View Full Version : What?!

05-31-2004, 12:28 AM
i went to a port length calculation website and it said wiith a square port in a 4.5 cube box if the port is 2 inches wide and one inch long then the length should be .75 inches. the MDF is .75 inches. so could i just cut a 1" by 2" hole in the box?

another question. the resonant frequency of my subs is 22.5 hertz. shouldnt i port it right at the resonant frequency so the subs move as much as possible? this seems to be a really low frequncy to tune at though. help!

05-31-2004, 04:02 AM
i would think that is correct, with so very little port area, only a small length is needed to get to the proper tuning

im not sure about this but generally people say not to go below 25ish

06-01-2004, 12:32 AM
Compressibility would be so much of a factor with a port that small that the sub would basically act like it were in a sealed enclosure. The air would compress to the point that it would effectively plug the hole.

06-02-2004, 03:44 PM
ok thanks guys, i forgot about that response curve.... if i ported that low i would have terrible response at higher frequencies. thanks

06-02-2004, 03:46 PM
11" by 3" area, by 17.2 inches long. 30 hertz. that sound better?

06-02-2004, 04:13 PM
wait, i didnt notice that that site says i have to have a minimum of 138 inches of port area. that is rediculous, if the port area is that big i would have to have a port length of almost 50 inches. why is it saying this

Randy Savage
06-02-2004, 04:15 PM
You want anywhere from 45-60 in of port for that size box...

06-02-2004, 04:34 PM
actually i calculated it to be 7.23 cubes for both, 3.615 cubes each. how am i supposed to fit all that length into a 3 foot box? angle the port?

Randy Savage
06-02-2004, 04:56 PM
Are you doing a common chamber? I generally use anywhere from 10-15 in of port per cubic foot.

If the length is longer than the depth/height of the box (whichever side the port fires), you will have to make a bend in the port. With this, you have to take end correction into consideration here...

06-02-2004, 05:06 PM
You don't count the length of the port do you? It's justy width by heigth right?...

Randy Savage
06-02-2004, 05:08 PM
Port area is the LxW of the port opening.

Another thing, the port displacement has to be calculated and subtracted to the total displacement of the box...so if you are shooting for say 4 cubes net, you need to take the port, as well as woofer displacement into account...

Randy Savage
06-02-2004, 05:21 PM
Don't forget about bracing ;)

True- but depends on what kind of bracing...:)

Randy Savage
06-02-2004, 05:25 PM
I know, silly...

But say you use threaded rods...that will be tenths/hundredths of a cube, which won't change anything...:)

06-02-2004, 11:57 PM
yes common chamber

06-03-2004, 12:05 AM
ok.... if i used 10 inches per cubic foot in a 7 cube box i would have more port than box to fit it in.

06-03-2004, 01:27 AM
This is why I like sealed boxes.... I **** at teh Vent calculating..... Anything else I understand...

I always hear people say X wide by Y Long for the vent, but I never hear them talk about how deep inside the box to make the vent....

06-03-2004, 01:33 AM
ported is very complicated to me too, you just can't slap a box together like a sealed.

06-03-2004, 02:05 AM
four hundred posts....... only 9 thousand behind you jmac. lol

06-03-2004, 04:52 PM
what happens if i dont make the port 10 inches for every cube.... even if it is still tuned correctly?

06-04-2004, 12:04 AM
wait! when it asks for box volume on the port calculator, is that the volume of the box after i subtract the volume of the port?

06-04-2004, 10:06 AM
Yep, it is the volume of the enclosure after displacements (vent and sub).

06-04-2004, 11:39 AM
how are you supposed to know the volume of the port before you calculate it

06-04-2004, 11:40 AM
and what would happen if i didnt use 10 inches of port for every cubic foot?

06-04-2004, 11:42 AM
Basically you should figure out how much airspace you need for the enclosure. Calculate the size vent you need for that volume at the freq that you want. Once you have those numbers, you can figure out how big the box will be total. If it won't fit where you want it, you go back to square one and start all over. This is one of the main reasons that designing and building a vented enclosure is more difficult than a sealed.

06-05-2004, 12:38 AM
oh i see, figure the volume then the port and add them.... itwas difficult for me because i had allready built a sealed box and i was trying to cut a whole and stick a port in there.... it just took a lot of guess and check but ive got a design now. thanks everyone for your help!

06-05-2004, 02:58 PM
Most people figure the enclosure backwards. Here's how you do it correctly.

Decide on the amount of airspace you want.

Decide on a tuning freqency.

Figure the depth and surface area of the port.

Add the displacement of the port (Length * (Width +.75) * Depth for slot ports)

Add the displacement of the sub and any internal bracing.

Now find an outer length, width and depth that equals the product of your previous figures.

If you try to figure the outer dimensions first instead of last, you're going to have to do a lot more and more difficult math in the long run.

06-05-2004, 11:34 PM
yea, well i did it the dificult way but i finally got it workking, although my port only has an area of about 100 square inches