5 Ways to Strengthen Your Box
  
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Why strengthen your box? A flimsy enclosure can resonate, reduce sound quality and pressure, crack, or worst case scenario, blow apart. Adding strength can make your system sound better and louder, and better sounding bass is a great reason to strengthen your enclosure. Here are five great ways to reduce flexing and add strength to your box.

1. Increase Thickness

MDF: Thin walls are for speaker cones and heat shrink, not sub enclosures. If you are planning on using anything less than 3/4" thickness for your MDF enclosure, upgrade now. A full inch is better, but anything less than 3/4" flexes too much for effective acoustic reproduction. If you have a very large enclosure design (over ~4 cubic feet) or overly powerful subs (RFL, W7, etc), you should think about doubling up your MDF (to result in 1.5" thickness) for added strength. For fiberglass applications: When using fiberglass in your enclosure, thickness needed will depend on several factors (see below). Generally, 3/8" thick or so for an average to small sized enclosure with proper bracing should be sufficient. A good way to test fiberglass is by pressing on it to see if it flexes. The more flexing in the enclosure the more chance it will break, and the more sound is lost. If you can press pretty firmly on the fiberglass wall without any flex, you can then really test it. With a properly built fiberglass enclosure, you should be able to stand and jump on the enclosure without it flexing (or busting).

2. Bracing

MDF: For a single sub enclosure, especially sealed, with an average subwoofer should be fine without bracing. When you start adding subs, ports, or volume, bracing is necessary. Again, the point of this is to reduce flexing to add to the quality of your system. Several types of bracing exist, scrap MDF lying around works great. If you are using separate chambers in the enclosure, the separator is acting as a brace, and can be very effective. You can use this same technique without using separate chambers by cutting holes in the separator. For fiberglass applications: Bracing is especially important in fiberglass enclosures. Braces are a great way to quickly strengthen fiberglass and can be implemented in several ways. You can use a brace that spans the entire enclosure, but may need to be cut at an angle or curve, you can brace parts of the enclosure at a time, with the use of dowel rods or strips, or you can use a core material, covered below.

3. Add a Core Material (composites only)

Several core materials exist for use in fiberglass enclosures, varying in price, effectiveness, thickness, etc. If you are building a fiberglass enclosure, and weight is an issue, then this option is for you! A "Sandwich Core" material is a lightweight material that, used with composites, increases the flexural strength of the completed part verses just using the reinforcement (fiberglass) alone. Types of cores include foam, honeycomb, and certain types of wood. A cheap substitute for an expensive core material is scrap wood (say 1/2" x 1/8" x any length), or cardboard (any size that fits). Apply core material to wet resin and immediately cover with an extra layer of fiberglass cloth or mat, being sure to get out any air bubbles. A larger surface area in relation to the core material touching the fiberglass is more effective at adhering, and generally, the thicker the core (depending on the material), the stiffer the end product. When using a core material in a curved enclosure, however, make sure it is flexible enough to bend and not break when you are installing it.

4. Joints

Different joints used when constructing your MDF enclosure will provide different amounts of strength. The typical butt joint is OK for most applications, but when you need more strength and a better seal, try a new joint. The strength of your joints is the wood glue you use in construction, the following (as well as other) joints provide more surface area, giving you a stronger bond. Make sure to use proper wood glue and screws when constructing your enclosure.

Butt joint. The typical joint used in car audio enclosure fabrication.	Dovetail Joint. Requires a lot of accuracy, but the joint is very strong.
Lap joint. Adds surface area by rabbeting one wall of the enclosure before joining.	Miter Joint. Angle both walls at 45 degrees, not the preferred joint to use.

5. Use a stiffer material/combine materials

If you are using particleboard or plywood, it is a good thing you are reading this. The minimum and standard in the industry is MDF, medium density fiberboard. Anything less than this is sub-standard and should not be used. It is OK to combine MDF and fiberglass, and is commonly done. What is less common is to reinforce one with the other. Adding fiberglass to the interior of an MDF enclosure is a good way to seal everything up and add strength at the same time. Using a core material (mentioned above) adds strength to composites without all of the weight. Some installers have even been known to brace MDF with angle iron to strengthen SPL enclosures. Use your imagination, it may take trial and error to come up with the combination that works for you.

A special note about composites

Fiberglass is predictable when it comes to strength. Once you have worked with it a few times, you will come to realize curves increase strength, flat surfaces require much more bracing and reinforcement. If you are wanting a large flat surface, use MDF - not fiberglass. Also, when you are fabricating, do not use excessive amounts of resin, use just enough to wet out the reinforcement and scrape off the rest. Be sure also to get all of the air bubbles out of the mixture (they do make rollers for this). These simple tips will help you get the most out of your fiberglass enclosures.

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