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View Full Version : Awkward box shape + ported?



Trendkill
07-08-2007, 04:36 PM
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.

http://img413.imageshack.us/img413/787/boxfx7.th.jpg (http://img413.imageshack.us/my.php?image=boxfx7.jpg)

btlking
07-08-2007, 04:45 PM
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
07-08-2007, 05:12 PM
Here is another image of the rough shape in 3D:

http://img165.imageshack.us/img165/323/box3dqw1.th.jpg (http://img165.imageshack.us/my.php?image=box3dqw1.jpg)

Trendkill
07-09-2007, 01:59 PM
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
07-09-2007, 09:25 PM
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
07-10-2007, 01:36 AM
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
07-10-2007, 04:52 PM
I would suggest just using aero ports because the are far easier for oddly shaped boxes.

Trendkill
07-10-2007, 05:18 PM
Can you tell me more about aero ports? What are the disadvantages to them?

steinkea
07-11-2007, 02:22 AM
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
07-11-2007, 11:17 AM
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
07-11-2007, 08:05 PM
people use slot ports cause they are free and easier for a rectangular prism style box. where areoports are 13 a piece.

Trendkill
07-11-2007, 09:43 PM
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
07-12-2007, 01:09 AM
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.

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
07-12-2007, 04:29 PM
Ok, next question. Is it better to isolate 2 subs in a ported design, or should I have them in the same chamber?

Trendkill
07-12-2007, 05:21 PM
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?

helotaxi
07-13-2007, 08:01 PM
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?

Change the power in the signal tab to the neighborhood of what you will be sending to the subs and watch the port velocity go through the roof. You're gonna want much more port area than what you are considering. Run too small a port and you're gonna get a nasty whistle from the port. With the port way too small it will cease to function as a tuned port and the will end up as nothing more than a leak in a tuned box.

Rawr-DQ
07-13-2007, 08:15 PM
Also, you want a flat response? that box wont give it to you, you'll have all sorts of peaks and valleys in response all along the frequence chart, a sphere will give you the flattest response.

But really, when you need a flat response it takes all kinds of tuning and adjustments with an EQ/ect. to get a flat response, and is mainly done with the desire of an all out SQ system...that and a Home audio system.

helotaxi
07-13-2007, 09:02 PM
Also, you want a flat response? that box wont give it to you, you'll have all sorts of peaks and valleys in response all along the frequence chart, a sphere will give you the flattest response

That really only holds true for a full range speaker. A sphere won't hold a standing wave. Standing waves aren't an issue in the sub bass range in a small room.

Rawr-DQ
07-13-2007, 09:36 PM
such as a car =p

Trendkill
07-13-2007, 11:45 PM
Yeah I wasn't expecting perfect, just as close to an in-car-sealed-box version of "flat" with a ported box instead, if it's possible, which I've read it is. Thanks for the tip about the signal tab. That was definitely the missing factor. Now a few more questions:

1. Why is it that when I change the end correction, only the length of the port increases (and only slightly) but the air velocity doesn't budge? I thought the flared ends were supposed to reduce air velocity.

2. How useful is the "1st port resonance" stat

3. Is the "110ft/s" guideline only for audible frequencies? Or would subsonic frequencies produce audible port noise when pushed through the port too fast?

Thanks.

helotaxi
07-14-2007, 01:52 AM
Flanged ports only mean that the port is flush with a baffle of some sort. Free ends are the other option in WinISD which means that the port ends are in free space. WinISD doesn't do flared port calcs.


such as a car =p

Exactly. Standing waves aren't an issue in the sub bass region in a car, and a spherical enclosure is pointless for sub bass. The response will never be perfectly flat, but the response curve in the bass region is usually MUCH smoother than in the upper freqs.