Porting a dual 12" box

[Bumpin Expo]

Well I wanted to make a ported box for 2 12" Subs and i have some questions. For now, the plans are to port 2 12" Mtx Audio 8000 Subwoofers. each are going to see around 500 rms. The reccomended space for a ported enclosure is 2-3 cu/ft so I used 2.5 for calculations. Now on the questions

1. I used a calculator on the12volt.com and for a box that would be 2.5 cu/ft and tuned to 32 hz would need 13.23 inches of port lenth with a 4 inch diameter. Now from what I remember from asking about ports in the past, there is "Port Noise" Now what exactly is port noise and how would I over come this problem? Its not as simple as putting a 4" pipe in 13.23 inches long?

2. Say i did build this box, what would be better...2 chamber with 2 seperate ports or one chamber with one port? A box with 5 cu/ft tuned to 32 hz only needs a pipe 6.6 Inches. But what about port noise?

Try to explain in noobie terms if you can. Im trying to figure this out!

[MDIFYTD]

1. Port noise is basically air rushing through the port too fast. The way to overcome this is by increasing the 'surface area' of the port i.e. using a larger diameter port. The problem with using round ports is usually you need a pretty big 'pipe' to get your port mach,speed of the air, to an acceptable level. So the way to get around this is to use multiple round ports or a slot port. A slot port uses one wall of the box along with the top and bottom with an additional wall contructed inside the box. This allow you more square inches of surface are than round ports.

examples of slot ported boxes:

http://community-2.webtv.net/MarvsPlace/MarvsPlace/page2.html

2. There's advantages to doing both common and seperate chambers. For instance with a common chamber box your port will not have to be as long. However, if one of the speakers were to fail then the other would probably fail because the enclosure is so 'uncontrolled'. This wouldn't happen in a seperate chamber box but the your port length will be much longer. Another thing to consider with a 5 cu ft common chamber box is you'll have to brace the crap out of it.

As for the size port you need: those two subs tuned to 32hz require 43.47 sq in minimum. A 4" round port is about 12.3 sq in. So you would need 4 4" ports to get adequate surface area each of them would have to be 23.5" long. Or you could do a single slot port 15"x3" 16.7" long.

[Bumpin Expo]

Hey man, i read multiple ports are bad. It doesnt give the same effect as one big port. I read it on the MTX website under education then box design. So on a box that is 5 cu/ft the port would be


L=16.7"
W-15"
And it will be 3 inches deep inside the box?

[MDIFYTD]

15" tall 3" wide 16.7" into the box

you can wrap the port around the inside of the box to get the length you need.

[Bumpin Expo]

And is this the kind of port you are talking about?

http://www.mtxaudio.com/caraudio/images/integratedVent.gif

but instead of it going in like that it will be a long vent only goin in the box 3 inches rite

[Bumpin Expo]

ahh i c now, sorry about that last comment...

[Bumpin Expo]

ok now that I got that down I have a few more questions

1. Do i need to use the wall as part of the port or can i have two subs and have the port in the middle?

2. Do i Just make the ports out of like 1/2" MDF board to save room?

3. How do I know how much air space this port and internal bracing will take, any calcs u kno of

[MDIFYTD]

1. no you don't have... to you can constuct the port in the middle it's just usually easier to usean existing wall.

2. I would stick with 3/4"

3. no calcs for this one man. for the slot port it would be L x W x H and don't forget to account for the .75" the wood displaces. For the bracing it's basically the same thing you'll just have to measure the size of the hole you cut out and subtract that from the amount the whole piece of wood displaces. So it would be L x H x W(.75") - the hole in the middle...

[Bumpin Expo]

Hey man i think I found a calc to measure the bracing

http://www.eatel.net/~amptech/elecdisc/spboxnew2.htm

Its towards the bottom.

So tomarrow im going to start a box thats going to be 5 cu/ft after bracing and the space the port takes up. The port is going to b 15" tall 3" wide 16.7" into the box. This will tune the box to 32 Hz. All this is correct right??

Also, If I make the subs face the rear...What way should the port b facing? Maybe ill point the subs up and the port fire up also? Does it matter?

[MDIFYTD]

So tomarrow im going to start a box thats going to be 5 cu/ft after bracing and the space the port takes up. The port is going to b 15" tall 3" wide 16.7" into the box. This will tune the box to 32 Hz. All this is correct right??
yep

Also, If I make the subs face the rear...What way should the port b facing? Maybe ill point the subs up and the port fire up also? Does it matter?
Try to have it face the same way as the subs



Hey man, i read multiple ports are bad.
Hmmm I've never heard that and I couldn't find it on their site. Can you show me where you read that?

[93ex]

modifytd got ahold of this one lol

Altho the infamous crx spl box has port and subs facing diff directiosn, if I recall correctly

[Bumpin Expo]

My bad i saw it on diy subwoofers http://www.diysubwoofers.org/faq.htm

Quoted:
1.06 - Can I use two short ports to replace one long port of the same diameter?
No! The resonance frequency (Fb) is proportional to the cross-sectional area of the port, and inversely proportional to the length. If you increase the effective cross-sectional area by adding another port, the length will have to INCREASE as well, otherwise Fb will go up!

[MDIFYTD]

right so the sound isn't affected just the lenghth of the ports

[Bumpin Expo]

ahh whoops nvm i see what it means now, it means if i need 1 12" port with 4 inch diameter i cant use 2 6"...nvm my bad. Im makin plans for that box rite now, thanks a lot man. Ill post pics when i get almost done!

[MDIFYTD]

let us know if you need any help...