Any inherent problems with this enclosure design?

[Lothar34]

I had another thread in car audio about exactly what to do (What to do - small enclosure, odd shape) and while I thought I had made up my mind to go with sealed boxes, I really hadn't. Good thing I didn't actually start building.


I have the TB W8-740C 8" subs. From what I can tell, a small ported enclosure is the best for these but every time I ran the numbers, I came up with an insanely long port length. After messing with some of the parameters and going with a less-than-optimal volume and a little bit lower tuning, I was able to come up with a box of 10.9L and a port with 5.5in^2 and about 30" long (34.8Hz).

I came up with what I think is a design that will work, then downloaded Google's Sketchup software so I could model it to show to you all what it would look like and ask if there are any problems with it. I've never had to build such a strange box before so I want to know if I'm screwing it up.

When you measure the port length, do you just measure a line through the middle of the port path?

The port is 2" wide and 2.75" deep. The chamber with the driver is 4.25" deep. I will round off all the edges of the turns in the port path and will put a little bit of fill just around the driver.

I can't figure out how to attach more than one picture, so they're here:
http://home.houston.rr.com/lothar/GT...x.ported.1.jpg
http://home.houston.rr.com/lothar/GT...x.ported.2.jpg
http://home.houston.rr.com/lothar/GT...x.ported.3.jpg
http://home.houston.rr.com/lothar/GT...x.ported.4.jpg
http://home.houston.rr.com/lothar/GT...x.ported.5.jpg

[Volenti]

Originally posted by Lothar34

When you measure the port length, do you just measure a line through the middle of the port path?

Yes, looks ok to me.(nothing obviously messed up)

[Lothar34]

Hey am I not supposed to include the port's volume in that 10.9L total Vb? Because I was. And I just saw something that made me think I wasn't supposed to, and if I'm not, then that changes things...

[looneybomber]

Yeah, you don't include the port volume, you must subtract it. Sorry.

[Lothar34]

I think it'll be OK. The chamber where the driver is mounted is supposed to be 10.9L (or 665 cubic inches). The way it is in that model, it is 466 cubic inches net. That's about 70% and I can make that up with fill. I still have an inch laterally to play with too, so I could stretch it out and get it to 515 cubic inches (77%) and use less fill. I'll probably do that actually. I can't wait to get started on this...

[jmiyake]

Fill does not work quite the same on a ported design. On a sealed design you can stuff it and make up the lack of volume. On a ported design the stuffing must not obstruct any area between the driver and the port.

Lining the walls helps on a ported design, but I don't think you can make up anything like 30% as you might be able to on a sealed.

[Lothar34]

Originally posted by jmiyake
Fill does not work quite the same on a ported design. On a sealed design you can stuff it and make up the lack of volume. On a ported design the stuffing must not obstruct any area between the driver and the port.

Lining the walls helps on a ported design, but I don't think you can make up anything like 30% as you might be able to on a sealed.

Well will it make up 23%? I knew better than to block the path to the port, but I thought it still worked the same as in a sealed enclosure. I should be able to get a good amount along the walls and in those corners. If the baffle is a lot larger than the driver, are you supposed to put some on the back side of the baffle too? On the other boxes I've done the baffle was barely bigger than the driver and it wasn't really possible, so I'm not sure if you're supposed to put any there or not.

The way that picture is, the blue top portion is 11.75" wide across the top, but when I build it it will be 1" wider.

Thanks for the comments.

EDIT:
I just checked in Unibox and it says there's a bit of a loss between 30 and 60Hz when the box size drops that much, regardless of the amount of fill. So I will try to figure something else out.

[Svante]

After a quick look I think it looks ok, it will work. However, remember that this port, just as any, will benefit from rounded edges at the in- and outlets. You may also want to make it possible to tune the port slightly when you find that it does not match the simulation perfectly. This can be done by adding a piece of MDF on the walls of the port to make it narrower (lowering fh) or by making the openings bigger (raising fh).

And remember to round the edges of everythng that is passed by a high air velocity.

[Lothar34]

Yeah I've got a router and am going to round off all of the outside and inside corners along the port path. Once the thing is assembled though, I won't be able to get it apart easily to change the port dimensions.

I made a different design last night where I can get the full 10.9L plus the port, but it's horribly complicated and there's just too much room for error with all the cutting. I'm going to try to see if I can come up with a simpler design tonight.

[joe carrow]

I see the first quarter wavelength pipe resonance in the port is around 110 hz. Over what range do you plan on using this sub? If it's a 4th order crossover at or below 80 hz, it shouldn't be too bad.

That's the tradeoff with longer ports... they let you have higher diameters that help reduce port noise, but at a certain length you have to worry about pipe resonance.

WinISD Pro Alpha shows the first pipe resonance for you. I just calculated 110 hz by hand based on a 30 inch port.