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Building a Fiberglass Subwoofer Enclosure Box

written by: William Busse•edited by: Rhonda Callow•updated: 12/15/2010

As home theater sound systems become increasingly more sophisticated, the use of highly specialized speakers continues to grow in popularity. For serious home theater enthusiasts, knowing how to build a subwoofer enclosure can provide a custom look perfectly matched to the theater room environment.

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    Fiberglass Enclosure Design

    Although there are numerous commercially produced subwoofers available from a variety of manufacturers, those on a budget or serious audiophiles may choose to assemble their own components. Designing and building a custom enclosure can create a subwoofer that is perfectly matched to the system and room dimensions.

    Since fiberglass is a very durable material when hardened, it is an excellent choice for speaker enclosures. It is also visually appealing when prepared and finished properly. While choosing the speaker and electronics may be uncomplicated, many find that the intimidating part of the process is learning how to build a fiberglass subwoofer enclosure.

    When considering the type of cabinet to make, it is important to establish the correct dimensions. Since subwoofers push a specific amount of air, improper sizing will diminish performance. Subwoofer speaker manufacturers can provide the exact specifications for building a box with the necessary free space.

    A fiberglass enclosure can be constructed for under $200 and includes the following materials:

    * Resin and Hardener

    * MDF Board

    * Fleece

    * Fiberglass Mat

    * Body Filler

    * Primer

    * Acrylic Paint and Clear Coat

    * Sandpaper

    * Gloves, Brush and Trowel

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    Building a Form

    The first step in building a fiberglass enclosure is creating a form that will serve as the substructure. This is most often made from subwoofer enclosure medium density fiberboard (MDF) or sometimes foam. The advantage of MDF is that it is rigid, has true corners and the fiberglass materials are easily attached. Foam is often used in auto applications but may not be suitable for home theater enclosures.

    Wherever the speaker will be mounted, a mounting ring should be attached in the center of the form. Holes for the back panels and mounting ring should be pre-drilled and dowlings used during the resin process.

    It is also suggested that the openings for the speaker, ports and wiring terminals are precut into the mold prior to the application of the fiberglass process. It will be much more difficult to cut into the enclosure after the resin has been applied and hardens.

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    Applying Fiberglass Resin

    After the mold has been created, a material called “fleece” is then attached directly to the enclosure. The fleece should be applied in a manner that makes it flat and wrinkle free. This includes stapling it at the center and working outward, then smoothing and stretching the material before another staple is placed into the next area. When the process is completed, the fleece will completely envelop the enclosure.

    After mixing the hardener and resin according to manufacturer’s directions, a thick, even coat is directly applied to the fleece. It will take approximately two to three hours to harden depending on the temperature and humidity.

    When the first layer is sufficiently dry, a fiberglass mat is then attached to the fleece and resin layer. The mat is best applied in small pieces about a square foot each and slightly overlapped. Another layer of resin should be first applied to the fleece and the mat piece embedded into it. It is important to remove any air bubbles and wrinkles that might appear, especially in the corners.

    Additional layers of fiberglass mat and resin can be added for better durability. Usually, there should be no fewer than three layers of mat to ensure sufficient strength and abuse resistance.

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    Filler, Sanding and Painting

    When the resin has hardened, a layer of body filler is used to cover gaps and depressions. Body filler also gives curves and unusual shapes a smooth professional look. 40 grit sandpaper will take off excess material and each successive sanding should be done with finer grit all the way to 80 or 100.

    Base primer will cover small cracks and indentations and prepare the enclosure for final painting. Acrylics give the best shine and luster and a “factory” appearance. 120 grit sanding will remove any streaks and runs if necessary. After three coats of paint, a clear coat can be sprayed to provide a truly professional look.

    The process of learning how to build a fiberglass subwoofer enclosure is completed by installing the electronics and attaching the back panel.