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Introduction
Do you ever see a car go by and find yourself assaulted by a thud-thudding noise? That's probably because they have a sub-woofer inside. Sub-woofers generate those low frequency vibrations which you can feel, which are more akin to changes in air pressure (like from an implosion.) In my living room, not only can it be felt directly in one's body, but one's feet can feel the floor vibrating underneath, (and I imagine the neighbors can hear it outside.)the Ideal Sealed Box EnclosureProduction on modern CDs often keeps the frequencies of the bass separate from the rest of the sounds to facilitate use with a sub-woofer. If you listen to music (especially R&B) without one, you are missing an important part of the music.
What's the deal with the sealed box?
This is where we get into physics. Consider a speaker sitting face up on the table in front of you. If the membrane moves in one direction (up), it pushes air molecules up. At the same time, it pulls air molecules in from the bottom. This causes air to move around the speaker from top to bottom (around the edges) and back again with each vibration. This movement also gives rise to waves that move outwards from the speaker.It's similar to waving your hand back and forth, and pushing air around. As you can imagine, the slower you move your hand, the less of an effect you have, because it is easier for the air to get out of the way without giving rise to much disturbance. And this is the problem with low frequencies.
The trick is to somehow prevent the air from getting from the top of the speaker to the bottom around the edges. And this is solved by using a sealed enclosure. Air that is on top is pushed up and pulled back down again without having anywhere to go. Air on the bottom is compressed or expanded in a sealed chamber, and not allowed to detract from the actions on the air on top. The effect is similar to having an object in your room that gets bigger and smaller as it vibrates. It literally causes a change in air-pressure in the room, starting from the object, and moving outwards in a compression wave. I believe the pattern formed is a three dimensional cardioid.
Fig 1.0 Comparison of Open Speaker and Enclosed Speaker
From this, we can arrive at some design constraints for an ideal enclosure for a sub-woofer speaker:
 | It should be air-tight |
| It should either be rigid or thick and soft (absorbing, not transmitting internal vibrations.) It should not vibrate on the outside with changes in internal air pressure. |
| It should definitely be heavy, so it resists any sort of vibrations, whether coming from inside, or imparted by the speaker pushing against the box. |
| There should be some sort of damping material inside to prevent internal reflections of sound, that could resonate at certain frequencies. |
| Lastly, it is recommended that the box be at least 3 cubic feet. My guess is that as the speaker moves, it imparts some vibrations to the box, regardless of how you damp it, and this is a recommended size for accommodating the frequencies wanted. (I built mine considerably smaller, about 2.4 cu ft, and am happy with how it sounds.) |
Building the Box
Fig 2.0 - Inside a Typical Sealed Enclosure
I bought a 4 ohm, 12", 300 watt sub-woofer speaker at a local surplus store for a mere $25. It was missing an SKU number and probably had been used as a display. The first question I had when I got home was: how am I going to power this thing? After some experimentation, I discovered that my guitar amp (an old Crescendo tube-kit I got for 10 dollars someplace second hand) was sufficient to power the speaker --provided the input to the amp was at line level. After much experimentation, I found that a couple of 470uF capacitors across the speaker input (parallel) removed the high-frequencies, and voice. (I experimented with inductors as well, as per the usual filter scheme, but found that they tended to resonate at certain frequencies and did weird things to the sound, like garbling it.)
I built my box out of pine, although for a comparable price, one could build one out of MDF (Medium Density Fiberboard), a much more rigid, and thus better, material. Not being familiar with working MDF, I chose pine. I bought one 16" x 8 feet (16" x 6 sides) sanded pine board from the store for about $40.This, I cut into six sides. Some of these sides were made smaller than the others so I could build a box. The box is over 16" high, and about 16" wide. This is smaller than the recommended 3 cubic feet, but the sound turned out fine. I cut a hole in one of the faces for the speaker. I think you will find that building a square box is the hardest part of this project. Finishing it, as below, is the easy part. And somewhat fun. (Now that the thing is done, I'm almost tempted to remove the screws, cover it in leather, and put them back in for appearances sake.)
I assembled the box by first putting construction glue on the edges, then screwing them together with large screws. I reinforced (braced) the inside of the box, with a hardwood dowel going one way, and a two-by-four going the other. These should not touch the speaker or each other, but should be close to the center of the faces they are bracing. I also doubled up the back of the box with an extra sheet of wood. Once the glue was dried, I put silicone into every crack inside the box, so it was air-tight.
When this was dried, I did the wiring. I drilled two tiny holes in the back, and had two screws on the outside of the box. I drilled two holes on one cross-braces as well. The wires were wrapped around the screws, passed through the holes, and wrapped through the cross brace, before they reached the speaker. I sealed around the wire holes with silicone, and used duct tape to hold the wires in place on the brace.
When this was done, I cut some old bed-foam (initially put on a bed to make it softer) into the proper size and glued it into the enclosure, including on the cross-braces. Interestingly, this foam had the sort of dimples on it that sound-proofing requires. I finished the box with a couple of coats of Tung Oil. And some more loose foam inside, pressed up against the back walls. And rubber feet on the bottom.And that was it. The only thing left was to make up some patch cords, put a new outlet on my guitar amp (which disconnects the internal speaker and routes the sound to the outlet when something's in it.) The chords were: For the speaker, a 1/4" mono chord that plugs into the amp output, and has the capacitors wired in. A 1/8" stereo phone jack which goes to a 1/4" mono jack (one channel is thrown away.) This goes between the personal CD player and the amp.