slot vent length

[alteredbeast77]

hello all

need a little help with with vent length.

single 15"driver
enclosure is 48"w x 18"h x 24"d,all panels and port walls 1.5" thick throughout
gross volume 8.2
net volume 5.0
tune 20Hz
one chamber
2 slot vents/ports 1.5"w x 15"h x 49.5" long at either end of enclosure exiting to the front.
left,back,right side will be the outer vent wall with divider at vent entrance behind driver in the center of the box.

my broblem is that every formula or online calculator,winISD beta/pro,i have tried them all,i get a different length,some differ by 6", what is the most accurate way to calculate vent length.

thanks.

loren

[Wheels]

http://audio.claub.net/software/kougaard/ubmodel.html

a lot of people here use Unibox. Unless I have been very mistaken you need the driver specifications for any of the software to give you an accurate result.

[alteredbeast77]

I am familiar with unibox it is the first one I used.

let me ask my question in a different way.
with all t/s parameters entered correctly which program yields the most accurate results for the length of slot port.

for example winISD Beta Calculated a Port length of 49.5 Inches.
winISD pro calculated a port length of 55 inches.
unibox was some where between .

which one do I trust ?

[---k---]

The difference is in the assumption of the amount of stuffing used, which affects the volume calculation. The math is solid. All of them are doing it correctly, they are just using a different stuffing coefficient.

I have always liked Unibox. Unibox allows you to choose between no stuffing, medium, and heavy stuffing. Guess what you are going to use, probably medium, build, and then listen and adjust the stuffing to get the perfect tuning.

[alteredbeast77]

thanks ---K--- your right, i didn't thank to mess with the stuffing setting in unibox ops: , winISD pro and unibox are with in .25" of each other on the length now, that's good enough for me.

time to make some dust

[sdl2112]

I don't have much experience myself (yet), but I have generally read that port lengths need to be shorter than calculated for a given Fb. Best to leave some flexibility. Troels has a nice write-up on the subject here

[alteredbeast77]

sdl2112 that write up help clear up a lot of questions, thank you for posting a link,

[Undefinition]

I learned most of what I know about slot ports from this site (and my own experimentation)

http://mobile.jlaudio.com/support_pages.php?page_id=165

[Wheels]

Thanks Undefinition, looks like I was very much mistaken on how to calculate port tuning.

[alteredbeast77]

undefinition, i ran across that tutorial a few days back on JL's website , the thing is that i can not figure out that formula :M.

[Undefinition]

undefinition, i ran across that tutorial a few days back on JL's website , the thing is that i can not figure out that formula :M.

Oh yeah. The formulas you need for speaker design are pretty complex. But that's what MS Excel is for. Basically, when learning speaker design, do your best to understand the formulas, but don't sweat it too much. It's the "general idea" that matters. (says me)
Reason being, from that point once you start plugging values into a program like Unibox, you'll notice patterns. That's where the real learning begins, I think.

Here's how I do a slot port. I wish I could say it's easy, but you'll notice that it's not exactly a linear process, and there's some "bouncing back and forth" involved.
1. Figure out what a round port would be first. Because basically all a slot port is is a rectangular-shaped round port. So you need to find a reasonable-sized round port that isn't too big to fit in the box--and not too small that it chuffs.
2. You need to figure out the "surface area" of the opening of that round port. (For instance, a 3" dia port has a "surface area" of 7.068 sq in (using pi*r^2).
3. Here's where the Jr. High algebra comes in. You have to decide on a rectangle that has a final surface area equal to that diameter of the round port. I'm going to keep it easy and make it a 7" x 1" slot port, because 7*1 = 7 in sq.
4. Of course, we're off a bit from our original surface area, but not by much. So I can recalculate a round port by using some high school algebra:
7 in sq = pi*r^2. Solve for r. Double it to get the new diameter, which in this case is 2.98". (I made an excel sheet for these calculations because I got tired of doing them by hand. It's available for download on my site)
5. Now I'll plug that revised diameter (2.98") back into my box modeling software. Whatever length it gives me is the length my slot port needs to be--sort of.
6. You see, with slot ports, the wall of the cabinet can "extend" the length of the slot port more than intended. I follow the advice on JL's site, and merely subtract 1/2 the height of the slot port from the final length. (Since my slot port is 1" high, then I can subtract .5" from length Unibox gives me to get the precise correct answer. I don't really think this last step is critical. Reason being, you're pretty close, and at this point the speaker designer may be adjusting the box tuning up or down by a few Hz to get a bass response that sounded right to me (versus just "looking right" on screen). Of course, with a round port, adjusting the bass tuning is simply a matter of using a shorter or longer tube. With a slot port, you've somehow got to give yourself a bit of "adjustment room" in case you want to make the slot longer or shorter. I won't give you a precise way to do this, but I will say that hot melt glue is a nice way to temporarily hold wood pieces in place, that can be removed later by tapping on it with a hammer.

Well, that was a mouthful, but hopefully it helps.

[alteredbeast77]

thanks undefinition, i really appreciate the tutorial

i think i have this all figured out, took a lot of back and forth math to keep the vent width and height the same and only adjust the length, your excel sheet came in handy for that task.

if anyone has time to give this a once over for mistakes i would really appreciate it

box front will be 2.25 to flush mount and roundover on vents.
1.5" mdf for everything.
48W x 18H x 24D
8.2 ft^3 gross
Vb 5.0 ft^3 net
Fb 20 Hz
2 vent's 1.5"w x 15"h x 57"

Qts 0.45
Vas 100L
Fs 23Hz
Re 3.45
Le 4.17
Xmax 20.5mm
Qms 7.71
Qes0.48
SPL 87 db
Sd 775 cm^2

thanks

[Wheels]

Quick question, When you calculate Vb, I know the port volume is idependant from this, but what about the driver volume? I had been told a few years ago you were supposed to subtract off the driver from the Gross vol, but I can't always find the volume of the driver. Sometimes an equivalent volume is listed, but I don't think that is the same thing. It seems like that could effect port tuning quite a bit. What do you guys do?

[alteredbeast77]

you subtract driver displacement and port displacement from gross volume to get net volume, as well as anything else that will be put in that gross volume must be subtracted from gross volume to get net volume/Vb.

[alteredbeast77]

well if i did the math correctly my enclosure is .8 cubic feet to small now due to the extra 6.5" in length that unibox added to the vents :banghead:

the enclosure is as big as i can make it, any bigger and it wont fit in my media center as the tv stand .

well back to the drawing board

[Wheels]

I'd change the TV stand

[---k---]

more stuffing? raise your tuning?

[alteredbeast77]

wheels, the space that the tv stand occupies between the towers is the only place in the room that a box of this size will fit

i think what i will do is just stick with my original design with 49.5" vents witch should yield something like 21.?? Hz tune, i think, and line with heavy insulation and also stuff with polly fill until i get what i want :unsure:

[Wheels]

ah, bummer. well, good luck with it!

[alteredbeast77]

I learned most of what I know about slot ports from this site (and my own experimentation)
http://mobile.jlaudio.com/support_pages.php?page_id=165

undefinition, i understand how to work the JLaudio formula now, and thanks again for the tutorial

[alteredbeast77]

how close is to close ?

http://www.htguide.com/forum/attachment.php4?attachmentid=&stc=1

the silver erea represents the woofer, the vent's are 5" from the motor.

1 will this effect the air flow of the vent's being this close ?

2 can i extend the vent divider up to the motor to use it as a brace, or would this effect vent length/tuning or act like a chamber divider ?

the motor structure has a radial vent, it is not vented out the back side of the motor.

this is the first time i have ever posted a photo in a forum, so this may not work the first time around ops:

[---k---]

If you use a bigger port size, it will make the port longer. But, what happens if you make it bigger and use just one? Will it fit from the back left corner to the front right? Or something like that?

What driver are you using? post the pic of the frequency response model too.

[Undefinition]

I imagine he likes the symmetry of two slot ports.

I have no idea if your port length is right, because I haven't done your number crunching. However, the actual physical cabinet design of yours looks good. I wouldn't make the center brace any longer. 5" is plenty of distance between a vent opening and the driver.

[alteredbeast77]

:coffee: ok, i reworked all the math with the JLaudio formula and unibox, and now i'm showing 2 vents at 26" give or take, what confuses me in unibox is going from round ports to slot vents,i don't think i'm doing something right

Undefinition, yes i want to keep it symmetrical so it blend's in, with a grill on it the vents are all you will see

---K---, the driver is, alpine SWR-1542-d type-R

http://www.alpine.com.au/files/current_owners_manuals/SWR-1542D.pdf

i cannot for the life of me figure out how to post a pic. of the graph, still learning excel bare with me, but if i understand it correctly it is flat down to 20Hz @ 115db

i really appreciate the help