End correction on ports

rpsands · #1 03-28-2010, 06:29 PM

Hey folks,

So, I wound up with a spare 3012HO sitting around and so I decided to make a cabinet out of it. I went down to the home depot and got some nice looking BC 3/8 ply (just for kicks, gonna brace the heck out of it) and had them cut it to my measurements.

The internal volume before the shelf port is 1.64 cf (before accounting for the volume of the driver, so realistically abou 1.5cf). Borrowed similar dimensions to the Barefaced midget since that is a beautiful form factor.

Anywho, there is one piece of cabinet construction I cannot figure out about shelf ports. How do I figure out exactly how long to make the shelf port, assuming I want to use a bottom shelf port with one division for bracing - akin to the fEarful?

I know I want around 20 square inches of port area at least for the 3012HO to do its thing, but beyond that Winisd is giving me a lot of guff.

I have 12 and 3/8" of room to play with across the front of the cabinet.

So, in Winisd I plug in two square ports of 1 3/4" high by 6 and 3/16ths wide, and come out with...14.94" deep shelf. This fails as the cabinet is ~12" deep past the baffle. For reference, the cabinet internal volume drops to ~1.35 cubic feet with this shelf port, I believe.

What I gather is that I am missing a calculation regarding end correction since I am using the bottom and side wall for each port but I don't get how to calculate that. Anyone know?

rpsands · #2 03-28-2010, 06:34 PM

gratuitous build pic:

rpsands · #3 03-28-2010, 08:56 PM

Help a brother out

I tried some various port calculators on the net, and I'm getting two 1 1/2" x 6 3/16" ports, 8 and 3/4" deep for around 50hz tuning (cuts the cab volume to around 1.4 cubic feet). 18.5 square inches of port area is slightly less than I'd like.

The other option seems to move the tuning up to 54hz, and enlarge the ports to 1 3/4" by 6 3/16" and 9" deep.

Anyone have any more input or care to check my numbers?

JimmyM · #4 03-28-2010, 08:58 PM

i would help a brother out if i knew what to tell you. i guess all the speaker guys are out having lives. sorry, rp..it happens.

OldogNewTrick · #5 03-28-2010, 10:55 PM

Glad you asked... here you go:

click

rpsands · #6 03-28-2010, 11:05 PM

Thank you!

BruceWane · #7 03-29-2010, 01:17 AM

You could extend the port to needed length by running it vertically up the back wall, as shown. It'll use up some internal volume, so you'll need to recalculate a bit. Since you're already at a pretty small volume, it might be hard to get to a low tuning within your given external dimensions......... a smaller volume needs a longer port, but the longer port makes the internal volume smaller, which needs an even longer port, which eats even more internal volume..... small cab design can lead to a self consuming circle.

rpsands · #8 03-29-2010, 09:56 AM

I've got it down to a 9.5" shelf and 52hz tuning with all the internal space calculated closely. Thank gosh I'm not using any cup handles or anything (just a strap). Spent quite a time doing the greenboy method with a spreadsheet. Coincidentally it matched up very very closely with the shelf port calculator I was using (which suggested 9.5" would be 54hz with my box).

The tuning might be a smidgen high but I think it'll work well enough. Can't wait to weigh the finished box, as my measurements suggest somewhere in the neighborhood of 23lbs

bobcruz · #9 03-29-2010, 11:43 AM

Someone needs to invent a port that pops out from the cabinet when in use so it doesn't take up internal volume, then slides back inside the cabinet for transport.

rpatter · #10 03-30-2010, 11:23 AM

End correction for slot ports is a tough one. When I first started using BassBox Pro I actually called tech support because I thought the program was calculating port length incorrectly. I would build the port according to the program and the box would always be tuned much lower than target (port too long).

So now whenever I do slot ports, I use the cut and try method. I make the port a little longer than I think it should be, screw it in place and then run an impedance sweep to see what it's actually tuned to and make adjustments until I get it right. In lieu of that, I think you're better off using round ports - the results will be a lot more predictable.

Ralf

Quote:

Originally Posted by rpsands

Hey folks,

So, I wound up with a spare 3012HO sitting around and so I decided to make a cabinet out of it. I went down to the home depot and got some nice looking BC 3/8 ply (just for kicks, gonna brace the heck out of it) and had them cut it to my measurements.

The internal volume before the shelf port is 1.64 cf (before accounting for the volume of the driver, so realistically abou 1.5cf). Borrowed similar dimensions to the Barefaced midget since that is a beautiful form factor.

Anywho, there is one piece of cabinet construction I cannot figure out about shelf ports. How do I figure out exactly how long to make the shelf port, assuming I want to use a bottom shelf port with one division for bracing - akin to the fEarful?

I know I want around 20 square inches of port area at least for the 3012HO to do its thing, but beyond that Winisd is giving me a lot of guff.

I have 12 and 3/8" of room to play with across the front of the cabinet.

So, in Winisd I plug in two square ports of 1 3/4" high by 6 and 3/16ths wide, and come out with...14.94" deep shelf. This fails as the cabinet is ~12" deep past the baffle. For reference, the cabinet internal volume drops to ~1.35 cubic feet with this shelf port, I believe.

What I gather is that I am missing a calculation regarding end correction since I am using the bottom and side wall for each port but I don't get how to calculate that. Anyone know?

rpsands · #11 03-30-2010, 11:31 AM

I think I've got this one really close to right, but in the future I think I'll just wuss out and use tube ports. My preference for slot ports is mostly because of the free bracing and the cosmetics - but a good performing cabinet is a good performing cabinet really

And the shelf does give you some dimension limits that tube ports might not (particularly baffle size).

rpatter · #12 03-30-2010, 11:44 AM

If you have access to a tone generator a large resistor and a multimeter, it's not difficult to manually tune the box. If you're in doubt, just make the port 20-25% shorter than WinISD says it ought to be.

Ralf

rpsands · #13 03-30-2010, 11:48 AM

Thanks for the info Ralf. that's roughly what I did (divided by 1.5, per Greenboy's suggestion, and calculated the volume of air behind the port)

GregDunn · #14 03-30-2010, 12:55 PM

Sorry I didn't see this earlier... I'm more concerned that the cab is made out of 3/8 thick panels. For relatively low power audio use it's generally accepted that 3/4 is about the minimum you want, to keep resonances below the audible range. For a bass cabinet, even more so. I know you're going to brace it but you'll probably end up with a cab weighing even more once you fill it with heavy timbers. Not to mention that'll eat up even more of your internal volume.

Don't mean to rain on your parade - and I hope it works out for you- but I fear you'll be getting some unexpected coloration due to flexing of the panels.

rpsands · #15 03-30-2010, 01:06 PM

Go check out Barefaced bass's website. I believe he uses 8mm composite panels or so, possibly smaller now.

I'm going to brace it with 3/8" ply in ways that make it extremely rigid; the panels will be at least as rigid as a bass cab with no bracing and 3/4, if not more so.

Amongst the DIY crowd, 1/2" is the standard and 3/8" is common. 3/4" is almost completely relegated to mainstream folks and those who can get a hold of 3/4" poplar plywood somehow (about the same weight as 1/2" baltic birch).

I do appreciate the concern of course; one concern is that the cab might be as heavy as braced 1/2" ply but I don't think it will. This stuff weighs about as much as 1/4" baltic birch.

Rick Auricchio · #16 03-30-2010, 01:46 PM

The 3/4" ply is favored by manufacturers, who want to save labor by not needing bracing. They don't have to lug the cabinet around once they've shipped it out.

rpsands · #17 03-30-2010, 09:06 PM

Updateroo. I had some issues with not having wood long enough for a port so I kludged it; this is mostly a for fun thing for me (my first from scratch cab) so I'm learning lessons - like make sure you have another piece the size of the bottom of your cab for making into a shelf.

To make the two pieces of the shelf line up perfectly I used some 1/2" poplar dowels I had left over as a runner for the top of the shelf, and clamps to hold the pieces very firmly together.

You can see the first section of braces on the bottom. The other side will be the same, and there'll be a "+" criss-crossing the top, notched to fit together. think it will wind up very rigid. You can see I've been extremely sloppy with the PL so far, first time using it. I'll probably cut some of the excess out with a knife. One thing I'm learning is that unlike silicone caulk, it expands in a nasty fashion -- so you can't treat it quite the same.

rpsands · #18 03-30-2010, 09:08 PM

The most valuable player of my tool collection so far is the 20 dollar clamp set I bought from Lowes. Absolutely vital and awesome. I've used it for clamping cut guides, gluing, and everything.

Rick Auricchio · #19 03-30-2010, 09:16 PM

The PL Premium is most easily cut away when it's still a bit rubbery, not rock-hard. I used a small triangular paint scraper. There's no need to clean up the inside of the cabinet, where nobody will see any extra PL...

PL Premium is designed to expand quite a bit, so it will fill up to a 1/4" gap. With a bit of practice, you'll know how thick a bead to lay down.

rpsands · #20 03-30-2010, 10:08 PM

Quote:

Originally Posted by Rick Auricchio

The PL Premium is most easily cut away when it's still a bit rubbery, not rock-hard. I used a small triangular paint scraper. There's no need to clean up the inside of the cabinet, where nobody will see any extra PL...

PL Premium is designed to expand quite a bit, so it will fill up to a 1/4" gap. With a bit of practice, you'll know how thick a bead to lay down.

Thanks for the tip! Thankfully I did a good job cutting for once and had no gaps larger than 1/16". I think you're right, I'll just leave the PL. It's rock hard now

Thankfully I was also very neat about the outside of the cab, not much to clean up there that won't go with some sanding.