Hi! I'm designing my box for my alpine type x and using the calculators, I found that I cannot use less than 6,2 inches of port diameter to get no port noise cause it has 19mm of Xmax. (so 30,18sq in. of port area).

Does that make sense?
Can I use 2 smaller ports if togheter they have the same area?

 

or this calculator is probably wrong because I just noticed alpine recommends a 12 sq. in. port. But that's for a rectangular port.

 

Sounds about right with max rms power that port diameter allows max port velocity to be just under 20m/s...unfortunately it needs to be 56 inches long to tune the box to 32 hz. What is the volume of the box? how much power are you going to use with it? The Alpine recommended box works fine for music and the way most people listen to their systems. The truth is you'll mostly be in the 50-300 watt range which will deliver no port noise at all. However at max output there could be port noise..though you'd have a hard time hearing it with the sub pounding. Remember Alpine (and most manufacturers) want to make the subwoofer most attractive to consumers..telling them that it should be in a 2.5 cubic foot box with a 12" x3.5"x 38" port makes it less attactive, and for most people unneccessary. I've used those subs in a few systems and they work very well with the Alpine box for all types of music. I like my system tuned a little higher though...34 hz, and a bigger box 1.75 cubes net (after driver and port displacement) lined with 1 pound of polyfill (this increases virtual volume to about 2.0 cubic feet) and with a 12 x 2.25 x 27 inch slot port. This gives more output between 25-55hz and the same output above and below. The port velocity is below 30 m/s @ 1000watts. I used that set up in two vehicles with excellent results...tight, clean, deep and loud bass.
Here is a plot of the Alpine spec box (yellow) vs. mine (red):

 

It's a fiberglass enclosure of around 1.5 cu ft., I'll measure it exactly when I have the time. And I am only sending 800 watts rms to the sub.
Does adding polyfill changes the calculations of the port? I mean do I use the "virtual volume" tu calculate the port lenght?

I wasn't even planing doing a ported enclosure when I began my fiberglassing, but now I realize that I might have enough volume to make a decent one. But I won't be able to fit a 27 inches port or so, that's why I was thinking of two 3inches round ports or something like this. But would I get port noises so bad?

 

yes the virtual volume is what needs to be considered. unfortunately, theres no way to predict exactly how much virtual volume is created by the stuffing. Luckily, there are some guidelines from guys that have tested and measured enclosures with differing amounts of stuffing. The above box, I've actually measured myself, so i know what it tunes to (that's why I was very specific with the volume, weight of stuffing and port dimensions). See this article for more info regarding polyfill:

Ultimate Polyfill Subwoofer Enclosure Resource [audiojunkies]

you can't just divide the port diameter in 2. You have to divide the port area in 2. for example; area of a 6" port is approx 28 square inches, to keep the length the same, you'd need two 4.2" diameter pipes. Given what you are using (1.5 cube without stuffing, fibreglass enclosure), I'd suggest a single 4" x 14" port (36hz) or similar. Two 3" ports has slightly more port area compared to a single 4" port (using Pi(3.1415) x radius squared you get 14.13 square inches for two 3" vs. 12.56 for a single 4")To reduce port noise to nearly inaudible, and to keep your port as small as possible, you can use a flared port like these:

Precision Sound Products

hope that helps.

 

umm what size woofer?

if you only have 1.5 cubes to work with, stay sealed

 

It's a 12 inches

 

I just measured my box and it is 1.7 cu.ft.

Would I be better to stay sealed? Alpine recommends a 1.5cu.ft. for a ported box but like veman said this might not be the best box...

 

Is that the internal volume? what type of music do you listen to? how important is output overall...both will sound good, but the ported will be significantly louder. For me and what i listen to (pretty much everything from opera to hip hop), I'd go with a ported design, as I had 2 12's in a sealed box with 1000 watts and switched to a single 12 in a ported box with 500 watts. It was much louder in the range it played (25hz-60hz) and sounded just as good, but with much less stress on my electrical system. With 1.7 cubes plus 1.5 pounds of polyfill, you'll get a virtual volume of about 1.9 cubes net which will work well with that 12, you'll need a 4" x14" port tube preferably flared. Here's a plot of the sealed vs. ported:

 

Yes it's the total internal gross volume. I really listen to everything like you. I tried the box how it is (sealed) and I found it lacks of output...
What frequenc would it be tuned with the 4x14 tube?

Btw, thanks a lot you're helping me a lot