Ported sub cabinet basic questions

[alec_cat]

Hi everyone. I'm going to be starting my first subwoofer build (a 3 cu ft ported box) and would appreciate help with some basic cabinet questions.

1. Plywood! I went to Home Depot and Lowes and didn't find anything very nice, and my local lumber mills only sell wholesale. Can anyone recommend any good online sources at all? Or would shipping costs just be nuts?

2. Bracing - I've seen some very elaborate but completely disparate bracing techniques on various build threads. There doesn't seem to be a single approach. What would folks recommend as straightforward and no nonsense? Front to back and side to side 2x4s?

3. Lining and stuffing - do subs need acoustic foam and acousta-stuff-like stuffing? What's the purpose of it?

4. Affixing a plate amp and the driver - how do folks get a good airtight seal? Are screws enough? If you glue it and something goes wrong, how do you remove it?

5. Port location - does it matter where this is? I was thinking of putting it under the driver (front-facing).

Thanks for any help!

[jsr]

I'm not am expert, but just some thoughts.

0. Diysoundgroup makes a 3cf sub box thats braced and all, might be an easier option unless you just want to make one yourself.

1. Yes, wood panels are heavy and large and usually expensive to ship.

2. People get elaborate to not block the rear of the driver and such. I don't find any worse perforce with simple front to rear and side to side bracing personally. But I don't use high end sub drivers that costs hundreds so my ears may not be as refined for the"best" bass.

3. Acoustic stuffing is usually for absorbing midrange and higher frequencies. I personally don't see any need for a sub where a filter will roll off output far below where the stuffing is effective. For a sealed box, stuffing can simulate a larger box (what I've read), and the usage for a vented box seems debatable depending in which site you visit.

4. Most plate amps come with a foam installed around the edge. It's the same stuff used for drivers. If you feel you need more, you can add some foam tape used for drivers. Don't glue it or you'll never be able to take it out.

5. I've not found port location to be problematic. As long as the port has some room to breathe and the air velocity isn't too high. If the port is front firing and the velocity is high enough to make noise (~30m/s), youre more likely to hear it them a rear port.

The experts will correct my errors.

[mdocod]

HI Alec,

1. Home depot and lowes both usually carry a finer grade high ply count (i've heard the lowes version is better) furniture grade plywood in thicknesses up to 3/4". You may want to go back and have another look, maybe they were all out that day or it was stacked in a funny place? plan on ~$40-50 for a 4'x8' sheet. Not bad really.

2. One of the "goals" with bracing is to shift panel resonances well above the passband of a sub and/or and, ideally, well into the territory where there isn't enough energy to excite a thick panel into a resonance anyways. Another perspective, is that you want your driver to be as rigidly coupled to the box as possible, and the box to be as rigidly coupled to the floor as possible. Bracing and double thick front baffles are common to get very rigid driver to box coupling. Excessive internal bracing is often more of an advantage because of the weight it adds, which prevents the box from moving out of phase with the driver. A few decorative marble bricks could serve a similar purpose.

3. Dampening material: personal preference IMO. Try it out to taste.

4. See jsr's response.

5. More important than location IMO, is dealing with air velocity and compression properly in the design. If the port is a good match to the system, then location doesn't matter much, if it is a poor match, and prone to make chuffing noises, then bottom or rear mount ports are often chosen to help "hide" these problems.

Eric

[69FireBird]

i looked at diysoundgroup subwoofer boxes but it looks like they are all made for a certain woofer? they are cheap though but only if you are using one of a few woofers for sale?

[jeff_free69]

4. Affixing a plate amp and the driver - how do folks get a good airtight seal? Are screws enough? If you glue it and something goes wrong, how do you remove it?



Thanks for any help!

I don't - I keep the amp under in the stereo rack. no hole cutting ( which means you're not locked into the same amp forever).

PLus having it plugged into to the same outlet helps reduce the chance of getting ground loop hums.

Makes life a little easier when it comes time to tweaking it too.

[alec_cat]

Thanks very much for the clear answers, everyone! I have a feeling I'm going to like this forum a lot

I'll also go back to Lowes and check what they have more carefully.

[johnastockman]

Thanks very much for the clear answers, everyone! I have a feeling I'm going to like this forum a lot

I'll also go back to Lowes and check what they have more carefully.

Bracing is important, but with individual preferences and implementation, sometimes no two sub-bracing methods look alike. I've used 2x4's before, in the first speaker cabinets I built in 1977; the cabinets were for a floor-standing 2-way similar in size to the Large Advent. The 2x4's were just some scrap, 2 side-to-side and one front-to-back between the woofer and tweeter. Very effective. For a subwoofer enclosure, a couple side-to-side, maybe one top-to-bottom and one front-to-back works well for me. I've achieved a noticeable SQ improvement by just adding some internal bracing to commercial subs. I line the inside walls with some of that egg-crate style foam, open-cell variety. If it's ported, I don't use any other stuffing. You can use some of that weatherstripping foam tape that you get at a hardware store for a gasket around the flange of the sub amp to seal it well. Make sure your port is at least the diameter of the port away from the inside walls. General rule 'o thumb is for a 10" driver, not less that a 3" port, 12" driver, a 4", 15" driver, a 6". Just generally though, some drivers with high Xmax might require a larger diameter. Use at least 3/4" thick material, whether it's plywood or MDF. For heavy drivers, like the RS series, I double up on the front baffle thickness, or sandwich 3/4" with 1/2". With the port location, as long as it's not directly behind the driver, you're OK. There are some simple-to-use programs that will help with box volume and port dimensions, all you need are the T/S parameters. I use this one, WinISD:

http://www.linearteam.dk/default.aspx?pageid=winisd


Unibox is another:

http://www.pvconsultants.com/audio/boxmodel/unibox.htm


They will inform you if the port air speed is too high, the result of using a port diameter that's too small. IIRC, with WinISD, it'll be stated in a mach number. I think it should be below .16 or .15 mach with WinISD. Good luck with your sub, let us know if you come up with any other questions...and post up some photos if you can.


John A.


PS I go to local lumber yards instead of the usual suspects of big-box stores. Not the wholesale-only ones, but you might be able to find a retail lumber supplier in your area. They seem to have a better quality of goods overall than my Lowes or Home Depot. Especially the quality of MDF...and the accuracy of their panel saws if you need full-size 4x8 sheets cut down to a more manageable size.

[alec_cat]

Thanks again, everyone. I really appreciate the advice.

I have a small update, I went back to Lowes and Home Depot and decided I'm just going to go with 3/4" MDF

I'm going to start drawing the cab and bracing layout for my woodworking friend, and then get started ordering extra bits and bobs.

I'll post pics for sure!