View Full Version : Port Help Again

05-09-2005, 01:27 AM
I asked for help on my last box but i can't get it to fit in my trunk so now i have a new box and i need help with the port size.

the box INTERNALLY measures: 14.875 W x 12.5 H x 14.5 D

it is a box for a single 12... can some one help me with the port size?

I want it to be tuned to 30 hertz the internal volume is:

1.5602394386574074074074074074074 cu ft

the port face is 14.875 x 3


I know the formula.. i just can't seem to get the right answer.

Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv is port length (what your solving for)
Av is your port area (height x width of the port face)
Vb is the internal volume of your box
Fb is the tuning frequency you want

subtract half of your port width from your port length(Lv) to get physical Lv

help plz

05-09-2005, 07:08 PM
any help?

05-09-2005, 07:20 PM
wow thats some serious stuff. I wouldnt even know which gets done first in that equation.

05-09-2005, 07:47 PM
wow thats some serious stuff. I wouldnt even know which gets done first in that equation.

Thanks alot... if some one can please break down each step of the forumla i can't figure out what the ^ means

05-09-2005, 07:54 PM
it means to the power of

05-09-2005, 07:56 PM
winsid pro seems to come reccomended for figuring this stuff out.

05-09-2005, 07:58 PM
Thanks alot... if some one can please break down each step of the forumla i can't figure out what the ^ means

what the person said above

2^3=8 and so on

^= to the power of

05-09-2005, 08:09 PM
with winisd it shows that with a 2x3 port face 10" long i will have a tuning of about 30 hertz

Is this right?

05-09-2005, 08:12 PM
I got ~80" long port when done by hand.....
BBP pumped out....~84".....
id probably just go with lest port area especially if it is just for 1 12" cuz that would be like a 4 cube net box b4 displacment......

05-09-2005, 08:25 PM
Heres how i break the formula down.....
first of all to make things less confusing just use the cubic inchecs and skip the * 1728 in the formula...cuz that makes it more confusing....so for ur box ur Vb is 2696.09

its pretty easy tho....
multiply ur port area (Av) by 1.84*10^8
then divide that by

ur desired frequecy * 6.2893 (same as Fb/.159) squared, times ur volume again.....

then minus .823 * the square root of ur port area

So.....the formula i use....which is only slightly diffrent then urs looks like this

Lv= ((Av(1.84*10^8)/(6.2893*Fb)^2 * Vb)) - .823(sqrtAV)

so for ur problem u get

X= (44.625 * 1.84*10^8) (= 8211000000) / (6.2893*30)^2 *2696.09 (=95980170.53)

8211000000/95980170.53 = 85. 5489
85.5489 - (.823*(sqrt44.625) (=5.4978)

85.5489 - 5.4978 = ~80.5.....but i just round to whole numbers because i usually figure that the diffrence is to small to worry about....i just dont think its necisarry (sp? lol) to get too exact with it......

05-09-2005, 08:30 PM
is it possible to have a 80.5 inch port in a box that internally measures 14.875 W x 12.5 H x 14.5 D???

05-09-2005, 08:34 PM
yes, but your NET volume will be severely decreased.

05-09-2005, 08:36 PM
would i get close to a 30 hertz tuning if i used a 2x3x 10 long port?

05-09-2005, 08:36 PM
2x3 port is toooo smalll even for a single 12
but to answer if its right or not

x = ( we want this to be ~10) ((6*1.84*10^8)(=1104000000)/((6.2893*30)^2 * 2696.09( = 95980170.53)......1104000000/95980170.53= ~11.5 - (.823(sqrt6) = (2.01)....so 11.5-2.01 = 9.49 final Lv = 9.49"
so yes 10" is close enough to yield ~30 hz tuning......

05-09-2005, 08:38 PM
and Dbfan.....is it possible to get a 80.5 " long 3x14.875" port in a 1.5 cube box......seems like it would be impossible.....iono.. u know more than me...but that just seems too big

05-09-2005, 08:38 PM
6"^2 is nowhere near enough port area. I wouldn't give a 12" any less than 25"^2 of port.

Moe Lester
05-09-2005, 09:41 PM
new box time...

05-09-2005, 11:27 PM
NEW BOX IT IS, heres my new box design:

14.875W x 12.5H x 18.5D INTERNAL

1.99 cu ft - INTERNAL

30 hertz tuning

14.875 x 2 port face

Is 2" enuff port?

if my calculations are correct i believe i need to have a 38" port ??

if thats correct how do i go about making the port that long if the height of the box is only 12.5 inches internally? Do i make 4 walls 9.75" with a 2 gap between each?

05-10-2005, 12:07 AM
alright time for a new thread!