DIY ported Subwoofer box
I built another subwoofer box for my boat , and decided to do a write up for other DIYíers. I didnít get as many pictures as I wouldíve liked (I get in the zone and forget to take picsÖlol). Anyway the box makes the sub. Iíd rather have a crappy sub in a proper box than an awesome in an improper box. 9 times out of 10 prefab boxes arenít made to the correct specifications. This write up is for ported enclosures utilizing Precision Flared ports. This is how I build my boxes. Iím by no means a professional expert, but Iíve built quite a few boxes and have always had excellent results.
Calculating and Planning
Things youíll need:
Torres Calculator http://www.stevemeadedesigns.com/boa...ng-calculator/
Precision Port Calculator http://www.psp-inc.com/tools.html
First you need to determine how much space you have or are willing to use for your enclosure. Write down your max dimensions of height, width, and depth, but allow one dimension as a variable that you can add or subtract from. Calculate the volume in cubic ft of your space available. You can use the Torres calculator if you don't like math. Be sure to account for the thickness of the wood you'll be using.
-I have 26"width, 17"depth, and 18"height as my variable dimension. I've predetermined that I'll be using a double baffle, so .75" will be added to the depth (baffle is where the sub mounts, doubling up adds strength and allows you to flush mount the sub). My gross internal volume is 3.6^ft.
Pick your sub(s). Some subs are best suited for sealed enclosures, others ported. If you're going with (1) sub try to go with the larger one (if output is what you're looking for). More cone area typically equals more output. Look up the manufacturers recommended enclosure size. Some subs require huge boxes, others don't...be sure to take this into account. You also need to find the displacement of your sub by calculating or looking it up. The volume of your sub will be subtracted from the internal volume of the box.
-I have a 15" sub. The displacement is .22^ft, and the recommended ported box size is 3.0-3.5^ft. The OEM's box size recommendations are the internal volume after you subtract displacement of the subwoofer, bracing, and port.
Adding internal bracing to your box is a good thing to do, although it does take up internal volume. You'll want to brace larger boxes that house (1) 15, (2) 12's, etc.. and subs that you're putting a lot of power to. Dowel rods work just fine to brace a box and are easy to use. If youíre not going to brace internally using a double baffle is highly recommended.
-I decided I'll need to add another inch to my variable to get the correct internal space. Now my external dimensions are 26"w, 17"d, and 19"h, which equals 3.84^ft.
-I need to determine how much internal space the braces occupy. I'm using (2) 1.25" and (1) 1" dowel rods for bracing. The total length of the 1.25" dowel rods is 33"...so radius x(.62) radius x(.62) x 3.14 x length(33) /1728 = .023^ft. Total length of 1" dowel rod is 24.5" so radius x(.5) radius x(.5) x 3.14 x length(24.5) /1728 = .011^ft. Total brace displacement is .034^ft.
-I need to subtract the sub displacement of .22^ft and bracing displacement of .034^ft from my gross internal volume of 3.84^ft which gives me 3.58^ft. I still need to figure out the port displacement and subtract it from 3.58^ft.
My previous box was a slot port box. It worked very well, but slot ports require more space and are more difficult to build and calculate (especially so if you do 45's on the corners, round over edges, etc). I built a box using Precision Flared ports over the winter and was very pleased with the results. Precision ports are widely used in an array of home theater and car audio enclosures. Straight tube ports can create port noise, flared ports typically don't. Tube ports can reduce the overall enclosure size and allow for more flexibility, although the port length may be too long for smaller enclosures. You can mount the entire tube port external to the enclosure which can be helpful if you're limited on space. You can mount the port on any side of the box, but it's best to have the sub and port on the same side to avoid cancellation and phasing issues.
Decide what tuning frequency you want, and will be best for your sub. 35hz seems to be best on boats.
-I decided to go with (1) 6" Precision flared port tuned to 35hz. I need to determine the length of the port. Don't use the Torres calculator if you're using flared ports, use Precision Port calculator
So I enter 3.58^ft of volume, tuning frequency 35hz, port diameter 6", (1) port. The calculator tells me the port needs to be 14" long. When I install the flare port, I'll need to cut the tube down to 14" of total length (from flare end to flare end). Radius x(3) radius x(3) x 3.14 x length(14) /1728 = .23^ft. So .23^ft Ė 3.58^ft gives me 3.35^ft after all displacements which is within the manufactures recommended ported enclosure of 3.0-3.5^ft.
-Now I need to make sure everything fits. Pull up the Torres calculator; enter your dimensions to get the wood cut list. I have enough room to install the sub and port on the same plane. Since I'm using a double baffle the port will be 3" from the internal back wall. The port will be a bit too close to the back wall. I'm going to make an adapter for the port to raise it up a bit and give me more space. General rule is the port diameter should be the distance from the back wall. So a 6" port should be 6" away, although I've heard Precision Ports can be within 2" of the back wall and still work fine. Everything fits, Iím ready to assemble.
Things youíll need
Titebond III wood glue
Epoxy Resin http://www.uscomposites.com/epoxy.html
Drill with bits
Jigsaw or router
Terminal cup (optional)
Decide what wood you want to use. Most use MDF. I use Arauco plywood. Itís lighter and stronger than MDF and is more resistant to moisture than MDF. Some say MDF is best for its acoustic properties, this may be true, but Iíve built boxes with MDF and havenít noticed a difference sound with Arauco wood. Baltic birch is another option but itís a bit pricier. If you decide to use a wood other than MDF be sure to look for a sheet with no or minimal voids/knots. If you use MDF be sure to thoroughly epoxy resin the box inside and out and wear a respirator mask when cutting. .75Ē should be the minimal thickness.
Take your cut list the Torres calculator and start cutting, OR go to Lowes/Home depot with your cut list and have them cut the wood for you. If you cut out the sections on your own itís best to use a circular saw or table saw.
-I already had the wood, so I measured and cut all the pieces. When I was done I made sure everything correct and would fit together properly.
PREDRILL your holes for the screws with a small drill bit. If you donít do this youíll split the wood when you drill the screws in. Pre-drilling also allows you to sink the screw below the surface line for a better appearance and tighter bond.
Start gluing and screwing!
-I use clamps and screws. The clamps make assembling easier and provides a good bond between the surfaces when waiting for the glue to dry. If you donít use clamps you can use ratchet straps around the box while the glue is drying.
Draw the cutout of your sub and port with a protractor or you can use a string by finding the center of the cutout and tracing around. Cutout your circles. If youíre using a terminal cup, make your cutout for that as well.
-I used a router and a jig I made to make the cut outs.
-I cut my dowel rods to length and glue and screwed inside the box
-I also used a router and jig for the 2nd baffle and the port adapter. The cutout for the 2nd baffle needs to be the outer diameter of the sub so it will flush mount.
-Once the glue dried and cured I sanded all the external edges and used wood filler over the screw holes.
-I used silicone caulk on all internal seams. I donít have to use caulk since Iím epoxy resiníing the box, but if you have any openings between the seams the resin will soak through without providing an air tight seal.
Apply the epoxy resin. Be sure to wear a respirator and apply in a ventilated area.
-I only applied epoxy internally as Iíll be using bed liner on the external surface.
Apply your external material. Iíve used carpet, vinyl, and bed-liner. I like bed liner the best as itís easy to apply and provides a durable water resistant finish. If youíre applying carpet use contact cement to adhere the carpet to the box.
-I applied 2 coats of duplicolor roll on bed-liner.
DO NOT assemble your port before the box is built. It needs to be assembled inside the box. Cut the port to the required length, glue/tape the port ends, and screw it onto to the baffle.
-I cut my precision port and glued one side with PVC glue and duct taped the other side so I can remove the port without destroying the box. Once I test out the box Iíll remove the duct tape and permanently glue the other side.
Wire your sub(s) and screw it in.
-I didnít use terminal cups. Iím using 8ga power wire for my speaker wire. The wire runs from the amp directly to the sub. I drilled 2 small holes in the box for the wire and siliconed around it to seal it up.
I tested it out, and was pleased with the resultsÖ.I pissed off all my neighbors when adjusting the gain/crossover..thatís usually a sign of good output :)
If you want to apply sound dampener, the roll on roof patch roll at Lowes/Home Depot works well. I have used dynomat and second skin, the only noticeable difference is the price.
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