DIY Acoustic Room Treatments & Superchunk Corner Bass Traps
Achieving the best performance from your audio equipment is not possible until you take the last step to build acoustic treatments into your room: acoustic panels and superchunk style bass traps. How do you know if your room needs acoustic treatments? That's easy - it does! Trust me. Still don't believe me? Hold your hands up in the air and clap - just once. Hear that echo? That is the sound of your clap bouncing from wall to wall in your room. In an especially "live" room (one with lots of hard surfaces such as bare walls, no carpet, large glass covered picture frames hanging on the walls, etc) the echo from your clap will take a long time (hundreds of miliseconds) to dissipate. Try doing this in several places in your room and you'll hear differences in the decay time of the echo.
So how do you get rid of the echo? You either ABSORB it or DIFFUSE it with acoustic room treatments! Using a combination of absorption and diffusion works very well though you do want to be careful not to over-do it as too much absorption can degrade audio performance, too.
The point is that sound produced by your speakers should not bounce off of the walls, the floor, or the ceiling before reaching your ears. The goal state is to have your speakers deliver sound directly to your ears without echoes and reflections that destroy the coherence of your music and add harshness that makes you want to turn it down. Each time the sound reflects off of a hard surface, you are adding resonances that are delayed by a small amount of time (just a few milliseconds is all it takes to ruin the sound). These small time delays between the sound traveling directly to your ear from the speaker and the same sound reading your ear only after bouncing off of the wall are readily detectable. They help your brain and two ears determine the source of a sound. Its a very handy evolutionary survival mechanism to hear your predator sneaking up on you and to know which direction to run so that you can escape! Multiple arrivals of the same set of sounds where each arrival is delayed from one another by small amounts (typically milliseconds) makes the music and movie dialog sound "muddy" and less clear. Each time a reflection point is removed (or substantially reduced), the dialog and music becomes more clear.
Several different types of acoustic treatments have been built into my theater room to help control unwanted sound reflections. These include the curtains in the front corners of the room, the tiles used for the ceiling, a thick carpet and carpet pad, wall-hanging acoustic panels, and bass traps in the rear corners of the room.
For more detailed and technical reading about room acoustics and treatments, have a look at Ethan Winer's web page.
In the image below, you can see two different forms of acoustic treatments that have been installed in the theater. The first is a set of curtains and drapes that have been hung in the front corners of the room. A lightweight black curtain was mounted on the front wall to frame out the screen and then the heavier blue drapes were added to the front and side walls. Together, they provide both a visual and an acoustic benefit by helping to reduce the amount of echo in the corner of the room. Since the material is relatively thin (compared to the wall hanging panels), the acoustic impact is not very large (this is intentional since I'm using dipole speakers). As an added benefit in flexibility, the acoustic impact of the curtains can be "adjusted" simply by sliding the curtains back and forth across the curtain rods that hold them to the walls.
After an extensive web search that began as an effort to avoid painting more traditional drop-ceiling tiles (I figured this process would have been tedious, messy, and consumed vast quantities of paint) I came across a company in California by the name of Ceilume. I used two different tiles: Dart in the center and Stratford as a border tile - both in black, of course. Installation was simple, the border tiles cut easily with a set of kitchen scissors. The finished ceiling looks great!
As I was about half way done with the installation, I noticed a rather large improvement in room acoustics compared with the insulated rafters that are above the tiles. This was a rather nice surprise, since my initial interest in these tiles was for their appearance. There was a significant improvement in musical clarity due to reduced sound reflection when the ceiling tiles were installed. The Dart tiles don't absorb echos like the curtains and the wall-hanging panels do, rather they make a rather effective diffuser. The image below on the left shows the pattern looking at the side wall and the image on the right shows the pattern looking at the front screen wall.
The Dart tiles are really great - they even have an unexpected benefit for my theater room. When you orient the "darts" in the pattern so they run parallel to the front wall where the movie projector shines, they work very well to reduce the light reflected from the screen to the ceiling and back to the viewer. When I began installing them directly in front of the projection screen, I oriented a few so the darts were parallel to the screen wall and a few so the darts were perpendicular to the screen wall. It only took two seconds with an image on the screen to determine which way they needed to be oriented... Not only do these tiles look great (I get all kinds of great compliments from visitors), but they also help with room acoustics and light control in the theater! There is one drawback to these tiles with my particular configuration, though, and this may not be an issue for other people. Because the tiles are so lightweight, my subwoofer (which is quite a bit more substantial than the "run of the mill" subwoofer) causes them to bounce around and rattle in the ceiling grid that holds them up. I tried a few remedies and the best solution that I found was to purchase several sheets of standard drywall and cut it into squares. The back of the Dart tiles has a raised lip that is perfect for accepting one square of drywall as added mass. If you cut the drywall into 23.5" by 23.5" pieces, they fit snugly into the lip and since the weight rests on the ceiling grid, it doesn't place any stress on the tile itself. This layer of drywall also helps to further isolate the noise upstairs from the theater room in the basement. So there you have it, a great looking, rattle free, sound diffusing, and light-reflection-preventing solution!
You've gotta love it when a plan comes together!
A quick search on the web reveals a number of people that have documented their DIY acoustic panels. So, I figure I'll add mine to the mix. Overall, the process is pretty simple: 1) build a frame, 2) fill it with fiberglass insulation, 3) wrap it with fabric, and 4) attach it to the wall. The point of these panels is to absorb mid- to high-frequency energy and keep it from echoing around the room. For absorbing low-frequency (bass) energy, see the section on corner bass traps below.
I started at my local hardware store - they were kind enough to cut my wood and order the fiberglass for me. For the fiberglass, I ordered Owens-Corning #703. It is a rigid, dense, compressed fiberglass panel and is available in sheets that measures 2 feet wide, 4 feet long, and either 2 or 4 inches thick (4" of thickness is ideal here). What are you looking for is the ability to absorb high frequency acoustical energy and this requires density. The703 pressed fiberglass is ideal because it is more dense than regular R13 or R19 rolled "pink fluffy" insulation, even if you compress it yourself. Some people have used Roxul insulation bats that you can also get from local hardware stores, but it's a bit less rigid than the pressed fiberglass (though it is less expensive), so it might be more challenging to work with. 703 comes pre-packaged, twelve pieces per carton for about $140 - this was enough for me to make seven panels. The wood was cut into 4" strips that I chopped up with my mitre saw in order to construct 2 foot by 4 foot panels. I waited until I actually had the fiberglass in hand so I could measure it accurately before cutting my wood. This way, I constructed several frames that were just large enough to accept the fiberglass and still have a snug fit.
In the image above, the red arrows indicate sound waves. As the sound wave from the speakers travels through the absorbing material on its way to the wall, its magnitude is reduced. The amount of reduction is related to a number of factors (explore some of the links on this page for greater explanations) such as the frequency of the sound, the thickness of the material (4 inches of absorptive material is typically sufficient to absorb most midrange and treble frequencies), the absorption coefficient of the material (typically related to the density of the materials - this is usually provided by the manufacturer), and the angle of incidence for the sound wave. Some part of the original sound from the speaker makes it through the absorptive layer, reflects off of the wall behind the absorber, and then must pass back through the absorptive layer a second time in order to reach your ear. Not only does transmission of the sound wave through the panel cause attenuation of the original signal, but there is also additional attenuation each time the sound wave transitions into a different medium (this is the impedance mismatch between the air and the panel, the panel and the air behind the panel, the air and the wall, the wall and the air behind the panel, the air behind the panel and the panel again, and between the panel and the listenin room). This impedance mismatch causes the original sound wave to lose additional energy.
Thus, the reflected sound wave that reaches your year is ideally reduced to the point where it is no longer noticed in contrast to the magnitude of the sound that reaches your ear directly from the speaker. Due to the placement of the acoustic panels on the walls, a 4" thick panel in reality provides the same level of attenuation as nearly 11-12" of 703 suspended in the middle of the room between you and the speaker! Placed on the wall between the speaker and listener, the angle of incidence is close to 45 degrees, so the path through 4" of material is closer to 4 * sqrt(2) or about 5.6 inches. Since the sound waves must pass through the acoustic panel twice to reach your ear, the path through the panel is 5.6" * 2, or about 11-12" - not bad for a single, rather unobtrusive, panel hanging on the wall!
By the way, a great source for panel materials is www.atsacoustics.com if you are having trouble sourcing things locally.
Looking back at my construction, I would recommend choosing a hardwood framing material. I picked MDF (medium density fiberboard) which has the drawback of not always accepting screws very well - especially on edge. If you don't pre-drill appropriate sized holes, the fiberboard just splits (I expect that plywood would likely behave in a similar manner). Thus, I carefully pre-drilled holes just a bit deeper than the length of the screws and countersunk the holes for the screw head. I then glued the joints and added small hardwood triangles to each corner. The triangles were glued and tacked along each edge with wire brads (always pre-drill). Adding the triangles to each corner made the entire structure much stronger. As a final step, I used a 1/2" round-over bit in my router to round the entire outer perimeter of each panel. You can see the result in the image above. A quick sanding with a sanding block or sponge worked quite well to provide a smooth and even finish. This pays dividends when stretching the fabric over the completed panel - it provides a more clean and professional looking result.
Adding the triangular corners bestows an added benefit of providing a neat and easy place to install a hanging wire so the completed panel can be hung on the wall much like a framed picture. For this, just drill a small hole in the triangle, push some picture hanging wire through (readily available at Wal-Mart), and tie a sturdy knot in the end. Then do the same on the opposite corner and its ready for hanging. Well, almost...
Next, it's time to insert the fiberglass. For this step, I recommend wearing a breathing mask and gloves of some sort to protect your lungs and hands. I started by folding a double layer of quilting batting (also available from the giant W) and cutting it just a few inches larger than the fiberglass panels as shown below.
The next step is to gently insert the fiberglass into your empty frame. Start by placing your completed frame on the floor with the rounded edges facing upward. Then, place the batting and fiberglass board on top of the frame and gently press it down into your frame. I found using my T-square useful to press the fiberglass into the frame so I didn't de-form its shape or crease it as it went down into the frame. This method is especially useful for making sure the surface of the fiberglass is flush with the edges of your frame.
As you press the fiberglass panel down into the frame, the batting gets folded up around the edges. This helps make a snug fit to hold the fiberglass in place, and covers the back side of the fiberglass so it isn't left exposed. When this step is complete, you have the top of the fiberglass level and even with the top edge of your wooden frame, and the backside of the panel neatly covered with your folded layers of batting. The result is displayed below.
The next task is to choose a fabric to wrap around the front and side edges, stapling it across the back. Essentially, you can choose just about anything you like here. The one recommendation I have is that you choose a fabric with an "open" weave so that high-frequency sounds will pass through the fabric rather than reflect off the surface. An example of a "closed" weave fabric is a drape with a heavy lining or backing that also blocks light. If you hold an open weave fabric to your mouth, you should easily be able to breath through it. For my panels, I chose an inexpensive curtain set (again from the giant W). Each curtain set provided enough material to wrap two 2x4 foot panels. Just iron the fabric to remove the wrinkles, stretch it across your panel, and staple it to the backside. Start in the middle of each edge and work your way out to the corners, stapling every two inches or so and pulling it tight as you go. The corners are the most challenging part, so you might want to play around with various folding techniques until you get something you are happy with before you staple the corners down. I chose blue and tan to compliment the wall and carpet colors. Finally, cut a few pegs to 1-2" lengths and glue them to the back of your triangle supports on the back of the trap. Along with some self-adhesive felt, these feet will provide the necessary air gap behind your traps for improved attenuation.
The next trick is determining the best place to hang your panels. The goal is for them to absorb sound at the "first reflection points" from your speakers. In the image below, I have marked these points in blue. In order to do this, you need to have already determined the placement of your speakers and the primary listening position(s). Here is where things get easier when you enlist the help of someone else in the house. You'll also need a mirror (a small makeup mirror works well) and a laser pointer (a laser pointer cat toy is available at Wal-Mart for $5). While sitting in the primary listening spot, have your assistant hold the mirror flat against the wall. The mirror should be the same distance from the floor as your ear when you are seated. Shine the laser on the mirror and move the mirror/laser around until the laser dot shows up on your speaker (specifically, aim for the tweeter). Mark this spot with a small piece of tape or a pencil mark. You will want to mark two locations on each side wall: 1) the reflection point for the left speaker and 2) the reflection point for the right speaker. Next, repeat this process for the opposite side wall. If you have a rectangular or square room, the results should be fairly symmetric. Do the same for the back wall behind the seating position. Since my panels have picture hanging wire on the back, I just nailed up two heavy-duty picture hanging hooks with about 12-15" between them and hung the wire across both hooks. Done! If you have a drywall ceiling, some people will also suspend another panel from the ceiling where the tweeter will reflect from the ceiling back to the primary listening area. To hang the panel, you'll need a few screw hooks to tap into a beam, then just use a little wire to hang your panel.
Treating the places marked in blue will make a VERY dramatic improvement in the sound quality that you hear. When done properly, your speakers should "disappear" in the room when you close your eyes and listen to music. The vocalist will no longer be glued to your left and right speakers, but will appear to be standing right in front of you - between and just in front of your speakers. Good recordings will make you want to get up and touch your center speakers to make sure that your preamp/receiver is still, in fact, operating in Stereo mode rather than Pro-Logic mode. As the instruments play, you should be able to point to each one in space somewhere behind your speakers. Each instrument should have its own "location" that is distinct from each of the other instruments. When you hear this - you've done it right. You may need to "toe-in" your left and right speakers a little (move each one so that the front baffle points toward your listening location) in order to get this effect. Move the speakers around a little: out from the front wall by a few feet, out from the side wall by a few feet. Be patient and keep trying until the center image snaps into focus - you'll know when you get it! Your ears will be amazed! Now enjoy the rest of the music! Visitors comment on the "seductive" and "velvety-smooth" music and vocals that result when the first reflections are tamed and effectively removed from the room. Another visitor used the word "clarity" to describe their amazement at what they heard.
The image below is a set of measurements that I made with REW from 100Hz to 20,000Hz. I set up a small speaker on a stand and directly pointed it at my SPL meter which was mounted on a tripod. The speaker was placed two feet away from the meter. The gold line is a direct measure of the speaker with nothing between it and the mic. The purple trace is the response of that same speaker setup, but with a single 4-inch acoustic panel placed directly between the speaker and the mic. The 4-inch thickness of Owens Corning 703 reduces SPL by more than 15dB from 400Hz to 20,000Hz, with the advantage increasing as frequency increases beyond 6kHz. The blue trace on the bottom was made with two 4-inch acoustic panels placed together between the speaker and the mic. The 8-inch thick layer of OC703 show increasing absorptive benefit in the 250Hz-4kHz range.
The picture below of my side wall shows where my acoustic panels ended up being placed. The blue panel takes care of the first reflection point for the left speaker and the tan panel takes care of the first reflection point for the right speaker (not seen in the image). The opposite wall looks the same - one blue panel and one tan panel to help tame the first reflection points in the room. The contour of the ceiling tiles acts as a diffuser (not an absorber) to prevent coherent sound reflections from reaching the primary listening area, so I did not place any acoustic panels on my ceiling. What a difference it makes and its so easy to do! This is probably one of the least expensive tweaks you can do that will provide the greatest improvement to your listening experience!
Overall, I made five 2x4 foot panels and two 2x2 foot panels. All panels are 4" thick and are mounted on the side and rear walls of the theater. Use the same mirror & laser technique to find the reflection points on the rear wall. Having an acoustically "open" back on your panels provides a nice benefit. The sound waves that you wish to absorb (thus preventing reflections) travel 4" from the front to the back of the panel, reflect off the surface of the wall, and then must travel back through another 4" of the panel in order to get back into the room. Thus, your 4" thick panel actually provides 8" worth of acoustic attenuation. The net effect is not small and is clearly noticeable when listening to music. No more "clap echo" in my room! When I get some more time, I'll make some measurements of the impact of the acoustic panels.
There is one final detail to attend to: the need to tame the bass response in the room. Bass is tricky because the wavelengths are so very long (a 20Hz wavelength is 50 feet long!) and they interact with the physical dimensions of the room, especially at room boundaries. Each time two room boundaries intersect at 90 degrees to one another, you typically get a 3dB increase in sound pressure. You'll get between 6-9dBs of increase in the corners because you have three room boundaries coming together, so there are additional interaction effects. Depending on the room size and sound frequency, you'll get some standing waves piling up on one another in a summative manner resulting in huge "peaks" while others will interact in a destructive manner resulting in "troughs" in frequency response throughout the room. Bass traps (especially when placed in the corners of the room) can go a long way toward smoothing out these effects and producing more uniform bass response throughout the room. Part 2 of Paul Spencer's Bass Integration Guide and Art Ludwig's Room Acoustics are required reading on this topic!
There are several styles of bass traps that are readily available - some of which are very expensive to purchase. The good news is that you can build them yourself for a fraction of their retail pricing. The first type of corner bass trap is simply to take one of the acoustic panels from above and place it across the corner of your room. This works reasonable well, but leaves an empty hollow space behind the panel. The second type of corner bass trap completely fills the corners so there is no empty space between the front surface of your trap and the wall behind it. A Google Images search on "corner bass traps" will reveal tons of designs and varieties.
The most effective form of bass traps are known as "super chunk" style traps - they are triangular in shape and are made from light-weight pink fluffy rolls of insulation. In contrast to the wall-hanging acoustic panels, bass traps need to made from uncompressed insulation. The point is for the particles in the insulation to absorb the bass energy, thus converting the acoustic energy of the sound waves to kinetic energy inside the insulation. This damps the acoustic energy and goes a long way toward eliminating the build-up of bass in the corners of the room.
The first thing you need to decide is how large to make your bass traps. Here is where part of the work is already done for you since it is easy to find R-19 insulation that is intended for 24" on center stud spacing (23" wide). Since bass traps are most effective when they fill the entire corner of the room, the issue of how high to make them was easy to determine as well - floor to ceiling. So, off to the hardware store for a 40 foot roll of 23" wide R-19 unfaced insulation, some plywood, T-Nuts, 1/2-13 threaded rod, paint, craft paper, spray adhesive, and some more Wal-Mart curtains so my traps match the wall-hanging acoustic panels.
After cutting my top and bottom triangular plates from a 4x4 foot sheet of plywood, I made a jig for my drill press for drilling the holes in the corners of my triangle to hold the threaded rod. Each triangular plate measures 24" x 24" and about 34" across the front diagonal (17" deep). If you want to make smaller bass traps, just use insulation intended for 16" on-center stud spacing. This will result in a trap that measures aprox 16" x 16" x 24" across the front and is 11" deep (slightly less than half of the cross sectional area as the larger traps).
Since one of the rear corners in my theater room has a door that is used occasionally my traps need to be easily movable, thus I made the traps about 3.5 feet tall and stackable. This height also fits the dimensions for my fabric as well - some planning ahead will reduce the amount of waste and leftovers when your projects are completed. In order to make the corner traps stackable, the top plate of the bottom trap and the bottom plate of the top trap used T-Nuts to hold the threaded rod. Since the front edge of the wood would be exposed in my design, I painted it black after the T-Nuts were installed. The next task is to cut your threaded rod to length, attach it to the panel, and secure it on the other side of the wood with a regular nut. This way, you have a wooden plate with three rods sticking up in the air. The next step is to put a few nuts on the tops of your threaded rod and drop the top plate in place. This holds the rods in place (properly spaced from one another - see below) while you install the insulation. Use a T-square and a razor knife to cut 23"x23" squares of the R-19 insulation. Then cut the square in half along the diagonal so you have two triangles. In order for the insulation to fit inside of my threaded rods, I cut off 2-3" inches of each front corner and cut a small notch in the the back corner.
Then, take a few minutes to fluff up your insulation by gently pulling it apart top to bottom and place it on your wooden plate. In order to keep the layers from sagging over time due to gravity, I used some wire I had laying around (separated lengths of Cat5 bulk wire) to create a supportive mesh between each layer of insulation. I started by running the wire along the first dimension shown above (<-- 1 -->) and wrapping it around the threaded rod on each side. Then, bring each wire end to the last rod in the back as shown by the number 2 arrows in the image above. When you hit the third rod, wrap each wire a few times, and then bring them forward toward your first edge as shown by the number 3 arrow. When the ends of the wires hit the first wire across the front, gently pull them tight and wrap them a few times. A bead of solder holds them firmly in place so they won't come undone. Now just keep going until your frame is filled with insulation. I placed the wire mesh supports every 5 inches along the threaded rod. Thus, the threaded rods and the wire mesh is enough to keep your insulation in place as long as you handle it from the edges and with a bit of care. To get the last layer in place, you'll need to remove your top plate, reinstall it, and secure it with a nut on top and bottom. For me, one 40-foot roll of R-19 was enough to make all of my bass traps. A bit of black spray paint at this point will help hide the nuts. When you are done stacking the insulation, just cover the front with some kraft paper (available at Wal-Mart) attaching it with some spray adhesive. Since the bass traps are supposed to absorb only low-frequency sound energy, adding a layer of kraft paper prevents the bass trap from also absorbing high-frequency energy. Many people have reported decreased ambient quality to music when there is too much high-frequency absorption in the listening room. The job of the craft paper is to reflect the high-frequency sounds so you don't end up with a "dead" room.
Construction on the first one is complete, now just build three more. Most of the hard work is done at this point - all that remains is something to dress them up so they match the rest of your room. For this, I used the same fabric that I used for the wall-hanging sound panels.
After ironing the curtain, I spread the fabric so the bass trap was centered along the length and I attached the fabric to the bottom of the top plate with a staple gun. Turn your bass trap upside down before doing this so you are actually attaching the fabric to the top edge of your bass trap. This will keep your fabric wrap from drooping over time. You can see a close-up in the image below - I drove the staple through the folded seam at the edge of the curtain so it would help keep the fabric from pulling through over time.
Then, turn the trap right-side-up again and pull the rest of the fabric together behind the threaded rod that will go into the corner. Here comes the really technical part - use a good ole-fashioned desk stapler to hold the fabric together in the back. Make sure you have both sides of the fabric in the stapler (you may need to cut off the excess first), give it a pull to make it tight, and staple away. Move down a few inches and staple again. Don't worry too much about the pulls that you will see on the sides, they won't be visible on the front. When you are done, the completed trap looks great!
Here they are all stacked up in the back corners of the room. Note the door on the right, hence the movable design. Moving them out of the way is simple, just grab it by the edges and move them one at a time. They are very light weight and easy to move, yet they do their job nicely!
So after all of the effort to build the bass traps, what difference do they make? I made some before and after measurements using REW and the differences are pretty clear, though equalization is needed for overall best performance. Below is a comparison of frequency response plots before and after the traps. The difference does not appear to be very large in terms of frequency response alone (though the room mode at 45Hz is reduced by a full 5dB), but time-based plots below show a different story.
Below is a before (purple) and after (yellow) comparison of the waterfall plots for the bass traps. Note the reduction in bass energy across the entire spectrum for the subwoofer (less cross sectional area of the graph is visible in the plot on the right). Also, notice the "holes" in the bottom of the after graph at 30Hz and 60Hz that indicate much reduced overhang of bass energy.
The graphs below show the before and after impulse response for the traps. Note the overall cleaner appearance of the impulse response curve with the traps. Without the traps, it takes over 600ms for the impulse level to remain below the -20dB mark. After adding the traps, the impulse response hits the -20dB mark by 400ms. Also note the reduction of the light and dark banding of the impulse response curve after the bass traps are added.
The spectral decay plots also show improvement - note the increased distance between the traces for each successive 20ms time period. The difference made by the corner traps reduced the decay enough that I had to slide the scale down by 10dB in order to see the entire graph.
Finally, here is a shot of the back of the room with the new bass traps in place and three of the wall-hanging acoustic panels. What a difference the bass traps make! They have eliminated the reverberant "boom" from the bass effects in both movies and music. The result is clearly distinct bass notes from music instead of notes that all run or blur together. The complete story of fine tuning the IB subwoofer using the bass traps in combination with the Behringer Feedback Destroyer parametric equalizer is on the page for my infinite baffle subwoofer. It is a remarkable transformation in sound quality!
Wall hanging acoustic panels and corner bass traps are perhaps the least expensive tweak that you can add to your system that will have such a significant impact. For less than $200, I made half a dozen custom wall treatments and placed them at the first reflection points along the side and back walls. The bass traps for two corners of the room cost less than $140. The improvement in room acoustics is immediately clear. Before the acoustic treatments were added, standing in the middle of the room and clapping your hands resulted in an echo/reverberation that seemed to linger forever (relatively speaking). After adding room acoustics, the echo is gone. A clap sounds like a clap - just one. Similarly, notes on a bass guitar are clear and distinct. A kick drum sounds clear and tight with no reverberating overhang. So there you have it, another DIY project with great results - all for a fraction of the equivelant retail cost of comparable products. Commercially available 4" thick 24" x 48" wall hanging panels sell for $70-$200 each and the smaller 24"w by 48"h corner traps for $130-$300 each. I made two floor-to-ceiling bass traps for less than the cost of getting just one half-height trap delivered!
Here are a few great sites with DIY materials for making acoustic panels and other room treatments: Acoustimac.com, ATS Acoustics, Real Traps,
There also also great threads in various forums about building corner bass traps and wall hanging broadband absorbers: corner bass traps, more about bass traps, and more corner traps. GearSlutz has an entire forum dedicated to acoustic treatment issues and AVS Forums has an Acoustical Treatments Master Thread. It is a long, long read, but there is tremendous knowledge contained there!