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Discussion Starter #1 (Edited)
I'm starting this thread to show visually what I mean when I refer to "optimizing" ported sub designs by maximizing port size by maximizing port length.

For these examples, I'm going to keep gross volume and tune the same. I'm operating under the assumption that this is the size box and tune that the designer wants. I'm reducing net volume for the volume taken up by the port, and I'm going to show why the effect of losing that volume is negligible compared to the benefit gained by increasing port size. I'm also using small boxes for these examples to make the negative effects of losing volume as significant as possible, in order to show that it still doesn't really matter.

So we've got an SQL-12 in a 6-6.42cf net box tuned to 15Hz, and a UM12 in a 4-4.5cf net box tuned to 20Hz. How much of a difference does the lost volume from the bigger ports make? As we see here, hardly any difference at all.




So everything looks roughly equal here, but it ignores port velocity, so what about when we look at that? Now we see it is a major issue in the SQL-12 boxes, and a minor issue in the UM12 boxes.



So what if we add some PEQs to the models to bring the SQL-12 velocities to around the same ~32 m/s, and to reduce the smallest port UM12 model to around 20 m/s?




Now we see the larger port versions of the SQL-12 boxes have a big advantage over the smaller port versions, and even in the UM12 case, if we compare the 4cf version with the 4.3cf version, we know port compression is already kicking in at 16 m/s, so I'd still prefer the 4cf version with the bigger port and lower port velocity.

So hopefully this shows visually what I mean when I am always talking about optimizing a ported design by maximizing port size, and the way to do that is to maximize port length. Decide how long of a port you are comfortable with as far as first port resonance, and try to use every bit of that length to maximize the port size for a given box size and tune. That will allow you to "optimize" the design and get as much real-life output from it as possible.

In the SQL-12 boxes, by increasing length from 26.4" to 35" to 45.5", I was able to increase the port size from 4" to 4.5" to 5" square, and the reduction in port velocity and resulting real output capabilities speaks for itself. In the UM12 boxes, by increasing length from 26.5" to 34.8" to 46.2", I was able to increase the port size from 4.5" to 5" to 5.5" square. I used this particular UM12 case to try to go toward the extreme of a smaller driver in a smaller box with a higher tune to try to reach the limits of where increasing port size is beneficial, and yet we still see even at that point, the lost volume vs the reduced port velocity is still close to a wash, so there's little downside to increasing port size, even in this extreme case. In the more typical cases we see with boxes with more capable drivers like a UM18, and box sizes that are 6cf+, maximizing port size just becomes more and more important, otherwise the design is unnecessarily limited by the port, and thus what I would call not yet optimized. Since it has no effect on the outside box size and tune, why not simply make the port bigger and get more output?

So even if someone is only comfortable with a first port resonance of, say 160Hz and thus a max port length of ~42", or a first port resonance of 180Hz and thus a max port length of ~37.5", or whatever, they should still be targeting that length in order to make their port(s) as large as possible, otherwise they are just leaving free output on the table.

Is it possible to go to even more of an extreme with a small box and small/weak driver, and high enough tune that it isn't necessary to go all the way to the max length? Sure, but it's going to be a really extreme case. For the vast majority of ported designs, it's going to have a huge impact, and the purpose of this thread is to help those in those scenarios.
 

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Displacement and motor force can really wreak havoc on port velocity. Going with the longest vent your pass band can withstand is a good rule of thumb for infrasonic tunes. Vent area seems to trend at roughly 1.3cm2 for each liter of chamber displacement when tuning in the upper teens to 20Hz. It can be a challenge finding the sweet spot with displacement and vent area, since both regulate tuning frequency and are at odds with each other. Add more displacement, velocity creeps up, add more vent area, the tune starts slipping up. Even though the PA460 and the 18DS115 are both pro 18" woofers, the requirements for each could not be further apart when it comes to venting. Adjustments usually have to be made on a case by case basis.
 

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Discussion Starter #4 (Edited)
Displacement and motor force can really wreak havoc on port velocity. Going with the longest vent your pass band can withstand is a good rule of thumb for infrasonic tunes. Vent area seems to trend at roughly 1.3cm2 for each liter of chamber displacement when tuning in the upper teens to 20Hz. It can be a challenge finding the sweet spot with displacement and vent area, since both regulate tuning frequency and are at odds with each other. Add more displacement, velocity creeps up, add more vent area, the tune starts slipping up. Even though the PA460 and the 18DS115 are both pro 18" woofers, the requirements for each could not be further apart when it comes to venting. Adjustments usually have to be made on a case by case basis.
You're talking about changing drivers and tune, but I'm specifically talking about for a given driver, box size, and desired tune, in other words, for a particular desired design, the need to maximize port area by maximizing port length to minimize port velocity, while illustrating that the volume lost by making the port bigger and longer rarely has a significant negative impact. It still needs to be decided if the resulting response and port velocity is acceptable, but first we should at least "optimize" that particular design to see how low we can push port velocity.

Essentially, by holding box size, tune and port length (maximum acceptable) constant, port size is determined for us, or at least the maximum port size supported by that box size, tune, and maximum acceptable port length. Whether that tune strikes the best balance for a particular driver and box size is a separate question, and something I plan on addressing in another post where I outline a good approach to comparing multiple tunes and trying to hone in on the balance someone is trying to strike with their goals, which is more along the lines of what you're talking about.

This topic has come up multiple times in different threads, such as the threads talking about whether lowering tune increases or decreases port velocity, or threads about whether plugging ports is a reasonable way to lower tune. What I've mentioned and shown with models in those threads is that when we are always maximizing port length to our acceptable limit, port velocity goes up as tune goes down, and that plugging ports is a perfectly reasonable way to lower tune if each port is at max length.

In those threads I've used the big comparison graphs that I posted previously for my SonnySub designs to illustrate these two points, as in these examples external volume is held constant and port length is held constant at 48", and the only thing that changes is the port area (essentially just like plugging ports) and resulting tune and each design is already minimizing port velocity as much as possible due to its maximized port length, and we see velocity goes up as tune goes down.



 

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It's been understood since the days of Harry Olson that it's best to use the largest port possible consistent with port tuning and achievable port length, especially with higher force-factor drivers.


Impractically-long port structures gave way to passive radiators, as they present more concentrated moving mass than the air in a port.
 

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Discussion Starter #6 (Edited)
^^^ This isn't thought to be a revolutionary idea, and it's not intended for the modeling veterans. It's just that I've seen so many people, many of them somewhat new to sub modeling, that don't fully realize the relationships involved in modeling subs and thus don't maximize port length. So this thread is intended to be a PSA to try to help those sort of folks, and to actually show why with examples and illustrations so that it is fully understood.

Although if I'm being totally honest, I see an awful lot of commercial subs with surprisingly short ports, as well.
 

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Good post, seems like the fundamentals are missing for a lot of the people here, even the regulars -which isn't slinging mud, but really stating just how useful posts like this can be. You obviously spent a lot of time putting it together, nice work. I believe that, for a given box size, the exercise of thinking of the max port length/lowest tolerable vent resonance one can deal with, then holding that constant while manipulating other data will force people in general to visualize what will end up as a more optimized enclosure.

Chris
 

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Vent area seems to trend at roughly 1.3cm2 for each liter of chamber displacement when tuning in the upper teens to 20Hz.
Guess you mean 1.3dm2(130cm2). My rule of thumb for pro audio cabs is 100cm2 for every liter of displacement, which is of course optimized for space-efficiency. Anything below that point quickly gets critical, even with tunings at or slightly above 30Hz.
 

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Guess you mean 1.3dm2(130cm2). My rule of thumb for pro audio cabs is 100cm2 for every liter of displacement, which is of course optimized for space-efficiency. Anything below that point quickly gets critical, even with tunings at or slightly above 30Hz.

Here is an HR grab as an example. That is good for an 18Hz tune.



 

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Guess you mean 1.3dm2(130cm2). My rule of thumb for pro audio cabs is 100cm2 for every liter of displacement, which is of course optimized for space-efficiency. Anything below that point quickly gets critical, even with tunings at or slightly above 30Hz.

Here is an HR grab as an example. That is good for an 18Hz tune.



I think that‘s a bit excessive for a single 8“ driver!
 

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I think that‘s a bit excessive for a single 8“ driver!

That is for an 18" with stupid amounts of motor force. Running a BW 4th order high pass filter at 20Hz keeps velocity under 23m/s with 1000 watts. The 18DS115 and the 18DS100 are both fun like that... lol.
If you put a 18TBW100 for an example in there you can use a HPF filter at a lower frequency with a shallower slope and probably be 5m/s lower.
 

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An unsolicited homage

Something makes me want to say : It's about time! seeing this presentation / discussion of the subtlies and nuanced details of (ported) in a most informative / illustrative / user friendly format.

This peek at the the “ability” for behind the scenes “number crunching ,TSTL, will make any DIY wannabe / experienced sub builder appreciate and hopefully pursue , as external factors allow, the extra effort to get the best dialed in response no matter which REQ or Sub EQ there is to work with.

This reminds me of the 2 year time frame between the Submaximus V2 and the V3.
When @LTD02 proposed the V2, the DIY kool-aid drinkers response was under whelming. 38 cu. ft.? The sound of folks taking 1 step backward was thunderous.
It is rather large but nobody could or would tell me no.
There were no build plans. That made it more fun . . .
At the time I had just finished some Martys with @ kevings and had 2 of the SI 18's for a build for myself.
(the story of my “wonderful 2x Klipsch Sub10's' fegeddabaudit!)
The success , even without much EQ, of my BFM 15” THTLP's made me think, yeah 18” FLH.
Bill Fitzmaurice had pics on his website of destroyed 18's and good arguments against feasibility.
The SI's got resold, pronto.
The UXL and Berry 6K , plywoods and yada, yada , yada. There's a thread / pics out there somewhere.
Folks liked the results
2 years later after some extensive behind the scenes high level discussions of modeling finesse among the cognizanti wrt large motor inductance tweak. Iirc, @chrapladm, and many others, there was the V3., only 40 cu. ft.
All I needed was 5 sheets of plywood. For only about $300 and < 3 weeks work,
what could possibly go wrong?


I've run out of room for more big boxes but looking at the data presented,
I bet I could replace the 65 cu. ft. of THTLP's in my mancave with “something” tuned bit lower
No good reason, just antsy . . . also 2 pristine UM 18's and an extra 6K amp
They've had a good 6 year run, and EQ'd thru the minidsp 2x4 with the BASH 300's far more than a humans should have to endure . . at least close to “only” 22 hz . . .JS

One of my favorite South Park lines: I think I learned something here today . . .

The curious DIY guy has taken the red pill and is a goal driven possibility thinker.

How lucky we are . ..

OP: Thank you.
 

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Which version of WinISD is that that allows multiple project windows?

I have 0.7.0.900 and as far as I can tell it doesn't.

Just downloaded 0.7.0.950 and it's the same.
 

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Discussion Starter #15 (Edited)
^^^ pro alpha. It's the main reason I use it.
 

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i think the natural question comes after this is how to do we "Decide how long of a port we are comfortable with as far as first port resonance"?
 

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i think the natural question comes after this is how to do we "Decide how long of a port we are comfortable with as far as first port resonance"?

Best practice is one octave above lpf. But many of us push that, using 130-140hz fpr with a 80hz lpf. Steep filters also help.

Chris
 

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Best practice is one octave above lpf. But many of us push that, using 130-140hz fpr with a 80hz lpf. Steep filters also help.

Chris
then how do you build a LT sub? without being 30CUFT? i seen some people running with 1st port resonance in 108Hz range without any problems.
 

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then how do you build a LT sub? without being 30CUFT? i seen some people running with 1st port resonance in 108Hz range without any problems.

By design principles, you don’t.

In my case, my port velocity is ridiculous with 10.5 cu at 11hz tune. No free lunch.

Chris
 
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