power is also a factor insomuch as it dictates excursion.




The whole principle behind a minimum port diameter is to avoid port noise, maximize output and minimize compression. When you input these values, they're assuming the Xmax number is what it's going to be doing at tuning. But as evidenced above, our subs don't get to max throw at tuning where vent mach and the chance for chuffing is highest.

With that said... if we entered the number from above- which looks to be ~ 5mm, we get a minimum port diameter if ~ 9". That's doable.

So where does that leave us? Modeling. I know WinISD and Bagby's software can calculate vent mach.

Below is the Ethos 18" again. Again on 2k. First let's do a 6" round port.



Now... it's generally accepted that 5% the speed of sound is the threshold for port noise. That's ~ 17 meters per second. A lot of guys will use a max of 30m/s and still consider that "acceptable". Either way... you can see that a 6" is way too damn small for a long stroke 18" on high power. Now that peak of ~ 55m/s is well below tuning and would be mitigated by our SSF anyway (hopefully), but even at tuning we're still looking at ~ 42m/s. Still too high.

Now if we bump up to an 8" port...



Bingo, bango... we're now below 30m/s to ~ 23m/s at tuning and probably 26-27m/s before our SSF catches it.

How big do we need to go to get to < 17m/s? About 10"





Let me close with this...

This is the Ethos (again), but this time on lower power (500w)



Now let's go back to the 6" port that was way too damn small...



HUH!?!?!? But, T-Fade... I thought you said a 6" was too small for an 18"!?!

Nope, I said it's too small for the displacement of a 30mm Xmax 18" on 2k. Looks to be fine for low power or less stroke.




It's important to note that nowhere in this diatribe did I mention enclosure volume. Displacement, people!!!
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Old 06-09-2016, 02:20 AM
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Again great write up Fade, So if i get this right your saying 17 m/s is the optimal port velocity, but anything under 30 m/s is still good? also does tuning affect this equation? Iv'e heard somewhere for spl tuning 35hz-up you would want the biggest port allowable and for low tuning 30's and under you can get away with a smaller port?
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Old 06-09-2016, 02:53 AM
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Quote:
Originally Posted by williamhung View Post
Again great write up Fade, So if i get this right your saying 17 m/s is the optimal port velocity, but anything under 30 m/s is still good? also does tuning affect this equation? Iv'e heard somewhere for spl tuning 35hz-up you would want the biggest port allowable and for low tuning 30's and under you can get away with a smaller port?
Tuning does, in fact, affect the port area needed. Because it affects displacement.

For every octave lower a sub plays, it needs 4x times the throw. Four times! Meaning if you need 10mm of throw to do 140dB at 60hz... you need 40mm to do it at 30hz. Is there any wonder why it's easier to do big numbers on the TL at higher frequencies?

So less port for lower tuning is exactly backwards. But perhaps the reasoning behind someone saying you *may* be able to "get away" with a less port on lower tuning is that there may or may not be as much low frequency content? Just depends on how low we're talking, your design and goals and what not.

17m/s is a great number to shoot for. Yes. Most builders will agree that 30m/s is still "ok". And there is evidence to suggest that going below 10m/s may actually be a detriment to output. But getting to 10m/s would require quite a lot of port.
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Old 06-09-2016, 03:22 AM
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How do you account for flares? I always assumed they helped keep flows more laminar upon exiting the port. Now I'm wondering what would happen if you had a large flare but a smaller port "body"? ie 8" flare with a 3" body. Is there a depth/ratio that a flare dominates the tuning of a port?

Sorry if this is an obvious question!
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Old 06-09-2016, 04:06 AM
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Quote:
Originally Posted by Souldrop View Post
How do you account for flares? I always assumed they helped keep flows more laminar upon exiting the port. Now I'm wondering what would happen if you had a large flare but a smaller port "body"? ie 8" flare with a 3" body. Is there a depth/ratio that a flare dominates the tuning of a port?

Sorry if this is an obvious question!
It's a great question. And quite a complicated one.

There is a very in depth white paper on the subject done by JBL. I don't know how many of you care to read a 62 page technical paper on ports, but it's a good read. And I won't even try to paraphrase the work done by JBL sooooo....

http://koti.kapsi.fi/jahonen/Audio/P..._PortPaper.pdf

But basically... there are two schools of thought on flares. Flare rate by length and flare by diameter. JBL deals with radius to length ratio and goes in depth as to the optimal flare rate for every scenario; including low THD and high SPL. Surprisingly enough... their findings indicate that a lower (or zero) rate is more beneficial to SPL while larger flares are better for low distortion. They also conclude that the inlet flare is more important than the exit. Which seems to be backwards of how a lot of people are doing it with the custom port like octo ports. It's prettier to have the flare on the outside, but the better performance would be to turn it around.

Here's a couple pics to illustrate the large and small radius vortex shedding and to show what's happening at port ends.

Large radius...



Smaller radius...

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Old 06-09-2016, 04:25 AM
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vent velocity isn't determined by the flair but the actual vent itself, so the flair size is ultimately is going to be limited by that of the actual vents velocity. typically the larger the better but as stated your going to run into compression of the port long before the flair. its also frequency related but i wouldn't get extremely worried about it. typically 40MM flairs are ideal for most applications. as far as actual "usable" port length 1/2 flair radius minus the actual overall length of the port.
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Old 06-09-2016, 04:26 AM
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oh nice. gonna check it out.
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Old 06-09-2016, 10:14 AM
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Yep.
The JBL paper is probably the most comprehensive paper on ports. Another good source of data is the work done by Collo investigating flares and pipe diameter. He tested at what air speeds significant compression and turbulence set in. Google search should bring it up. I'll see if I can link it when not on my phone. Basically his work shows that flares can help increase the useful velocity prior to severe compression. Also the main take away is that larger diameter vents allow higher air speeds before compression. So while a 2" vent is significantly over driven at 25ms a 6" diameter is more linear at the same airspeed. Unfortunately he stopped at 6" diameter but it stands to reason that larger vents would continue the trend. He even has a simple free program for looking at port performance. Very generic but it can be useful.

Like everything else port design is a matter of choosing your compromises.

Edit: Link to Collo's port and port flare testing.

Last edited by Ricci; 06-09-2016 at 02:26 PM.
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Old 06-09-2016, 11:08 AM
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Am i confused...so if you are overpowering a sub you actually need more port area? Ive been designing wrong. I get worried about reaching xmech so I run a little less port area.

I just designed a box for my death penalty 15 Input is 4k. I believe xmax is 30mm. Tuning is 33 and box is 4.2cu... I settled on 15.5^2... Not enough?
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Old 06-09-2016, 11:22 AM
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Quote:
Originally Posted by kr-15 View Post
Am i confused...so if you are overpowering a sub you actually need more port area? Ive been designing wrong. I get worried about reaching xmech so I run a little less port area.

I just designed a box for my death penalty 15 Input is 4k. I believe xmax is 30mm. Tuning is 33 and box is 4.2cu... I settled on 15.5^2... Not enough?
Compression actually limits Excursion so yeah you killing your driver with two little Port area..thermally.
Your good. Chances are your only going to see 15mm or so at tuning..

Last edited by bassfreak; 06-09-2016 at 11:32 AM.
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