where am i going wrong?

[eric12421 10+ year member]

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

Click to read more...

 

[lilmaniac2 5,000+ posts]

http://www.accelerator-pedal.com/ported-box.asp

 

[eric12421 10+ year member]

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 5,000+ posts]

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

Thats a whole lota numbers //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[j3bus2k3 5,000+ posts]

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

 

[eric12421 10+ year member]

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 10+ year member]

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

 

[Ingleside 10+ year member]

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 5,000+ posts]

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 10+ year member]

vb needs to be in cubic inches

 

[Ingleside 10+ year member]

EDIT: see earlier post.

 

[eric12421 10+ year member]

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

 

[j3bus2k3 5,000+ posts]

I edited my post for your answer...post #9

 

[eric12421 10+ year member]

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 5,000+ posts]

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"