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Constructing a Ported Enclosure
This is a discussion on Constructing a Ported Enclosure within the Stereo and Electronics forums, part of the General Help category; ...
10-20-2010, 01:50 AM #1
- Join Date
- Aug 1999
- San Diego, Ca
- 383 Procharged & N20 Vert
Constructing a Ported Enclosure
Related article: Benefits of Ported Enclosures and Calculating Port Size
The first step on constructing the enclosure is to make a cut list for the six pieces of MDF that will be necessary. Be sure to compensate for the thickness of the material when cutting the pieces. Usually, the surface of the enclosure that will mount the driver should be full width and height, overlapping the top and sides. This gives a flatter, stronger baffle to mount the speaker on. So, for this enclosure, the front and back pieces will be 9”x12”. The top and bottom pieces are usually the full width of the box, but sit inside the front and back in the depth measurement. That would make them 9”x 22” (1.5” less than the depth of the enclosure to compensate for the wood thickness). The sides fit inside both the depth and height measurements, making them 7.5”x 22”. A table saw makes quick work of cutting the stock, although a jigsaw or circular saw could work, and many home improvement centers will cut the pieces for you when you buy your MDF.
After the pieces are cut, we like to put a rabbet cut on the sides using a router. A rabbet is a notch cut into the outside edge of the piece. When assembled, this notch will create a slot that we can tuck upholstery into making the carpeting easier and cleaner.
Next, assemble the enclosure using carpenter’s wood glue and screws or staples. A pneumatic stapler is the fastest method, but drywall screws can be used as long as the holes are predrilled to avoid splitting the MDF. Here, Cody is using the stapler while aligning the box pieces on a flat surface to ensure everything will be flush. Staples or screws should be used every 4-6” but never closer than 1 ˝” from an edge to avoid splitting. Make sure your fingers are clear of the stapler before firing as the staples sometimes bend and exit the side of the wood.
Front and back pieces should be installed last as any misalignment can be corrected by twisting and flexing the sides and top as the front is nailed. Here we can also see the slot that the rabbet bit left around the perimeter of the side.
The port is made from standard schedule40 PVC, cut to length with the jigsaw. The inner diameter was then rounded over on the router to give a smooth entry and exit for the airflow.
The center point for the port and woofer holes was marked by drawing from corner to corner and drilling an 1/8” hole at the intersection of the lines.
A router with a circle jig is the preferred tool for cutting accurate holes. A straight bit is used and the jig has a pin that rotates the cutter like a compass drawing a circle.
We upholstered the box with trunk liner material using a spray-grade contact cement. Contact cement needs to be sprayed on both surfaces, allowed to dry, then pressed together to make a bond. Here, Cody is spraying using an inexpensive Harbor Freight gun, making sure to get good coverage on the routered grooves where we will make our seam.
Next, the carpet is sprayed. It’s important to get even coverage on both surfaces as contact cement only adheres to itself. If either surface is missed, there will be no bond.
At this point, the enclosure is rolled onto the glued carpet. The carpet is cut slightly long so there is an overlap on one side. To make an invisible seam, cut down the middle of the overlap with firm pressure and a very sharp blade, cutting both layers of carpet simultaneously. Remove the excess and you should have a perfectly mated seam.
The excess width is stretched around to the sides and tucked into the previously routered groove using a flat tool like a screwdriver or putty knife. Once tucked, the excess is cut off with a razor knife, and then the side pieces of carpet are installed, tucked into the same groove and trimmed.
Finally, the carpet is trimmed from the woofer and port holes and the port and woofer are installed. On our enclosure, we used Cyanoacrylate adhesive on the port and installed it from the inside of the enclosure to avoid damaging the carpet. Be sure to pre-drill the speaker mounting holes and use care not to damage the surround of the speaker with your screwdriver.
Once the speaker is installed it’s a good idea to verify the port is working properly. If errors were made in design or construction, the port could be tuned too high which could cause the woofer to self –destruct when played. To verify, plug a tone generator into your amp and sweep the bass range slowly with a moderate level of signal. At high bass notes, you will see minimal excursion of the cone. As the frequency sweeps down, you should note that the excursion begins to diminish and at one point the cone should almost become still. As the frequency goes below this point the excursion should once again increase. The point at which the excursion is minimal is the port tuning. See the video below on porting verification:
Contributor Joe padula is an instructor at Installer Institute in Florida.
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