Elfin Technologies

A Linear Speaker array you can build

Inspired by the BOSE L1 PAS, I built this Linear Array Add-On on the Cheap. You can too!

As a solo performer ( guitarist and vocals) I'm used to hearing terrible sound and acoustics in various club environments. But lately I've been pleasantly surprised to hear clear vocals and instruments in such environments. Time and again when I turn to look at the performer, I'll see one of these BOSE LI PAS (personal audio system). Basically they consist of a "base box" on the floor to handle the low to low-mid frequencies, along with a tall vertical speaker array of surprisingly small speakers to handle the mids and highs. There's a lot more of course... the two speaker components certainly have very precise individual equalization and amplifiers, with a goodly amount of power. But all those things are pretty common and can be had with minimal investment these days. The big innovation is obviously the vertical linear array.

Well unfortunately these BOSE systems start at around $1700 (USD). So I decided to do my research, and take a chance building my own. This photo shows a peek at the result. I already had equalizers, good power amps, bass speaker boxes, mixers, etc. So if I could successfully implement my own linear array, I'd surely add a nice boost to my own performance sound quality. So that's what I did and I'm happy to report that after some tweaking, the end result sounded great, surpassing all my expectations. And just as important to us underpaid musicians, the project barely cost me $300 to build. This of course excludes significant labor, but thats part of the fun.

During my research I discovered a few things that inspired me to document my work for the benefit of others. Apparently a lot of DIY builders would like to make one of these too, but there's a lot of disagreement about building them, and a good deal of the available information on DIY forums lean toward the home environment. This is an important point because quite often the performance version and the home versions are very different animals! And it became apparent that there were a lot of ways to go wrong and end up with something whose sound is less than desirable. So for all these reasons, I decided to at least offer a few critical photos and explanations of the dimensions, drivers, and components that went into this project. Whether by my own ingenuity or pure dumb luck (likely a combination), my array worked out very well and sounds great. So considering all the variables, I figure offering key details of my construction might help someone else have a fighting chance at success.

Before going any further though, please take note: This is NOT a step by step "how to build" procedure!. Rather, I'm offering some of the important details to consider in order to functionally duplicate my work, with some confidence in what you can and should not expect from it. In other words, I'm sharing some essential construction details, but you'll have to supply the rest. Note also that to attempt one of these as I've done it will require you to have some basic wood working skills as well as some important powered shop tools. There are ways around everything, but I'd suggest a table saw, jig saw, drill press, couple of electric hand drills, and a belt sander at the very minimum. I will, of course be happy to answer email questions (I'll start a forum of some sort if there is a lot of interest), and of course I would also entertain the idea of building these for you at a fair price.

A lot of the graphics on this page will expand with mouse click actions on most modern browsers. Of course you can also right-click and view the images in separate tabs on your browser, and save them. With mobile devices you can "tap" within the images to expand them, sometimes rotating your device to get a better view. Tap again to reset. Honestly though, as with most long articles on this site, a machine with a larger screen is a better way to view.

Now let me explain that this array was intended and designed to cover the audio range from about 200 hz up to around 20 Khz. It's drivers and inner acoustic air volume were chosen for this range. This means that for a full sound, you will have to use it in tandem with a separate bass speaker, usually placed at floor level. Remember, this was somewhat patterned after the BOSE LI system, and that's the way they do it. In my case, I had already built a pair of 2 way ported bass reflex speakers for performance, and I simply added an L-PAD rheostat to each cabinet to control their tweeters. This enabled me to lower or completely remove the treble component, letting the array handle all the mid-to upper frequencies.

Well any good speaker project starts with a choice of driver. For my project, I used 15 of these 2-1/2" full range TC7FD00-04, 4 ohm speakers made by Peerless/Vifa. These handle 20-40 watts (continuous and peak, respectively), and have a very good full range response. These are available from Parts Express, and again I used 15 of them in my array, which is a total of 48" in length. Assuming all speakers were wired for equal power, this would result in an array that could handle from 300 to 600 watts (again, continuous and peak, respectively). My array is also intended to be mounted approximately 2-1/2 feet off the ground, so the topmost speakers will still be slightly above the heads of all but the tallest people.

If you look at the Spec Sheet for this speaker, you'll see that its "on axis" response is pretty respectable for a full-range, from about 180 hz up through 20Khz. As expected, the off axis response is less then ideal at the high end when listening beyond about 30 degrees off -axis, but this will be mitigated. First, there is a horizontal dispersion characteristic that happens when speakers are packed into a closely spaced vertical array. This same effect also limits vertical sonic radiation, thus significantly cutting down on undesired reflections from ceilings. Second, and as with the BOSE system, my array alternately faces speakers slightly left and right (I also point the topmost one straight ahead). the end result is that from almost anywhere in the listening area, at least one or more speakers will be pointing directly at you. Look carefully at the first graphic at the beginning of the page, and you can almost see the way the speakers alternate their orientation.

So this brings us to the first of several construction challenges. In order to alternately orient these speakers, I had to build small sturdy frames for them, which would later fit comfortably into a recess I created in my frame. The images at left shows a few of these frames, and the right image also shows a group of them lined up on a board in the background. These frames are made from wood cut at 15 degree angles on a table saw. Mine are 3.2" wide in the dimension between the two angled sides, and 3.5" inches between the two flat sides. Note that 3.5" is the actual thickness of a most "2 x 4s". 1.5" is also the typical thickness. So during the cut, the thicker side came out to 1.44" thick, while the thinner side tapers to about 0.6". That 1.44" number isn't critical. It is simply the result of using the table saw to cut a 15 degree angle into a wood plank that is already 3.5" wide and 1.5" thick, adjusting the table-saw fence so that the thinner side is at least 1/2" thick when the cut is complete. (In my case, I went to 0.6, allowing for some touch up with a belt sander, still ending up with 1/2" minimum thickness.) The 3.2" width between the two angled sides is a little more important, as it permits 15 of these frames to be included side by side in a 48" space. Recall that 48" (4 feet) is the total length of my array. (Also note that I later decided to make the topmost frame without any angle. This , allowed for a balanced 7 speakers in each left/right direction, with the topmost speaker pointing straight out).

Making the holes in these cut frames is a little tricky. First you'll want to find the center on the top (angled) edge. Further, you'll want to drill your holes with the drill perpendicular to that angled surface. (This means that on the bottom 'flat' edge, you should expect the hole to end up off center). There are a few different ways to accomplish this. My method was to use some extra wood I cut with that same 15 degree angle under each work piece, so a drill in a press would cut into the surface at 90 degrees. For the actual holes, I used a 2-1/2" hole saw... the kind with a complete circular ring (Here's a link to one at ACE hardware.) These make clean holes ideally sized for the speakers. DON'T try to drill the entire way through each piece in one step! Instead, when you're about 3/4 of the way through, pull back on the drill press and turn your work piece over. This time, remove the extra angled wood, because the piece is now already lying on its 15 degree cut side. Using the pilot hole created by your first pass, finish the hole from the other side. Note that you actually will now be going in at a 15 degree angle, but the pilot hole resulting from the previous cut will guide you, and your 2-1/2" hole will end exactly in alignment with the cut from the other side. You'll also be finished with only a short distance to drill. The cut hole will now be much easier to remove from the hole saw then if you had gone completely through from the other side. Again, it is correct that the hole will NOT be centered on the flat (non-angled) side, but WILL be centered within the angled side.

Now, here are a few views of the main box construction that will house all these speaker frames. I was able to make this entire frame with a single 4' x 2' piece of 1/2" plywood (decent quality birch) and an 8 foot length of 1 x 4 pine (again remember that 1 x 4 is actually about 3/4" x 3.5".) The plywood pieces for the sides were cut to exactly 5" wide x 48". For the 2 ends, as well as 3 inside braces, the 1x4 pine was cut. Two pieces exactly 4" x 3.5" for the two ends, and a bit less (3" x 3.5") for three inner braces . The reason for the smaller inner braces was due to the way I decided to make the angled speaker frames fit into the main box. Using the same 15 degree setting on the table saw, I cut a couple of long 48" strips of wood about 1" wide, angled so that one side face was about 1/2", while the other was barely 1/4". Those strips were fastened 1/2" below what would become the front face, screwed, glued, and clamped in place. These would also line up with the end braces. Since the end braces were 4" long, and the plywood sides were 5", the end braces were placed to split the difference, recessed exactly 1/2" from both front and back. You can see in the second frame where I temporarily placed one of the speaker frames, to test how they would fit.

Incidentally, I angled those long strips so that I could provide a continuous 1/2" wide ledge of wood for the speaker frames to butt up against on one side, while minimizing the internal volume consumed on the other side. I couldn't do it exactly the same way on the back side of the box. I wanted all the braces there to come to within 1/2" of the surface. This allows for a stronger rear mounting with many more anchor / screw points when a single long piece of plywood is used for the back cover. Incidentally, those inner braces are placed at slightly uneven distances. I put one centered at 11-1/2" from the center of one end brace, the next one centered 13" from that one, the next one 12-1/2", leaving that last one about 9-3/4" from the other end. You want them uneven because we are already going to have to mitigate standing wave resonances from the distances between these braces, and it is always helpful when you don't have multiple resonances to deal with all at the exact same frequency.

For the back panel, I'll be cutting a piece of the 4 foot long ply wood, slightly less than the 3-1/2" inner width, to fit into the frame and up against all the recessed braces. I'll want the sealed cabinet to be reasonably air tight, so in the rear I used similar angled wood on the walls between the braces. This time tough, I had to cut them into shorter pieces of varied lengths, to tightly fit between the braces. Due to a few lost photos, you won't see that detail here, but its plainly visible in some of the latter photos where the wiring is shown. In addition to the usual gluing, screwing, and clamping for those pieces, some silicon caulking was used at the ends wherever there were gaps.

I said i wasn't going to offer a step by step guide, but at this point, let me mention a few construction details and materials that won't be obvious. You're going to need phillips head screw fasteners in several different lengths. 2", 1-1/2" and 1", the latter in both #6 and #8 widths would be a good start, and some smaller 3/8" ones later for mounting the speakers. You'll also want to pre-drill all your screw holes to ensure against splitting. (The smaller holes for the actual speaker drivers are the only exception.) I find it very worthwhile to have two electric hand drills ready, one with a phillips driver bit, and the other with an appropriate sized bit for pre-drilling holes. I'd also recommend you get some Elmer's Carpenters wood glue (not the typical white), and a caulking gun with a tube of silicone sealant, as you'd typically use for weatherproofing windows. You'll want to screw and glue every point where you put wood pieces together, and its always a good idea to clamp long pieces in place (as you can see from some of my photos, I ran short of clamps and had to get a bit creative.).

Some other misc things you're going to need very soon. First, as part of our anti resonance strategy, many speaker boxes need some kind of padding along the inside surfaces to help discourage reflections. In this case we don't have a lot of inside volume to lose, so my solution was to obtain a roll of craft foam, which you can get at Michael's and several other craft stores. Its thin but has a good damping effect on sound. Please forgive this awful blurred image on the left. It's the only photo I took when I first added this craft foam to the inner walls, and didn't realize the focus was bad. The craft foam is cut into reasonably well fitting pieces, overlapping where pieces join, and glued to the edges with 3M general purpose spray adhesive. Despite the blurry image, this will all become clearer in the next photos. I won't go too much into details on this next point, but you're going to want to plan on adding some vinyl covering fabric to the outside of this speaker for both durability and cosmetic enhancement, and you're probably going to want to spray paint pieces like the speaker frames, since these will be partially visible in the end. For simplicity and to blend with most other stage equipment, I just used black paint and coverings. Here are some photos to show various stages of subsequent construction.

So here, left to right, you see...

  1. A test fit with all 15 speaker frames. Note the top one has no angle. Here you can also see that each of the inner braces is held in place with 2 screw fasteners on each side. Of course, pre drill all your screw holes so you don't get any splitting, make sure to counter sink each screw so there won't be tell-tale bumps when you cover the outside.
  2. Here I started adding my vinyl covering. That same spray adhesive did an excellent job securing this material.
  3. Another test fit, with all frames painted.
  4. Here the speaker frames were finally mounted.
  5. A first view of the assembly standing on its own.

Obviously there is a whole lot of important detail in that 4th frame! First, I wanted a good seal for the speaker frames, but wanted to reserve the ability to disassemble too. So all the contact surfaces on the inside were coated first with silicone caulking, and the inside edges of those frames that would contact each other were coated too. Thats still a pretty firm hold, but it would be possible to separate them if necessary. Using the carpenters wood glue would make any such future revisions impossible. Next, in addition to the silicon caulk, notice that each frame is also screwed into place from the outside. This was done with 1" fastener screws, each with a recess washer, with each screw hole located to catch wood at 1/2" from the end of the frame. And of course you really don't want to forget to properly pre-drill those holes!!

The speakers have also been mounted at this point. There are three important steps here. First, you'll want to solder about a foot or so of wire to each speaker terminal. I used Red for all the positive terminals and white for all the others, to avoid errors in speaker phasing later. Next, you'll want to cut out a piece of that craft foam to the exact outline of each speaker, along with a 2-1/2" inch hole in the middle. These will allow each speaker to have a secure and air tight fit, and prevent any unwanted vibration. the craft foam is easy to work with and cuts easily with a safety razor. Finally, each speaker is situated into its frame along with the craft-foam inserts, and screwed down. I used #4 x 3/8" black flat head sheet metal screws, obtained from MicroFasteners.com , part number SMFPK0406". You'll have no trouble pushing the sharp screws right through the craft foam. Tighten these all with a hand screwdriver. Be mindful that the craft foam flexes, and so you want to tighten the screws a little at a time all around, as you would a car tire, and don't over tighten. The speaker frame metal should flex slightly, which is OK, but don't over-do it.

The next thing we'll want to address is the wiring, which means a lot of working from the back. But first, although its pretty safe at this point to rest the whole assembly speaker-side-down on a totally flat surface, I found it worthwhile to cut some pieces of wood to build a couple of 'U' shaped holders, that would allow the speaker to rest its weight on the edges of the frame.

So back to the wiring! There are many possible and equally valid ways to wire the speakers to form different total system impedances and power outputs. There are also variations for different weighting of power (sometimes called shading), such as wiring top or middle speakers to be louder then the lower ones. That said, trust me... you'll want to take the extra step of wiring all the speakers to terminal mounting strips, to make it easy to change your mind. All it takes are some small cut pieces of wood glued to the edges, to screw in the terminal strips. You cant see it, but I took the extra step of drilling small holes for the wires to pass through near the walls. It holds the wires in place better and makes the whole design neater.

In these two photos, you can also see the cut strips of angled wood mentioned earlier, fastened between the inside braces. This will allow the back to make a good seal all around. You can't see the angles of those wooden strips, but you can imagine how cutting those angles cuts the volume they take up by half of what they would, if simply cut straight. Notice also that the vinyl fabric covering is looped around the back areas. The back piece of plywood should be 48" x 3-1/2", but since it too will be covered and wrapped with this same vinyl fabric, I ultimately found I had to trim the back to about 3-1/4" to fit.

So now I'm prepared to wire and re-wire several different ways, without ever disturbing the wires actually soldered to the speakers. But the next step, before the actual wiring to combine the speakers, we have to plan for "stuffing" the insides of these speakers. Often speakers are stuffed with pillow stuffing or similar substances, because doing so helps to defeat resonances, while also giving the effect of a slightly larger box. But in this case, due to the short distances, we need to go some extra miles to defeat those resonances, and standing waves. So here are some materials needed.

Again, left to right, you see...

  1. Some flexible but firm packing material. This should be something that air passes through pretty easily. Since we will be packing the box with very loose pillow foam, this 1/4" layer will help prevent any of that loose stuff from actually getting down to the speaker cones.
  2. Next, this inexpensive pillow or toy stuffing worked very well. As there are many ongoing debates about the best material for stuffing speaker boxes, it's probably that different materials will affect the sound differently. So, since the whole point here is to help you duplicate something thats already a proven design, I'd recommend you use the same thing.
  3. Finally, this is poster board, available at most craft places. Its is basically styrofoam covered with cardboard, and is just under 1/4" thick. This stuff will play a very important roll, as you'll see in the next photos.

So what's going on here? The first frame shows the poster board cut into 3" strips, the exact same depth as the inner braces. The middle frame shows a layer of the firm but flexible packing material placed over a speaker group, followed by an insert of poster board, carefully cut to match the inner diagonal angle. You can see this repeated in the next frame. (sorry about the wiring mess, but I was already beginning to experiment with the wiring to combine speakers here.) These angled pieces of poster board are hugely important! They cause the standing wave resonances that would normally occur between all the walls and inner braces to be replaced by a continuous infinite band of resonances, covering all frequencies, since there are now infinitely more wavelengths accounted for. I'll explain more about that later.

The last step would be to stuff all the open spaces with that pillow foam. I don't have a great photo of that, but the rightmost photo gives you an idea. You might notice from that shot where I had added some holes between the inner braces, to allow me to more easily pass wires around without looping all the way under the foam.

Let me talk about the wiring a little more, because this again is a part of the project I didn't document well with photos. The terminal strips make it easy to prepare many wiring arrangements. You'll want to do all your wiring with small jumpers along with larger pieces of wire, all prepared with crimp on connectors, to easily route and re-route the wiring many different ways. The simplest connection is to wire 3 sets of 5 speakers, so that you have 5 speakers in series (5 x 4 ohms = 20 ohms), with all 3 groups of 5 wired in parallel. This will yield a 6.7 ohm total, with all speakers outputting equal power. Many modern amplifiers can offer greater amounts of power into lower impedances though. Some like my Behringer NU1000 can offer 500 watts/channel into 2 ohms! With such an amplifier, you could do the opposite... wire 5 groups of speakers in parallel, with each group having 3 speakers in series. 3 4 ohm speakers in series is 12 ohms, and 5 such groups in parallel would yield 2.4 ohms. Then there are wirings where some "shading" is done. One such arrangement I personally like is to combine the bottom 12 speakers into three groups, each with 4 speakers in series (20 ohms). Then, the top three speakers are wired in series (12 ohms) and all 4 groups are then wired in parallel. This yields approximately 4.3 ohms total, which my Behringer amplifier will drive to approx. 250 watts. What is interesting about this kind of arrangement is that the top three speakers output a bit more power per driver then the lower 12. that can be an advantage if you consider that the main use of the top three speakers will be to send sound to the people farthest away. And since the top three in this array are actually pointed in three different directions, the extra energy is well distributed.

Thats not the end of the possibilities either. Though there is no way to exactly balance 15 speakers left and right, a very interesting effect can be obtained with a stereo wiring arrangement with the 7 left and 7 right pointed speakers connected together separately, and brought out to a stereo source. (perhaps 4 in series, parallelled with 3 in series for each side). While some compromise would have to be reached for the topmost speaker (perhaps the old cross wired 'surround channel' trick), the end effect is pretty amazing and it can offer the illusion of stereo speakers, from a single box!

So to wrap this up, here are a few of the additions I ultimately made, to make this speaker ready for performance use.

So here, left to right, row by row, you see...

  1. A gutted recessed speaker panel was re-purposed to offer a standard pair of 1/4 jacks. They are wired in parallel, but could be used to offer some kind of stereo hook-up, as described earlier.
  2. Here's a shot of the back, with the panel mounted. A jig saw and drill was needed to make the cut-out.
  3. Here you see at least some of the grating that was added to protect the speakers. These are made from heavy gauge house gutter leaf guards, which you can get from Home-Depot (Unfortunately I think you have to buy a box of them). These are not long enough for the array, so two of them were judiciously soldered together after being bent into the exact curve needed to surround the speaker frames. They were painted black too. Then, some 1/2" x 1/2" angle aluminum pieces were cut to hold the grating on each side. These aluminum pieces were also painted and screwed into the frame. As it turned out, the grating is just held in place by tension, and a couple of screws at the top. Finally, a good sturdy handle was added to the side, to make this thing a little easier to carry
  4. An insert made of the same grating was added to the top and soldered. Here, two screws were added.
  5. Next is the hardware added make the array free standing. These are 1" pipes and parts I found in a local Lowe's. You can see where I made a pretty hefty base, which is held to the array with several screws. Mounted to that is a hardware piece intended to mate with a 1" pipe. It has a big allen key bolt for securing it. Not surprisingly, this arrangement needed a little "doctoring" to be really secure, but in the end it worked out well.
  6. Finally, here you can see the original free-stand mounting, where the pipe has been mounted to a mike stand tripod. Next to that is my bass speaker, along with the array on top, on an "alternative" mounting platform. Since the speaker is about 1-1/2 feet high, I can use that one in some situations where a more compact setup is required.

Before wrapping this up, I'd like to mention that the addition of those angled baffles (made of the poster board) was a really major breakthrough in the evolution of this speaker array. Despite sounding good during initial tests, an actual frequency sweep test pointed out some noticeable peaks and bumps at several middle frequencies. A little calculation of the wavelengths of those frequencies revealed that they matched up exactly to the distances between my inner braces and walls. Of course it is a little difficult to find the best microphone placement for these tests, given that the speakers point in many directions. But after reaching what I felt was a reasonably representative test setup, I went back and added those angular baffles and re-packed the foam. Eureka! It made a very significant difference in a positive way. No longer were there obvious and extreme midrange peaks and valleys, and I now had what I felt was a reasonable response. The graph here was the final un-equalized response, without my bass box speaker connected. There is still a bit of a dip around 12Khz, but this turned out to be very easy to correct with a graphic equalizer. Coupled with a separate floor speaker to cover the bass and lower mids, I can emphatically say this system has offered me the clearest and most detailed sound I've ever had the pleasure of playing through.

On that last point, it is important to understand that even the BOSE system has a very custom tailored equalization network for their speakers. When building an array, a good result is one like this, where the response is mostly flat between the frequency region it was designed for, where the number of corrections needed is reasonably small. But some corrections and equalization should always be expected.

Finally, despite the lack of full step by step detail, I hope enough essentials have been provided here to allow a seasoned DIY builder to have a good starting point, if an array with a similar purpose is what they are after. Good luck, and don't hesitate to ask if you have a question. I'm sure I've left a lot of details unsaid.