Build This Terrific Bass Trap
This awesome-looking bass trap design uses a sand-filled concrete form and is trimmed with stained wood and fabric. Its guaranteed to make your guests do a double-take.
A bass trap is designed to absorb low frequency sounds. These low frequencies are more commonly known as standing waves and can interfere with a room's acoustic characteristics making your movie's music and sound effects sound boomier than they really are.
Since standing waves are rather strong, you need an equally strong tool to stop them. Large, heavy items like a wall unit can sometimes help to natually absorb standing waves, but sometimes you need a little more muscle. This is where a bass trap comes in.
This base trap is built using a regular concrete form, also known as a Sonotube or a Quik-Tube. These are strong cardboard tubes used to make cement footings for decks, fences, and other stuff and will make up the body of your bass trap.
Here's what you'll need:
- 1 concrete form, 10" in diameter
- 1 compass (the geometry set kind)
- 1 piece of 3/4" plywood about 12" x 24"
- 1 (one) 1" board (birch, oak, etc.) about 16" x 24"
- Fabric or paint
- Water or oil-based wood stain
- 1 can of spray on adhesive
- 1 (one) 3" screw (Phillips head)
- 4 (three) 2" flat-head wood screws (Phillips head)
- 1 Phillips screw driver
- 1 drill bit
- 1 compass (the geometry set kind)
- A jigsaw and blade
- Stapler or hot glue gun
- Wood glue
Here's a quick run-down of how we're going to build the bass trap. We're going to fill the Sonotube with enough sand to absorb most of the standing waves that impact on it. To prevent the sand from getting out, we're going to plug both ends with plywood cut into circles. Thats basically it. The rest if decorative to make the trap look good.
Lets Get Started
If the inside diameter of the form is marked on the form itself, then great. Otherwise, measure the inside diameter of the form as accurately as you can.
Take that measurement and divide it by 2. For example, a 10" form may have a 9 3/4" interior diameter. Divide it by two and you get 4.875". Spread the arms of the compass so the distance from point to pencil is half the interior diameter of the form (ie: 4.875"). Use the compass to draw two circles on the plywood side by side using this measurement.
We're going to use a jigsaw to cut out the circles and we're going to end up with a pair of wooden disks. The jigsaw's blade will chip away a small amount of wood as it cuts, so be sure you cut just outside the line. Otherwise the disk won't be snug enough when placed inside the form. Later we'll be filling the bass trap with sand to increase its weight and this disk will act like a plug.
With the disks cut, take one of them and find its exact center. You already know where it is because it's where you put the pointed tip of the compass to draw the circle. Drive a 3" screw about half-way through the plywood. When you put the disk inside the tube, you might accidentally push it in too far. You'll be able to tug on the screw to adjust the position of the disk.
Coat the outside edge of the disk with wood glue and push it into the form until the disk is flush with the bottom of the form. If you push it in too deep, pull on the screw a bit to adjust. The wood glue will help seal the joint between the form and the disk to avoid any leaking sand down the road. Gently screw three holes on the outside of the form into the disk and drive in three 2" screws. Let the glue dry a few hours.
The Outside Fabric
Though you're free to paint the outside of your bass trap any color you want, I'm choosing to wrap it in fabric for a cool touch. Calculate the circumference of the form by multiplying its outside diameter times 3.14 (ie: 10" in diameter x 3.14 = 31.4" around). Cut a piece of fabric so its width is exactly the circumference of the form, and its length is about 2 inches longer than the length of the tube.
Apply some spray-on adhesive onto the outside of the form and gently roll if over the fabric so you have about 1 inch of fabric extending past each end of the form. Go slowly at this stage because you don't want any creases to show up in the fabric. I suggest you spray the adhesive a little bit at a time so you don't make a sticky mistake.
If your width cut was done right, the fabric seam will line up perfectly. If not, then its not too big of a deal as you'll see later. Carefully cut away any excess fabric that hangs over the edge of the tube at the bottom end where you installed the disk. Fold the loose fabric back into the open end of the form and staple it or use a hot glue gun. The bass trap is looking good already!
Now we need a base for our bass trap. Using a nice wooden board of your choice, use the compass to draw a circle about six inches wider than the outside diameter of the form, then cut out the disk. You can also use any other shape, like a square, etc.
To give the bass trap a cool, classy look, we're going to be staining the base so we want to use something nice like birch or oak. If you want to paint it, you can use plywood (don't forget to prime it). Use sand paper to sand down the outer edge of the base to make it smooth. Stain the base using a water or oil-based stain. Let the stain dry for a couple hours then apply another coat if you want.
Wait a few hours for the stain to dry. Remove the screw sticking out from the bottom of disk inside the form. Find the center of the newly stained base and drill a hole through it about 4/5 the width of the screw. Apply wood glue to the bottom of the form disk, line it up with the base, and drive a 2" screw through the center hole. Don't put too much glue because we don't want it to squeeze out and get stuck in the fabric. The screw should go through the hole and through both the base and the form disk. Tighten the screw so its flush with the bottom of the base.
Fold the loose fabric back into the open end of the tube and staple it or use a hot glue gun. Now that the bass trap can stand up on its own, its time to fill it with sand. Fill the tube up with dry sand about 3/4 of the way up.
Cap it Off
Now we want to cap the top of the bass trap. On the birch or oak board, use the compass to draw a circle the size of the tube's outside diameter plus about 2 times the thickness of the glued fabric. Now this extra thickness won't be alot. The design simply calls for the top cap to line-up with the fabric-covered surface. Use the jigsaw and cut out the wooden disk. This will be our cap. You can use a router to give the cap a nice edge.
Use sandpaper to smooth down its edges and then stain it. Use wood glue to glue the disk to the bottom of the cap exactly at its center. Once the glue is dry, just put the cap on the open end of the bass trap. The plywood disk should gently fit in the opening, and the protruding edge of the stained top should sit on top of the form. It shouldn't line up with the form, but with the extra thickness added by the fabric.
This cap doesn't have to be super snug because you may want to tune the bass trap later by adding or removing sand.
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