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eric12421
05-11-2006, 12:20 AM
hey,
im building a ported box for 2 12 inch swiss audio spl1290's.
i am trying to figure out what the length of the port should be but i just cant get it....im using this equation
Lv = [(1.463*10^7R^2)/(Fb^2*Vb)] - 1.463
ive searched and searched and read the entire sticky in this forum but i keep coming up with an outragously large number...

the box before the port is going to be
34.5 inches long
19 inches deep
and 14 inches high
andthe port is going to be 3 inches tall
i want it to be ported to 36 hrtz
the internal cubic feet is 4.5 giving each one about 2.25 cubic feet
im going to have the subs on the top of the box firing upwards and the port in the front like near the ground...

so using that formula i came up with that the
square root ((heightxwidth)/pie) because its a vented port.
square root ((3x33)/3.14) 33 beacsue tahts the inside measurment.
so i got 5.613615478.......thats the part that replaces the r casue its a vented box.

so....
Lv = [(1.463*10^7R^2)/(Fb^2*Vb)] - 1.463

Lv= [(1.463*10,000,000*31.51267873)]
--------------------------------------------- -1.463
(36^2)*(4.5)


Lv= [461030489.8]
------------------- -1.463
5832

Lv= [79051.87]-1.463

lv=79050.40 now obviously thats wayyyyy to big for the box???

anyone have any idea what im doing wrong??? like units wrong? the only one i can think of is the Vb i ahve it at 4.5 thats cuboc feet, should that be in inches or something, ikd im reallly confused.

thanks

lilmaniac2
05-11-2006, 12:30 AM
http://www.accelerator-pedal.com/ported-box.asp

eric12421
05-11-2006, 12:41 AM
yea i tried that
but like i want the port to be on the bottom of the box so its like 33 inches long. and like 3 inches high...i just need to know how long its supposed to be, and i cant figure it out

davidfre21
05-11-2006, 12:42 AM
u took the square root instead of makin that sharp left turn?

Thats a whole lota numbers :D

j3bus2k3
05-11-2006, 12:43 AM
1 3" port is NOT enough for 4.5ft^3....period.

eric12421
05-11-2006, 12:49 AM
i want a slot port....

the box is going to be 35.4 incehs wide and 17 inches tall after the port, and 19 inches deep. i am going to have the subs be mounted on the top of the box and the port running accross the bottom of the box, so the port would be 34.5 inches wide and 3 incehs tall, the only thing i cant figure out is how long should it be?

eric12421
05-11-2006, 12:50 AM
yea i spent alot of time on this so far like two hours, so im kinda pissed but whatever, practice makes perfectt

Ingleside
05-11-2006, 12:51 AM
i really dont know what the hell you did there. it looks like you're making a slot port box. but one thing is for sure.. you might run into problems with your 11:1 ratio for your port. usually you want to sick with something around 1:8.

read this to help you figure out this mess:
http://mobile.jlaudio.com/support_pages.php?page_id=165

EDIT: i did the work for you

if you are going to have a box be 4.5 NET volume, with a 33 inch wide by 3 inch tall port and you want it tuned to 36 htz, your effective port lenght is going to be somewhere around 37.5 inches.

j3bus2k3
05-11-2006, 12:52 AM
give me a few minutes to do the math...

The formula out of the Loudspeaker design cookbook gave me 39.4" if the port os 34.5"x3" and the net vol is 4.5 and desired tuning is 36hz.

EDIT: used 34.5" instead of 33...ignore^^^

maldecido33
05-11-2006, 12:53 AM
vb needs to be in cubic inches

Ingleside
05-11-2006, 12:54 AM
EDIT: see earlier post.

eric12421
05-11-2006, 12:57 AM
oh **** i figured that but i didnt know...thank you very much

j3bus2k3
05-11-2006, 01:00 AM
I edited my post for your answer...post #9

eric12421
05-11-2006, 01:00 AM
i really dont know what the hell you did there. it looks like you're making a slot port box. but one thing is for sure.. you might run into problems with your 11:1 ratio for your port. usually you want to sick with something around 1:8.

read this to help you figure out this mess:
http://mobile.jlaudio.com/support_pages.php?page_id=165

EDIT: i did the work for you

if you are going to have a box be 4.5 NET volume, with a 33 inch wide by 3 inch tall port and you want it tuned to 36 htz, your effective port lenght is going to be somewhere around 37.5 inches.


crap how could i madke the port shorter so it would fit inside the box, the box is only 19" deep, so im pretty sure that 37.5 inches long wont fit....what else should i change?

j3bus2k3
05-11-2006, 01:02 AM
decrease port area...

Make it 2" tall instead of 3", or instead of going along the bottom, have it be on the side from the top to the bottom somethin like 15.5"x4"

eric12421
05-11-2006, 01:08 AM
so if i had two slot ports one on each side that would work?

eric12421
05-11-2006, 01:14 AM
i think i might have figured it out...if i do that same thing but i make the port 2 inches tall and 33 inches wide then it would only ahve to 5.80 inches long, does that sound right?

j3bus2k3
05-11-2006, 01:16 AM
no, not at all....

EDIT: did the math and it came out to be 23.779" so you can roud to 23.75". Again, thats for a slot 33"x2"; Net volume is 4.5ft^3 and Fb=36Hz.

eric12421
05-11-2006, 01:22 AM
crap...ok how long should my port be if i want it to be 33 inches long, thats the only definate thing right now... it doesnt have to be tuned to 36 just somewere around there

j3bus2k3
05-11-2006, 01:23 AM
look up...^^^

eric12421
05-11-2006, 01:24 AM
oh yea....but the depth on my box can only be 19" cause i have a pretty small trunk opening

j3bus2k3
05-11-2006, 01:25 AM
so you use a second port piece...you bend the port. One port wall is 16.25" long and the second piece is 5"

eric12421
05-11-2006, 01:29 AM
oh
**** that would do it


thank you so much youve been a great help...

j3bus2k3
05-11-2006, 01:30 AM
Its late and I've been up awhile so I dont know how accurate those numbers are, not to mention I did them all in the dark with pen and paper....just as a disclaimer. But it all reality, if there off, you will not hear a difference and tunning would drop/rise by like .001 Hz...

eric12421
05-11-2006, 01:33 AM
no problem

j3bus2k3
05-11-2006, 01:33 AM
Just remember to maintain the width of the port all the way around too...and I changed the value of the second port piece. I typed 5.75" instead of the correct 5"....

eric12421
05-11-2006, 01:38 AM
so would the second port peice be 5 inches or 5.75 inches? sorry

j3bus2k3
05-11-2006, 01:43 AM
5"...I did it 3 times with a diagrams on Paint to try and help my sleep deprived mind get threw it, lol

eric12421
05-11-2006, 01:47 AM
my bad, thank you very much

j3bus2k3
05-11-2006, 01:49 AM
no problem mon!

/Jamacian Accent.

eric12421
05-11-2006, 01:53 AM
ahha youll never guess....i used to ahve dreads.. thats amazingggggggg

j3bus2k3
05-11-2006, 01:55 AM
lol thats awesome! Kinda twilight zone-ish though...

eric12421
05-11-2006, 01:59 AM
ehh..i loved em, you should have seen the stares i got when i had them,only abd thing got pulled over about 5 times with them and since only once

j3bus2k3
05-11-2006, 02:01 AM
LOL...Profiling FTL!!

eric12421
05-11-2006, 02:03 AM
yup it ****** but you know what... completly changed my outlook on life. so im gonna have to say it worth it.

eric12421
05-11-2006, 02:09 AM
alright finished drawing it up....do these number sound about right?

side= 15.25"x18.5"
top and bottom=19"x34.5"
back=33"x15.25"
front= 33"x13.25"
port 1=33"x15.5"
port 2=33"x5"

i hope that sounds right....

j3bus2k3
05-11-2006, 02:11 AM
uh...I really dont know without knowing how you want your pieces to fit together...Its all on you to make sure your final numbers are right. Measure 3 times, cut once. good luck man.

EDIT:ok, about the first port wall...I assumed the port wall was to be placed behind the baffle so the .75" was already taken into account. If its to be mounted with the baffle then ADD the .75"

j3bus2k3
05-11-2006, 02:20 AM
So If you want to place it behind the baffle, return the first port wall value to 16.25", if you want it next to the baffle then it needs to be 17"

EDIT: wait a second...what is the outer depth dimension?

eric12421
05-11-2006, 10:31 PM
i dont get what your asking?

j3bus2k3
05-12-2006, 12:14 AM
Your box, according to page 1, is to be 19" deep correct? Making the inside depth 17.5". Thus you were correct in your first port wall measurment of 15.5". I was mistaken earlier, It was late and I did it all in the dark, lol. And my original calculation for the second wall was correct of 5.75", lol. So in conclusion (lol), The first port wall length should be 15.5" and the second would be 5.75"