While it is always a pretty good idea to stay away from perfect cubes, they don't necessarily have to be avoided like The Plague. Due to the very small dimensions of most mobile subwoofer enclosures, there is little chance of generating standing waves in the enclosure (standing waves cause nasty response fluctuations). For a standing wave to exist, the distance between parallel boundaries must be 1/2 the wavelength of the frequency at which the standing wave exists. Considering that sub-bass waves vary from 56.4 feet (20 Hz) to 11.28 feet (100 Hz), the generation of a standing wave is going to be impossible....after all, the enclosures we're speaking of have to fit in the average sedan or hatchback!

Any standing waves that might be generated by upper ordered harmonics (caused by distortion) in the enclosure can be readily absorbed with the addition of damping material such as polyfill (available at your local cloth store--it is used to stuff pillows and quilts) or Fiberglas^tm (the pink stuff) and/or they can be broken up with strategically placed bracing within the enclosure.

In short, don't worry too much about shape. Make the box to fit the space you can allot to the enclosure and forget about it--there are more important things to worry about...like bracing.

Bracing and Strength

Of all the things to worry about when constructing an enclosure, this is probably the most critical element. If an enclosure cannot adequately contain the tremendous amounts of pressure generated by today's high-powered subwoofer systems, the results will be marginal bass quality at best or total destruction of the enclosure at worst.

A flexing enclosure is a lossy enclosure. If the panels on your subwoofer enclosure vibrate, you lose output (SPL) and clarity. The solution is two-fold: use only 3/4" or 5/8" thick medium density fiberboard (MDF) and brace (reinforce) the life out of the box. If MDF (or the brand name Medite) is not available in your area (it can be quite hard to find, but most custom cabinet making shops should be able to supply you with what you need), the only other real solution (barring exotic materials like sheet PVC) is to use a super high-quality plywood like birch or some other marine-grade plyboard. Avoid using particle board at all costs as it is too flaky (literally), doesn't hold screws well and swells like a sponge when water hits it. In short, particle board comes from the Pit of Hell^tm. Avoid it at all costs.

After the proper materials have been chosen for box construction, the subject of bracing must be addressed. Bracing is very important!

 

A true story We once heard of a fellow using a triplet of our 10W6's in a sealed box powered with a 1200 watt amplifier. The box was not braced at all, was one big chamber and was made of particle board. While demonstrating the system for a crowd of people, the box literally exploded in his trunk, coating his trunk with sawdust and enclosure shrapnel, leaving the subs to play in free-air (not cool). Not only is it frustrating (or really cool, depending upon your personality) to have a box explode in your car, but it can also be very dangerous....after all, you could poke an eye out!

If you remember just one thing about bracing, remember the following scenario:

 

The Jumbo Test Imagine, if you will, a rather bitter mid-sized African elephant named Jumbo (how original) that has been taking steroids and lifting religiously for the past 10 years. Jumbo's ex-girlfriend left him for a box (don't ask--because we don't know) and so every time Jumbo sees a box, he does his best to obliterate it. After building your enclosure, you should be able to place your box in front of Jumbo and let him tap dance on it all day long knowing that when Jumbo finally gives up (he never does) your enclosure will emerge unscathed. It sounds a bit extreme, but it's true: strength is key!

Sealing the Box

Whether you are planning to use a bandpass, ported or sealed box, sealing the edges is very important (isn't everything?). The first step to take in assuring a good tight seal at all joints is to use copious amounts of wood glue. Don't be shy with it--keep a wet rag handy to wipe up the excess. Like bracing, you can never use too much.

There have been some debates on rec.audio.car regarding the use of silicone caulk to seal enclosures since the caustic fumes (acetic acid) released during curing have an appetite for foam surrounds, but with a little understanding of what is going on, this problem can easily be avoided.

Fortunately, most quality audio subwoofers have a specially treated surround that protects them from hungry acetic acid fumes which is cause #1 not to be overly concerned with using silicone to seal your box. Secondly, the fumes are only released during curing (the time when the caulk goes from a free-flowing gel to an amorphous solid) so all you have to do to prevent damage to the drivers is to wait until the silicone has cured (8-12 hours usually) before dropping the subs in. As one member of the rec.audio.car newsgroup (who shall remain anonymous) can attest, it is not a good idea to stick your head inside the box while the silicone is curing unless you are in search of the world's most obnoxious buzz (don't try this at home kids).

Copyright 2001-2003  Peter Ferlow