WinISD and vent velocity... what am I doing wrong? - AVS Forum
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Old 07-30-2014, 11:23 PM - Thread Starter
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WinISD and vent velocity... what am I doing wrong?

I have been playing around with different models in WinISD and can't seem to grasp what I'm doing wrong with vented box velocity. Every box I build has excessively high vent velocity when I change the system input power to the speaker's rated power.

I'm currently playing with two ported LMS Ultras in a 16 cubic foot box tuned to 15 Hz. My vent size is currently 3" by 18" wide and vent velocity exceeds 34 ft/s with only 300 watts. The only way to keep the velocity down that I've found so far is to have a port so massive it would be bigger than the box!

TC's website only recommends 24 square inches of port area (for a single LMS Ultra in 8 cubic feet) and the port I modeled has 54 square inches, yet I still have a very high velocity... can anyone help explain what I'm doing wrong? Or am I doing it right and this is unavoidable?

Thanks!
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Old 07-30-2014, 11:36 PM
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Originally Posted by 95bat View Post
I have been playing around with different models in WinISD and can't seem to grasp what I'm doing wrong with vented box velocity. Every box I build has excessively high vent velocity when I change the system input power to the speaker's rated power.

I'm currently playing with two ported LMS Ultras in a 16 cubic foot box tuned to 15 Hz. My vent size is currently 3" by 18" wide and vent velocity exceeds 34 ft/s with only 300 watts. The only way to keep the velocity down that I've found so far is to have a port so massive it would be bigger than the box!

TC's website only recommends 24 square inches of port area (for a single LMS Ultra in 8 cubic feet) and the port I modeled has 54 square inches, yet I still have a very high velocity... can anyone help explain what I'm doing wrong? Or am I doing it right and this is unavoidable?

Thanks!
What units are you graphing in? You sure its feet per second? There's nothing wrong with 34 ft/s. Stay under 30 meters per second at max power and you should be ok.
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Last edited by notnyt; 07-30-2014 at 11:39 PM.
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Old 07-31-2014, 06:14 AM
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Double click the unit in the options and it will change the unit of measurement.

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Old 07-31-2014, 06:23 AM
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You're probably seeing that velocity at or close to 15Hz. Chances are slim that you'll actually put 300w into them at 15Hz, and that's what the manufacturer recommended port area is based upon.
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Old 07-31-2014, 10:45 AM - Thread Starter
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Originally Posted by notnyt View Post
What units are you graphing in? You sure its feet per second? There's nothing wrong with 34 ft/s. Stay under 30 meters per second at max power and you should be ok.
Yeah, it is in ft/sec right now. If I change it to m/s I get a peak velocity of 29.3 m/s at about 13 hz. I'm right at the limit, but under it

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Double click the unit in the options and it will change the unit of measurement.
Thank you, I changed it to meter/sec now.

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Originally Posted by Bill Fitzmaurice View Post
You're probably seeing that velocity at or close to 15Hz. Chances are slim that you'll actually put 300w into them at 15Hz, and that's what the manufacturer recommended port area is based upon.
Yes, the velocity peaks near tuning. That makes sense, I forgot power and excursion drop at the tuning frequency.

Thanks for the help everyone! Now I just have to decide if I should go overkill with the LMS Ultras or tone it down a bit with 15" LMS-Rs. The 15s modeled in the same 16 cubic feet aren't too far off from the 18s, are half the price, and require half the power...

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Old 07-31-2014, 10:47 AM
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Yeah, it is in ft/sec right now. If I change it to m/s I get a peak velocity of 29.3 m/s at about 13 hz. I'm right at the limit, but under it


Thank you, I changed it to meter/sec now.


Yes, the velocity peaks near tuning. That makes sense, I forgot power and excursion drop at the tuning frequency.

Thanks for the help everyone!
You may want to bring it down just a little if you can. Play with box size and port volume to get the speeds down a touch if possible.
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Old 07-31-2014, 07:02 PM - Thread Starter
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You may want to bring it down just a little if you can. Play with box size and port volume to get the speeds down a touch if possible.
Will do. I might not be able to go with a 15 hz tune, I'm limited to roughly 28-30" of port length with the box I've designed and I think the 3x18" slot port is nearly 32" long in WinISD. I might just have to bump it up to 16 or 18 hz to get my port length down a little bit.
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Old 07-31-2014, 07:06 PM
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Will do. I might not be able to go with a 15 hz tune, I'm limited to roughly 28-30" of port length with the box I've designed and I think the 3x18" slot port is nearly 32" long in WinISD. I might just have to bump it up to 16 or 18 hz to get my port length down a little bit.
you can bend a port around a corner.
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Old 07-31-2014, 07:12 PM
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What dimensions are your boxes? I'll help you. Like Not said.. fold the port. If it's tall enough it will make a 90 degree corner no problem.

Like this:



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Old 07-31-2014, 09:51 PM - Thread Starter
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That's the problem I'm running into, the spot I'd like to put the port is about 24" deep and will be about 10" tall. I suppose to get more room I could fold the port twice, once along the back wall and once along the top wall. The basic dimensions would be two boxes 30" tall, 24" deep, and 24" wide. There will be a 38" wide and 10" tall section that will attach the two boxes to make it look somewhat like a U shape.

The "box" is going to be an integrated sub system and TV stand in attempts to increase the GAF (we're not married yet ) I've attached a picture of a VERY rough MS paint drawing of what I'd like to do. Make fun of me all you'd like, I made that with a laptop mouse, plus I suck at MS paint lol. It is not to scale at all nor does it account for any accuracy in measurements... I just sketched it out to help me visualize it. The drivers are in grey and will be mounted on the side squares and I would like to put the port bottom center (black). I am somewhat OCD and if things aren't symmetrical it will bother me which is why want two drivers and the port dead center. The top section in the center (purple) will house a center channel one day. After the subs are done if I'm ever able to get some 1099s built that is what I'd like to put there as of now. The middle section in the center (blue) will be a space for my AV equipment, probably just my receiver and the blu-ray player. If I have room I'll stack the amps there too.

The bottom part of the whole design (right under the blue section) has to connect the two side boxes in order to give me the volume I need for the two 18s as well as give me a place to fit a port. I am very limited on these dimensions or I lose the WAF completely and I'll be stuck with the 10" sub I have now. I am starting to think the 18s are making this overly complicated... If I went with the 15s I could built a 40" wide TV stand and then build two separate boxes to put on the ends that would be big enough to house a 15 and a port. It would also be a hell of a lot easier to move if it were in three pieces.
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Old 07-31-2014, 10:00 PM
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Lol @ two LMS ultras ported that low inside a tv stand. I like you.

That might rattle the crap out of your tv and components.

The easiest thing would be make box just a tad bigger, and tune just a tad higher. Still awesome, actually probably more awesome and louder lol. 16hz makes more sense than 14hz IMO. Plus the chuffing of the port will only be problematic at extreme volumes and that certain frequency. Most of the time it will be non existent so a tad bit of wiggle room is ok too. I think your best solution is to balance all this stuff giving up a little of everything in order to get a nicer balance. You don't need to give up much.

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Old 07-31-2014, 10:04 PM
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That's the problem I'm running into, the spot I'd like to put the port is about 24" deep and will be about 10" tall. I suppose to get more room I could fold the port twice, once along the back wall and once along the top wall. The basic dimensions would be two boxes 30" tall, 24" deep, and 24" wide. There will be a 38" wide and 10" tall section that will attach the two boxes to make it look somewhat like a U shape.

The "box" is going to be an integrated sub system and TV stand in attempts to increase the GAF (we're not married yet ) I've attached a picture of a VERY rough MS paint drawing of what I'd like to do. Make fun of me all you'd like, I made that with a laptop mouse, plus I suck at MS paint lol. It is not to scale at all nor does it account for any accuracy in measurements... I just sketched it out to help me visualize it. The drivers are in grey and will be mounted on the side squares and I would like to put the port bottom center (black). I am somewhat OCD and if things aren't symmetrical it will bother me which is why want two drivers and the port dead center. The top section in the center (purple) will house a center channel one day. After the subs are done if I'm ever able to get some 1099s built that is what I'd like to put there as of now. The middle section in the center (blue) will be a space for my AV equipment, probably just my receiver and the blu-ray player. If I have room I'll stack the amps there too.

The bottom part of the whole design (right under the blue section) has to connect the two side boxes in order to give me the volume I need for the two 18s as well as give me a place to fit a port. I am very limited on these dimensions or I lose the WAF completely and I'll be stuck with the 10" sub I have now. I am starting to think the 18s are making this overly complicated... If I went with the 15s I could built a 40" wide TV stand and then build two separate boxes to put on the ends that would be big enough to house a 15 and a port. It would also be a hell of a lot easier to move if it were in three pieces.
Widen the port.
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Old 08-01-2014, 12:15 AM - Thread Starter
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Lol @ two LMS ultras ported that low inside a tv stand. I like you.

That might rattle the crap out of your tv and components.

The easiest thing would be make box just a tad bigger, and tune just a tad higher. Still awesome, actually probably more awesome and louder lol. 16hz makes more sense than 14hz IMO. Plus the chuffing of the port will only be problematic at extreme volumes and that certain frequency. Most of the time it will be non existent so a tad bit of wiggle room is ok too. I think your best solution is to balance all this stuff giving up a little of everything in order to get a nicer balance. You don't need to give up much.
Lol yeah ultras in a tv stand probably aren't the best solution, but I'm pretty limited on a subwoofer now. I'm hoping I can make it dense enough to avoid the rattling! If it rattles my electronics to death it won't be worth it. I am just trying to work with what I can... If I were single in my old home I would have enormous boxes and wouldn't care, but can't do that now unfortunately. I will have to play around with box size and tuning a little higher to see if the port will fit better.

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Widen the port.
Do you put any faith behind JL Audio's recommendation of keeping the port at a ratio no higher than 9:1? They state that ratio on their website, but don't give any reasoning behind it.

Last edited by 95bat; 08-01-2014 at 12:19 AM.
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Old 08-01-2014, 12:17 AM
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Lol I'm hoping I can make it dense enough to avoid the rattling! If it rattles my electronics to death it won't be worth it. I am just trying to work with what I can... If I were single in my old home I would have enormous boxes and wouldn't care, but can't do that not unfortunately. I will have to play around with box size and tuning a little higher to see if the port will fit better.


Do you put any faith behind JL Audio's recommendation of keeping the port at a ratio no higher than 9:1? They state that ratio on their website, but don't give any reasoning behind it.
No. Add bracing, now you have multiple slot ports and can keep your ratio

Just don't make them excessively thin.
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Old 08-01-2014, 09:06 AM
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... I forgot power and excursion drop at the tuning frequency.
Power is near max at Fb as it's an impedance minimum, or unless you use a HPF.

Excursion of driver decreases at tune, excursion of vent air increases.

Pricey, but PR's are a better solution.

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Old 08-01-2014, 10:25 AM - Thread Starter
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No. Add bracing, now you have multiple slot ports and can keep your ratio

Just don't make them excessively thin.
That would be ideal... WinISD shows longer ports with multiple slot ports. I think I'm just going to have to tune a little higher. With an 18 hz tune I can do three 3x6 slot ports and I get about 25" of port length.

With an 18 hz tune it still shows 115 db from 13.5 hz on up which is good for me The floor is on beams as well, which will hopefully add to the bass even more.

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Power is near max at Fb as it's an impedance minimum, or unless you use a HPF.

Excursion of driver decreases at tune, excursion of vent air increases.

Pricey, but PR's are a better solution.
Ah, my mistake on the power part. I'd use a HPF to help control excursion down low, but I would still have high power usage at the tune like you said.
The PRs are very nice, but they would add quite a bit to the cost of the build...
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Old 08-01-2014, 11:26 AM
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When you model winISD just do a single slot port. Don't mess around with multiple ports, it's too confusion. Use the total width of the cabinet minus the material on the sides (.75+.75 I assume) and the center brace down the port center (.75") so basically take the total width minus sides and brace and that is your port width. Then you just adjust the port height to achieve the level of air velocity you wish, and let winISD tell you how long to make the port.

If you reach the practical limit of your box either redesign the box so you pick up extra port length, or raise your tune to whatever is feasible.

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Old 08-01-2014, 11:36 AM
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Originally Posted by Mfusick View Post
When you model winISD just do a single slot port. Don't mess around with multiple ports, it's too confusion. Use the total width of the cabinet minus the material on the sides (.75+.75 I assume) and the center brace down the port center (.75") so basically take the total width minus sides and brace and that is your port width. Then you just adjust the port height to achieve the level of air velocity you wish, and let winISD tell you how long to make the port.

If you reach the practical limit of your box either redesign the box so you pick up extra port length, or raise your tune to whatever is feasible.
^ this.

Calculate the width of the port minus any materials in it. So if you have 3 braces like I did, its ( portWidth - 3 * 0.75 )
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Old 08-01-2014, 02:20 PM - Thread Starter
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I will work on the internal volume. The volume at the bottom of the box can be adjusted to make the total volume size closer to 16 cubic feet, which is why I've just been using 16 for now. The actual total volume is around 23 cubic feet if no wood/bracing/subs are taken into account. The bracing in the port still kind of confuses me. I read this article a long time ago when I made my first ported box for my old Pontiac Firebird back in my teenage years, it leads me to believe bracing inside the port would essentially make them separate ports that change the tune.

Here's the article, what do you guys think? Also, just wanted to say thank you for the advice! I've been reading the builds here for quite a few years now and I learn something new all the time

JL Audio Port Tutorial
Quote:
Multiple Ports
There are two widely used methods for calculating multiple ports for a single chamber. Only one method is correct, but, unfortunately, it is the least commonly used.
The first and incorrect method takes its thinking from the original port formula and basically says that if we take two ports and sum their cross-sectional areas, we can just plug this total into the port formula for Av to get our port length. This would sound reasonable, but it can lead to serious mis-tunings in some cases, as we'll see in an example below.
The second and correct way to figure out how long each port should be follows this simple three-step procedure:
  1. Divide the chamber volume by the number of ports you wish to use for that one chamber.
  2. Take the quotient and use that as your Vb (box volume) in the port formula.
  3. Do the number crunching and figure out how long each port should be.
Example:
Let's take an arbitrary box volume of 2.5 cubic feet that we want to tune to 25 Hz with a 4" diameter port. If we plug and chug with that big hairy formula (or let our favorite software package churn out the numbers), we'll find that Lv = 18.844 inches.
Now let's decide that we don't want just a single port because it looks boring. Let's put a 2" port in each corner of the box for a total of 4 ports and see what the two methods give us:
Method 1:
Each 2" port has a cross-sectional area of 3.142 square inches, so we multiply that by 4 to get 12.57 square inches. Plugging in 12.57 for Av in the port formula yields Lv = 18.844 inches for each port.
Method 2:
We want to use 4 ports so we divide 2.5 cubic feet by 4 and get .625 cubic feet. Vb now becomes .625 cubic feet. We are using 2 inch diameter ports so Av is 3.142 square inches. Plugging these numbers into the equation leads to Lv = 20.302 inches for each port.
Notice that Method 1 produces the same port length as did our single 4" diameter port, which is to be expected. (After all, we have the same total port cross-sectional area, which this school of thought proclaims is correct!) However, the first method is incorrect because it neglects the frictional losses encountered by using many smaller ports there is a higher port wall surface area to cross-sectional area ratio, which raises the total amount of frictional losses in the ports and, therefore, shifts the tuning!

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Old 08-01-2014, 02:40 PM
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I will work on the internal volume. The volume at the bottom of the box can be adjusted to make the total volume size closer to 16 cubic feet, which is why I've just been using 16 for now. The actual total volume is around 23 cubic feet if no wood/bracing/subs are taken into account. The bracing in the port still kind of confuses me. I read this article a long time ago when I made my first ported box for my old Pontiac Firebird back in my teenage years, it leads me to believe bracing inside the port would essentially make them separate ports that change the tune.

Here's the article, what do you guys think? Also, just wanted to say thank you for the advice! I've been reading the builds here for quite a few years now and I learn something new all the time

JL Audio Port Tutorial

It's pretty good. The effect length of slot port is important to read and understand. Winisd nails port length pretty much dead on with a slot port. I created my port short originally and set it up so I could extend if necessary since there was talk of it being up to 25% off... it was basically dead on, so ended up extending my port to the size it listed.
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Old 08-01-2014, 02:46 PM
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+1. It nailed both of my slot ports nearly Perfect. I'd measure but I did blast sine waves one digit at a time and watched the cone movement decrease then increase to figure out tune. It's was dead on my 20hz target.

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