Though it is a challenging undertaking, designing a bass reflex speaker for an audio environment is among the most satisfying tasks for a do-it-yourselfer. While there are many considerations from shape of the enclosure to size of the loudspeaker, we outline here the steps that will bring you the desired result.
Ported and vented are used interchangeably to describe the bass reflex "box" design. Air within the speaker cabinet is used to enhance low end response of the system. This is accomplished by the characteristics of the port, and the tuning of the enclosure to produce optimum low end response.
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Things you need
- Speaker driver
- 3/4-inch to 1-inch MDF
- 3/4-inch plywood
- PVC pipe
- Power drill
- Router or circular saw attachment
- Speaker wire
- Binding posts
- Connection terminals
Pick the size loudspeaker that will be housed in the cabinet. Popular sizes range from 8" to 20". Most probably, "real" home theatre or recording studio bass response will be produced by 12", 15" or 18" loudspeakers. Then, armed with the Thiele/Small parameters that dictate the enclosure size for a bass reflex speaker design, pick your cabinet materials.
MDF (Medium Density Fiberboards) is preferred over plywood because of it higher density, but whatever is chosen, 3/4" - 1" 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.
If the shape of the design is cylindrical rather than square, Sonotube is good for the enclosure, with the top and bottom being MDF.
PVC pipe is often very good for creating the port. The port or vent greatly aids in the reproduction of low frequency sound by extending the low frequency range of a speaker box. Bass reflex/ported designs therefore feature a lower cut-off frequency than a sealed enclosure system using the same driver, and also offer lower distortion and higher power handling. Instead, their sound can be more "boomy" than that of enclosed boxes, and that is where greater science and engineering comes into play.
Thiele/Small parameters are the mechanical and electrical measurements that determine the ideal design for the speaker cabinet. They are also know as T/S, and exist for any driver/loudspeaker. If such information is not provided with the loudspeaker (the driver + the cone), it is available online for every size component. Having those components and the T/S parameters dictates the interior volume of the enclosure, or box, and also dictates the resonating frequency that the enclosure is "tuned" to, the amount of damping for the enclosure, and the amount of bracing that might enhance the rigidity of the "box" inside.
Tips and warnings
- If this sounds a bit involved and technical, that's because it is. There is definitely science at work in creating any bass reflex subwoofer design.
- Ported designs are "tuned" to a resonating frequency of the driver, and are also adjusted in their interior volume by the size of the driver and the port. For this reason, bass reflex speakers tend to be larger than a closed enclosure.
- Always use generous amounts of glue and silicone sealant in constructing the cabinet. Positively no air should escape the cabinet except through the port.
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