1. ## where am i going wrong?

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

2. ## Re: where am i going wrong?

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

3. ## Re: where am i going wrong?

u took the square root instead of makin that sharp left turn?

Thats a whole lota numbers

4. ## Re: where am i going wrong?

1 3" port is NOT enough for 4.5ft^3....period.

5. ## Re: where am i going wrong?

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?

6. ## Re: where am i going wrong?

yea i spent alot of time on this so far like two hours, so im kinda pissed but whatever, practice makes perfectt

7. ## Re: where am i going wrong?

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.

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.

8. ## Re: where am i going wrong?

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^^^

9. ## Re: where am i going wrong?

vb needs to be in cubic inches

10. ## Re: where am i going wrong?

EDIT: see earlier post.

11. ## Re: where am i going wrong?

oh **** i figured that but i didnt know...thank you very much

13. ## Re: where am i going wrong?

Originally Posted by Ingleside
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.

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?

14. ## Re: where am i going wrong?

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"

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