Awkward box shape + ported?

[Trendkill 10+ year member]

My car is a 91 camaro. It has very limited space in the hatch. To get the most out of the space, a box has to be made in a very odd shape. So far all my design ideas have been centered around a sealed enclosure because I'm pretty sure that I want a "flat" response from what I've been reading. I've read that there are ways to get a flat response out of a ported enclosure, but supposedly it has to be done just right. So my question is: Given the general shape of the box in the picture below, can I design a ported enclosure that will not sound "boomy", and be able to keep up with sounds like really fast double bass drums, etc.? ...Or should I stick with sealed?

This is a view from the right side of the car. The box will be roughly 41" wide. The dimensions are shown just as a reference, not for calculation. It will hold two FI-Q 12's. I've done some rough calculations already and I'm quite sure I can meet the maximum suggested volume of 2.5cuft per sub, so that is not a concern. What I am concerned about is whether or not the shape is too cumbersome to bother with a ported design.

Click to read more...

 

[btlking 10+ year member]

buddy had a box exactly like that that was ported except where your sub is on there was the amp rack and twords the hatch latch was the sub

 

[Trendkill 10+ year member]

Here is another image of the rough shape in 3D:

 

[Trendkill 10+ year member]

Bump. I was thinking that I might be able to bring up the sub a bit to fit a port that would go down from the top of the box (to the right of the sub in the image) and then go under the sub.

The part I'm really confused about is how big the port needs to be. I'm using WIN ISD to figure out the length for various opening sizes, and I noticed that the smaller the opening I make, the shorter the length needs to be. Why do people have such huge openings then? Like if I put in a 2"x15" (which looks like alot of the ones I see pics of on these boards) opening for 28Hz, it says 45.18" for the length. Isn't that a bit absurd? Why wouldn't you just make the opening 2"x2" thus only needing 4.98" length? What am I missing/doing wrong?

 

[TheOtherSide 10+ year member]

The reason you don't use 2"x2" is the fact that the port noise would be awful. You need to use Win ISD to calculate the air velocity of the port, and keep it under 110 ft/sec. I think that is the number, somebody correct me if I am wrong.

If you go too small, your port will sound bad and if you go too big, your box will need to be huge. You need to tweak the port size to get the right box volume (net) with the correct port size to keep the noise to a minimum.

 

[Trendkill 10+ year member]

Ok thanks. Now a few questions on that.

1. I'm pretty sure the answer is NO, but do I include the port volume in the 1.8-2.5^3 recommended by FI?

2. What tuning frequency do I need to get as close as possible to the sound quality of a sealed enclosure? ("flat response" is what I'm talking about I guess)

3. Are there any other factors to building a ported box that would bring it closer to sounding like a sealed enclosure?

4. Will the shape of the box affect how it's tuned? I may indent that top sloped part to mount amps in, which would make the shape even more awkward.

EDIT: 5. Also, how do I tell what the air velocity will be? I can't seem to find anything about that in WIN ISD

 

[steinkea 10+ year member]

I would suggest just using aero ports because the are far easier for oddly shaped boxes.

 

[Trendkill 10+ year member]

Can you tell me more about aero ports? What are the disadvantages to them?

 

[steinkea 10+ year member]

I really dont think that there are any disadvantages to them. An advantage to them is you need less port area than you would if you have a slot port.

 

[Trendkill 10+ year member]

Well I would think that if they had zero disadvantages over huge slot ports, nobody would use slot ports. Yet pretty much every box I see on these forums is a slot port.

How do you go about tuning an aero port? Don't they come in pre-made sizes? Kinda seems like you are limited in your choices.

 

[ggk 10+ year member]

people use slot ports cause they are free and easier for a rectangular prism style box. where areoports are 13 a piece.

 

[Trendkill 10+ year member]

ah ok then. So assuming you don't care about cost, do they offer the same or better tuning capability as a slot port? I'm still confused as to how you would select your exact tuning frequency with a pre-manufactured part.

Also, where is a good place to buy them online? I'm having trouble searching for them.

 

[bushtree 10+ year member]

ah ok then. So assuming you don't care about cost, do they offer the same or better tuning capability as a slot port? I'm still confused as to how you would select your exact tuning frequency with a pre-manufactured part.
Also, where is a good place to buy them online? I'm having trouble searching for them.

http://www.partsexpress.com

and i believe you just cut the tube down to the exact length that you want, thus tuning to exactly what you want.

 

[Trendkill 10+ year member]

Ok, next question. Is it better to isolate 2 subs in a ported design, or should I have them in the same chamber?

 

[Trendkill 10+ year member]

Ok so i found the air velocity graph in WINISD. I'm getting max 34ft/s with a 2" diameter port that only needs to be 1.29" long. That's way under 110ft/s. This can't be right. It's almost like just poking a hole in the box, and I'm never seen anything like that. I'm starting to get really confused.

If I change it to "two free ends" it only increases the length to 1.75". Which if this is right, would mean there would be absolutely no reason to ever go with a huge slot port because a tiny piece of pvc would work just fine. And that can't be true. Can it?