View Full Version : port length and construction

01-07-2003, 08:00 PM
attached is a diagram of my box (all dem are inside), alreadt built. the sub is inverted and the cone inside comsumes 0.02ft^3 from the total 1.9567 ft^3 leaving 1.9367 ft^3 before port displacment. the sub will be running 600 watts, and have 3 --3" ports, i belive keeping the port air vel below 30 m/s.....now i pulled the equation from jl's website for port length but cannot seem to get a reasonable length from it (i get something around 5000 inches!) so here are my questions:

1) what is the defining equation for port length? please calulate what my port length should be tuned for 32 hz so i can confirm it when i calulate it with the new equations.

2) i belive the ports are going to be rather long (20+ inches)....so i would like to use 3" pvc, twp questions with this:
--will they rattle inside the box? if so what is a good aproach to prevent this?
-- how do i finish the port at the surface (where you normal see the flared tube)?

any other tips for porting this box will be helpfull, im ported it tommorow or the next day.

01-07-2003, 08:33 PM
ok....can you tell me how you get that so i can mess with different diameters?

secondly....can you sugguest the inside demensions of a slot port for that (i want it to be on b1 on that diagram (the depth is 20.5" inside). also an equation for the length of that port as well....can a slot port just be a cube, no bends or anything neccasary?

01-07-2003, 08:51 PM
ok jl suggest the port to be 1.75"*14" (internal) with a length of 24"...but tat length is dependant on the volume, so i would assume this would change in my case......can i bend the slot ports as well? would be be a poor idea to go with a tunning frequency about the suggested 32 hz? or should i just stay with that? i would probably like to keep that 1.75*14 demension, and just change the length to fit my volume? ide like the equation for it as well please.

01-07-2003, 09:06 PM
ok i got the equation for port length from jl's site to work....but i get 26.6" the equation reads something like this:

L= (Av * 1.84x10^8 * 0.159^2) / ( F^2 * V)

Av = cross sectional area of port
F = tune freq
V = volume in cubic inches

now they say when you do multiple ports you DO NOT add the cross sections, but divid the volume by the number of ports and use that as the volume. so for my 3--3" ports i used 0.646 *12^3 in^3 as the volume and 7.07 as the cross section....did i do this right?

i did the same with the 1.75*14 slot....24.5 cross section and 1.937 volume....what am i doing wrong?

01-07-2003, 09:37 PM
ok...sorry for all the changes.....i want a 14*1.75* xx slot port tunes in the 1.937 cu ft box (before port displacment). i NEED to know how to calculate it as well. i also need to have the shortest port possible, shown below is a diagram of what i can do for the slot port, it used to be to scale, but its not anymore (box changed as i built it :) ). please help!

01-07-2003, 09:44 PM
another option for slot port dependant on size:

01-07-2003, 11:24 PM
http://www.jlaudio.com/tutorials/ports/index.html .....no offense but i trust them over even you JMAC.....either way i am more conserned with how long this slot port needs to be and how to calculate it....a bit of help please (ide like it to be as short as possible...i dont think slots create noise do they?) thanks gents

01-07-2003, 11:28 PM
im not apposed to porting that front edge, but i know you are not suppose to have a ratio of h:w of more than 9:1 in a slot port, does that make the length different? if i have the room for a 1.75x14.....doesn't the length actully stay the same? or what happens is i make it like 4*20.5 ??!?!!? makes construction easier! :)

01-08-2003, 06:01 PM
ok i want to use something like the 1.75 * 1" port-- any reason not to?
how long does that need to be?
how do i calc that length?
does the chamber for air need to be as strong and sealed the same as the rest of the box (i was planning on using 1/2" mdf instead of 3/4 just for that) and then just wood glue...do i need to silicon?

01-08-2003, 07:28 PM
Originally posted by Jmac
1.75" x 1" port ?

Anyways, you can probably use 1/2" MDF for the port walls, but you might have to brace the long pieces .. Build with the same quality as you would with a sealed box ;)

1.75*14"....the 4 got cut off....thats the recomended demension for a ported 12w7 so i would like to use them. please show me how to calc it, so i decide to change from .5" or 0.75" mdf i can re calculate it.

01-08-2003, 11:38 PM
and what do i substitute for diameter in the case of a slot port?

01-10-2003, 08:49 PM
so my calcs are as follows...comes out somewhat short?
d = 14.31863cm --(2*sqrt(h*w/pi))*2.54
v = 55.4137 lit --1.9567*28.32

[ (23562.5*14.3186^2) / (32^2 * 55.4137) ] - (0.732 * 14.3186) = 16.2165 cm

16.2165 / 2.54 = 6.38 inches?!?!!?!?1 doesn't seem remotley close.

btw the volume is 1.9567 cu ft
fq = 32 hz
port dim: 1.75"*14"

01-10-2003, 09:10 PM
is it 23562.5 or 23542.5 you switched half way through?
i dont understand how a gross volume can be calculated, because the gross is dependant on the port length and the port length is dependant on the gross....anyways if i use that as volume, i get a port length of 29.1524 inches, which changes port displacment again...its circular!!

01-10-2003, 09:15 PM
also if i make that port smaller, 1.75*12, i get a port length of 24.7, much more managable, but is that to small....might that cause noise?

01-10-2003, 09:21 PM
ok...makes some sence now, i just need to know the tuning fq so that i can set the xo on my amp properly right? as long as i am close to 32hz i will be fine right?....does that smaller port mentioned earlier work ok?