Sealed M8a MK III Speaker Array
DIY Wall-Mounted Surround Sound Speakers
Designed by ThomasW and Jon Marsh at TheAudioWorx
This page describes the construction of my home theater surround sound speaker array. These speakers mate with my Avro open baffle front speakers to create a 7.1 surround sound system for my home theater. The speakers consist of the HiVi M8a 8" midwoofers and Focal TC120TD5 inverted tioxid dome tweeter drivers mounted in a ~1 cubic foot sealed cabinet and present an 8 ohm load to the amplifier. These speakers are one of the many variants of M8a bookshelf speakers that have been published online and in AudioExpress magazine over the past few years by Thomas and Jon. What makes these well suited to my wall-mounted surround sound needs is the lack of baffle step compensation in the crossover design. Most well-designed "traditional" speakers feature some sort of baffle step correction to deal with varying sound dispersion effects across the audio spectrum. Essentially, high frequencies from the tweeter tend to radiate out in a 180 degree arc in front of the speaker, whereas lower frequencies from the woofer tend to radiate out in a 360 degree arc all around the speaker. Thus, the job of the baffle step compensation circuit is to help balance the sound pressure levels between the drivers. This is a much boiled-down explanation of what baffle step compensation does for a speaker, for more detailed reading, do a few web searches on the topic - there is no shortage of reading material.
The original iteration of this speaker was designed using a Woodstyle WS803 Rev pre-fabricated cabinet. This cabinet measures 10.25w x 19"h x 12"d using 3/4" MDF construction with a net internal volume of 24.9 liters. This volume was then reduced by adding internal bracing to stiffen the cabinet. While starting with a (nearly) completed cabinet is an attractive way to plan a speaker project (it saves a TON of time), having a 12" deep speaker hanging on the wall at head height is not a good thing for my arrangement. Thus, I set out to create a more streamlined cabinet with a similar internal volume. What I settled on is a more triangular shaped enclosure that is deeper at the top and more shallow at the bottom, thus creating less of a head banging hazard while walking through the room.
Drivers for this project include the now-familiar HiVi M8a 8" midwoofer and the Focal TC120TD5 inverted tioxid dome tweeter. The tweeter for this project is somewhat expensive as far as tweeters go and can be difficult to find since it was recently discontinued. As of 2011, they can still be found at E-Speakers and sometimes on E-Bay. Focal drivers are well known, though, for excellent performance.
The crossover for this speaker can be found at TheAudioWorx web page so I won't duplicate it here. Implementing the cross over is a snap with components readily available from PartsExpress. With just a few hours worth of time (mostly spent on the physical layout for the first one - the rest are just copies and go much more quickly) the crossovers are completed and ready to mount inside the cabinets.
The tweeter XO and the Zobel network are contained on the same board (shown below). I used Dayton caps and Mills 12w non-inductive, wirewound resistors for the crossovers. I was able to order some of the coils in the exact values that I needed, others values aren't available so you need to order the next larger value and unwind a few turns from the outside of the coil. This is an impossible task without a decent LCR (inductance, capacitance, resistance) meter. When I ordered the caps for the crossovers, I also ordered a bunch of small-value caps such as 0.10uF, 0.22uF, 0.47uF, and 0.68uF. These are standard values and were used to "tweak" the measured value of the larger caps so the crossovers for the different speakers match one another. Typical tolerances for caps are 5-10%, thus a 20uF cap might actually measure somewhere in the range of 18uF to 22uF. The small values are added in parallel to the larger cap to increase the capacitance value so that the caps used in the same spot on different crossovers actually match. You can see the resulting caps in the images below. Do the same with your inductors for they, too, have manufacturing tolerances. Make small adjustments by unwinding little bits (a half to full turn at a time) and re-measuring them. It won't make a huge difference in the completed speaker, but it will help two speakers better match one another and it's very easy to do before you solder everything together and seal the boxes. Since this is likely to be my last speaker project for a little while, it was also a good opportunity to use all of my left-over crossover caps and resistors from previous projects where I ordered extras or made other changes.
The crossover for the woofer (shown below) receives the same matching treatment that was performed for the tweeter crossover (shown above). Since I needed smaller inductors here (because they need to fit inside the cabinets), I chose to purchase pre-made inductors rather than winding these myself as I did with the Avro project for my main speakers. The crossovers are mounted to 1/4" masonite board that has been pre-cut to fit specific locations inside the speaker cabinet. I used hot glue to hold each of the components down and some handy terminal strips from the local RadioShack for hookup. To reduce errors as your project progresses, mark each terminal as either input or output and mark the polarity as well. With the tweeter network, the polarity is reversed, so be extra careful here. I used 15ga inductors and wiring for the woofer crossover and 18ga inductors and wiring for the tweeter network.
Building your own cabinets adds a bit of time to the project, but allows you the opportunity to build something that better suits your needs. I played around with a spreadsheet to explore different dimensions and wall thicknesses in order to find the right balance of internal volume and a rigid cabinet. I have attached a small PDF scan of my hand drawn plans for the cabinets. The best part is that I was able to make two complete cabinets from a single sheet of 60" x 60" x 3/4" Baltic birch plywood (plus some left over scraps of MDF). Thus, I only needed to purchase two sheets of wood in order to make four speakers. The balance of the wood for the cabinets (doubled wall thicknesses) comes from scraps left over from various other projects.
The bottom corner of the side panels was rounded over using a sanding drum on my drill press. 50-80 grit paper is great for removing lots of wood in a hurry. The exposed edge of the panel was then rounded over with a 1/2" rounder over bit in my router. Some sanding with heavier grit paper on a sanding block and final touches with a fine-grit sanding sponge make the panels ready for some stain and paint. I didn't kill myself to sand the wood ultra-smooth since applying any type of paint causes the grain to raise.
Below you can see the two side panels stacked one on top of the other as they will be attached to the speaker. The entire speaker cabinet will be painted black. The larger side panel (bottom one) will be stained maroon to match the trim on my Avro speakers, and the smaller side panel (top one) will be painted black. These panels will make the side walls of the completed cabinet more stiff, so no additional wood was added to the inside of the side walls.
After the initial cutting, I glued and clamped the outer shell together. Getting everything lined up properly before tightening all of the clamps is usually a bit of a trick and goes more smoothly with an extra set of helping hands. The angle of the front baffle slants inward by about 14 degrees. Making this cut for the side panels is relatively easy: just clamp a thin (and straight!) scrap of wood to the wood you wish to cut and use this as a guide for a hand-held circular saw. Then, be sure to make your cut using a single pass without stopping or pausing until the entire cut is completed. For the top and bottom panels of the enclosure, I used my table saw to cut the necessary angle along the edge of the panel so it meets up nicely with the sides of the enclosure.
The next step involved making the cabinet more strong and rigid by doubling the inner surfaces of the walls (left cabinet below). I started by cutting up scraps of MDF and wood I had laying around from other projects. The scraps were glued to the inside of the cabinet. I doubled up the back panel of the speaker first, then the top and bottom panels. Next, I added another layer to the inside of the front baffle. The inside of the side panels only received a small strip that is used to create a stronger glue joint; the doubling for the side panels takes place on the outside of the cabinet, this is what forms the "wings" of my design (the rounded pieces of wood shown above). You can also see some small triangle pieces that join the side panels to the rear panel. This is to help further reduce cabinet resonances. More will be added before the cabinet is closed, but the crossovers have to be installed first.
Once the boxes are assembled and the walls are doubled up (top, bottom, and rear panels for now), I added the completed crossovers. They are held in place with Gorilla Glue and screws. I chose Gorilla Glue because it is an expanding glue that will foam up and expand to fill any air gaps under the masonite panel to prevent unwanted vibrations upon final assembly. You can see the hole in the top panel (which is actually the bottom of the speaker as it will hang on the wall) in the image below. This is for the input terminal and will be one of the last items attached to the speaker. You can also see the front baffle just to the right of the open cabinet. I glued some scraps of wood to the backside of the baffle to add some mass and help reduce vibrations. A third layer was added behind the tweeter, so it has its own sealed chamber. The back of the tweeter mounting flange is not a flat and solid surface, thus it does not lend itself well to forming the necessary air-tight seal required by the cabinet design. The tweeter chamber is just a few extra thicknesses of wood as shown that have been routed out to accommodate the entire depth of the driver.
The drivers have only been placed in the baffle at this point so I could have a listen to the speaker to make sure the crossover construction didn't have any errors. I have to say, I'm already impressed by listening to this speaker at this early stage. I was expecting this to be a relatively bass-shy design based on what Thomas had to say about it, but it seems to be a very capable performer! You can also see some additional triangle braces that I added between the rear and side walls. Similar braces will be added to the front baffle once its attached. Finally, the side walls will receive their bracing on the outside of the cabinet. What you see on the inside of the side walls in the image below are just small 1" strips that are designed to provide some added glue strength for the joint.
On the subsequent pair, I decided to reinforce the front baffle even further by making the third ply behind the tweeter mounting hole even larger and adding strips down the sides of the baffle next to the woofer cutout. The strips don't look like much, but due to the size of the woofer cutout, they actually represent an increase of about 20-25% in the wood stock used in this area. An additional brace will be added along the bottom of the woofer cutout once the baffle is mounted to the cabinet. This will help reinforce the bond between the top panel and the front baffle.
Below is a close-up of the tweeter mounting hole. This is a sealed chamber behind the tweeter so I don't have to worry about trying to seal around the tweeter flange itself once it is mounted. The holes for my router circle jig and the holes for the wires are also sealed up with glue. A little sanding around the edges to clean things up and it will be all ready to go!
Next up is gluing the front baffle to the cabinet. After a final coat of spackle and some sanding (OK, a LOT of sanding), they will be ready for some paint. For this stage, remove the actual speaker drivers and tape over the holes to keep the cabinets from filling with saw dust as you work.
Below are images of the assembled and painted cabinets. The holes in the top (which is really the bottom of the speaker as it will be mounted on the wall) are for the input terminals and the unpainted areas in the image on the right are where the side panels will be glued. Glue doesn't adhere well to painted surfaces, so these were left raw (unpainted and unsanded). I used the same flush-mounting technique for the drivers that I did with the Avro speaker project.
I had a bit of difficulty getting precision angle cuts for the side panels with my hand held circular saw (plus it's a little harder to clamp at an angle), so I used expanding Gorilla Glue to affix the front baffle to the rest of the cabinet. The baffle was purposefully cut just a bit larger than the rest of the cabinet, so I could use a flush trim bit in my router to get nice clean seams. This was followed by a 1" roundover bit on all four edges of the front baffle and the sides of the bottom of the cabinet. Little imperfections here and there in the edges of the wood were filled with spackle before final sanding. The paint is two coats (sanding between coats) of Rustoleum Latex Enamel Flat Black. The completed speaker will receive one coat of satin polyurethane.
Next, I used my router to make two cutouts in the back of each side-wall speaker to accept the keyhole mounting hardware. The recess is large enough to accept the entire keyhole piece of metal, thus the weight of the cabinet actually rests on the cabinet. The keyhole mounting hardware was secured to the cabinet with two 1.5" screws. I chose this method for the side wall surrounds because there was a wall stud exactly where I wanted to place my speakers. For the rear-wall surrounds, this was not the case. The desired mounting location turned out to be exactly between the wall studs, so I needed another mounting method. Someone online suggested using a French Cleat (available at nearly any local hardware store). The one I purchased was about 18" wide and could hold either 150 or 200 pounds (I forget which - either way, its plenty strong...). So I cut the cleat in half (yielding two 9" lengths) and mounted each half to a 1x4" header that I secured to the studs and painted to match the wall color. The complementary side of the cleat was mounted to the back of the rear-wall speakers.
I also added one more strip of Baltic birch to the backside of the baffle between the woofer and tweeter cutouts and then braced it to the back of the cabinet with another piece of wood. Plenty of glue on the ends and a few taps from the hammer and it is securely in place. Thus, most of the baffle is 2.25" thick and it is directly coupled to the 1.5" thick rear wall of the cabinet. Finally, and not shown in the images, I cut two more strips of wood with an angle to match the slope of the front baffle and glued them behind the top and bottom edges of the front baffle to better reinforce the joints with the top and bottom panels of the enclosure as well.
After all of this work, it was time to add the side accent panels. The maroon panel was glued and tacked in place with my nail gun. The black panel was then glued on top of the maroon panel. I stacked some weights on top to hold everything together until the glue set. Below is an image of the almost finished cabinet. I rotated the image so you can see what it will look like when hanging on the theater wall. The angled baffle works great to provide the needed internal volume while keeping the bottom edge of the cabinet out of the way so people can walk past the speaker without cracking their head on it. Overall dimensions are 14.5" wide at the top, 11.5" wide at the bottom, 12" deep at the top, 6" deep at the bottom, and 24" tall. Internal volume is about 20 liters or 0.7 cubic feet.
The side panels were inspired by the Legacy Audio Phantom-HD speaker, pictured on the right. The final look isn't 100% identical, but but this was a much more simple implementation and provides a color accent that matches my Avro main speakers. The last detail to attend to is acoustic stuffing inside the sealed cabinet. I found a deal on E-Bay for 5 lbs of raw wool (just search for "raw wool") for a total of $25 delivered. It took about 10-12 ounces of loose wool to fill the inside of the cabinet. With all of the braces and other pieces of wood sticking out from the side, I suspect it won't all slide down to the bottom of the cabinet as it hangs on the wall.
The end result is a cabinet that is massively stiff, resonant-free (sore knuckles, anyone?), and very heavy (52 lbs) that requires direct mounting to a wall stud.
The final touches to these speakers includes applying one coat of polyurethane and then mounting the input terminal cup, the tweeter, and the woofer. Before I actually mounted the woofer, I did a final wiring check to make sure everything was as it should be (no wiring errors). The next step was mounting this beast to the wall. This was quite a trick (due to size and weight) and required two people to line up both keyhole slots with the 3" deck screws that were secured to the wall stud. Using some masking tape on both the wall and the speaker along with a pen helped with getting the alignment for the keyhole slots. Mounting the rear-wall speakers with the French Cleat was a breeze by comparison, but still required the help of my son for lifting and balancing. Below is a panoramic image of the rear of the theater room with all four surround sound speakers in place.
Before these were mounted on the wall, I placed them next to my Avro speakers to give a listen and make a quick comparison. As expected, they are very close in sound to the Avros, but clearly don't have the same surface radiating area - so the midrange and bass are a bit shy compared with the Avro speakers. This improves as they are placed next to the wall (no surprise there, given these are intended for wall mounting). They sound very nice and clean no matter what I listened to and they cast a nice image as well. These are indeed very fine speakers. It almost seems a shame to use them "only" for surround duty since I'd wager they likely use higher quality drivers and feature better construction than the main speakers that most people have connected to their home stereo.
Their mounting height on the wall and the sloped baffle work together very well to keep these speakers out of the way so no one hits their head on them as they walk by. The baffle slope aims the speakers at the seating position for movie time. Hold on to your seats - the finished theater will blow you away!