Talk of making a slot ported speaker box is the same as the making of a vented or bass reflex speaker. Air within the speaker cabinet is used to boost low-end response of the system. This is produced by the characteristics of the port and the tuning of the interior of the box to produce optimum low-end bass.
Ported designs feature a lower cut-off frequency than a sealed enclosure system using the same driver. They also offer less distortion and higher power handling. Therefore, in making a slot ported enclosure, "tuning" of the box/enclosure is of equal importance.
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Things you need
- Thiele/Small (T/s) parameters
- 3/4-inch to 1-inch MDF
- 3/4-inch plywood
- Table or power saw
- Power drill
- Router or circular saw attachment
- Wood glue
- Wood screws
- Silicone caulk
- Speaker wire
- Binding posts
- Connection terminals
Pick the loudspeaker size that will be housed in the cabinet. Popular sizes range from 8 inches to 15 inches. The lowest frequencies for home theatre or recording studio bass response will be produced by 12- to 18-inch loudspeakers/drivers. Download winISD, a free speaker design software that is recommended widely. (The link to download winISD is below the steps for this project.)
Get the Thiele/Small parameters that dictate the enclosure size for a ported speaker design, slot or otherwise. If not provided with the chosen component loudspeaker, this information is available online. Plug those parameters into the winISD program. (The link to more than 5,000 T/S parameters is below the steps for this project.)
Determine your cabinet dimensions. The winISD software will do this for you. Note that ported designs and bass reflex designs are the same thing, and they yield a bit more low-end and are more efficient than sealed enclosures.
Choose MDF (Medium Density Fiberboard) over plywood because of its higher density, but whatever is chosen, 3/4 inch to 1 inch thickness is advised. Often in DIY enclosures, the top and bottom of the cabinet will be MDF and the sides, back and front will be plywood, as will be the slotted port.
Get your enclosure material. Buy the plywood or MDF and cut it yourself, or have it cut by the home products store where you buy for a small fee.
Dry-fit your sides, bottom or top, and bracing. Bracing is a piece to increase cabinet rigidity and aid in minimising vibrations. It's often the same size as the bottom piece, which is inset into the sides or walls of the enclosure.
Glue the sides, bottom (or top) and brace. Additionally secure the connections with wood screws. If the screws are to remain, countersink them. Clamp the enclosure with furniture clamps, and let it dry for eight hours.
Cut out the back panel. If you are building a powered sub, the cut-out should accommodate the plate amplifier. If you're building a 2-way or 3-way design, the terminal cup is what you cut out space for in the back panel. Dry-fit this panel, then glue, screw, clamp and let dry as in the step above.
Attach the top panel. This panel may be inset in a rectangular design or be larger than the enclosure itself if being used as a table top. In either event, attach with glue and wood screws, clamp and let dry.
Prepare and attach the front panel. This is the panel that needs cutouts for the loudspeaker and the port. Make those cuts. Before attaching the panel, caulk all inside seams with silicone caulk. Allow at least 15 hours for the caulk to dry. Silicone caulk emits gases that can erode speaker surrounds (the rubber portion around the cone of the speaker).
Construct the slotted port. Use ¾-inch plywood for all surfaces. Flare the ends of the port no matter what shape port you make. Glue the port to the back of the front panel, allow the glue to set; then caulk around the port to make a complete seal, and again let the caulk dry.
Glue, screw down and clamp the front panel. From the opening in both the front (for the loudspeaker) and rear (for the terminal cup or amp) caulk with silicone caulk all the seams of the front panel. Let this dry for at least 15 hours.
Sand the cabinet. Router the edges if you are to have rounded edges. Fill all countersunk screw holes with wood putty. Sand again. The cabinet is ready for the installation of the loudspeaker/driver and the terminal cup or plate amplifier.
Install the back piece, be it a terminal cup or plate amplifier. If it is a plate amplifier, all the terminals are on the rear-facing part of the amplifier. Make sure all parts clear internal bracing. If there's not enough clearance, notch out the bracing. Secure the back piece with glue. Once the glue is dry, caulk around the perimeter of the terminal cup or plate amplifier. Allow sufficient time for the caulk to dry.
Attach connecting cables between the terminal cup or amplifier and the loudspeaker/driver. Then attach the front speaker. Use wood screws and a non-drying caulk around the cone's perimeter to make an airtight seal.
Finish the cabinet with a wood veneer, paint or stain of your choice. There are many, many finishes to choose from.
Tips and warnings
- While virtually any driver can be used in a ported enclosure system, only drivers with a Qts value between 0.2 to 0.5 will generally give good results. If the driver has a Qts above 0.4, consider building a sealed enclosure or single reflex bandpass system instead.
- Allow silicone caulk to dry overnight before installing any loudspeaker component, as the gases given off during drying can harm speaker surrounds.
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