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Electronics If I had a hammer... Applying the finish
Electronics
If I had a hammer...
Applying the finish

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Small Big Monitor
Soffited sound -- but portable!
By Scott Dorsey

A NOTE FROM THE AUTHOR, MAY 2013:

Radian has discontinued the driver used in this design as of 2005 or so, because people were using them in PA speakers and damaging them. They are pretty rugged by monitor standards but not by PA box standards. The closest match from Radian today has a much stiffer surround and much stiffer spider, so the low frequency corner is at least an octave higher.

There is a driver from B+C that comes fairly close to the specs of the original, but it's nearly $600.  It sounds lovely and I may someday try and adapt the design to it, but not yet. As a result, the design in this article is now for reference ONLY and not for future construction (unless you happen to come across some used Radian drivers).  It's a shame, too, because there really isn't anything much in the way of commercial speakers for that application. -- SD

***

A lot of engineers who are used to mixing on the large soffit-mounted horn monitors popular in large speakers get somewhat disconcerted when mixing on nearfield monitors. However, in a typical project studio installation, installing soffit-mounted speakers is impractical.

As a result I looked into constructing some reasonably portable speakers that would come as close as possible to the sound of the popular soffit-mounted speakers, that would be practical for use in a project studio, and that would be reasonably easy to transport for the engineer who works in multiple studios.

I started out with a number of coaxial systems from a number of different manufacturers, and eventually settled on the Radian 5012, which is a pretty impressive device. This is a 12" woofer with an integral horn driver, much smaller than the 15" Gauss woofer used in the popular Urei 813s. Although the driver is smaller, it’s got a longer throw and can still move a sizeable amount of air.

The 5012 is actually a variant on the more popular Radian 5212 coaxial PA speaker, constructed with a foam surround. This gives it a reduced resonant frequency, greatly increasing bass extension over the 5212, but at the expense of efficiency and power handling capability. Even so, it is capable of very excessive control room levels. 

The coaxial design, if you haven’t seen it, is a fairly conventional woofer, but it’s designed in such a way that a compression driver tweeter fires through the center of the woofer. Theoretically this means that the two drivers effectively produce sound coming from the same point in space, so the response is constant as you move off-axis. In reality this isn’t quite the case, but it’s surprisingly close on these drivers. 

Compared with other horn drivers, the horn design is very much more advanced and the top end is therefore a lot flatter. There are some short high-Q resonances in the top octave, but not anywhere near as much as in older designs—and as a consequence the honkiness is much reduced.

Also, because the 12" driver is much less massy than a 15" one, the crossover region is a bit flatter and the overall midrange response is cleaner. I also tested some of the 15" Radians and didn’t like the midrange response anywhere near as much, and that combined with their increased size led me to reject them (even though they were more efficient and could be arranged for better low end). 

Design issues

Let’s first talk for a bit about the whole coaxial driver concept. The 5012 driver is a conventional woofer design, but with a horn driver mounted directly behind the magnet assembly and the horn mounted in the center of the bass driver (to some extent even using the bass driver as an extension of the horn). Since the horn system itself is pre-constructed, the only real engineering involved is to construct a cabinet optimized for the bass driver and a crossover to match the treble horn and the bass system.

In this case I went with a conventional bass reflex system. This is simply a resonant box that is vented to the outside air through a tube. The air inside the box acts as a spring, and the amount of venting adjusts the stiffness of the spring. Also, the vent itself means that both the front and back of the woofer are producing a wave that is coming from the front panel (either through the front of the woofer or through the vent). But because we can adjust the length of the tube, we can adjust the amount of time that it takes for the wave from the rear of the woofer to come out the front, and that allows us to set the resonant frequency of the cabinet.

The idea here is that as the bass response of the driver itself drops off, the cabinet resonance picks up and extends the low end by an octave or more. We want to set the response of the cabinet to extend the low end as much as possible. We don’t want to produce a tight, high-Q resonance that will result in one-note bass. We also want to avoid a broad resonance that overlaps too much with the driver’s natural resonance causing exaggerated upper bass response. Nor do we want too small a peak, which would result in wimpy low end. 

So we basically have three parameters that can be manipulated: the box volume, the cabinet vent diameter, and the cabinet vent length. Plugging and chugging into the Thiele-Small equations, we can get reasonable optimum starting values for all of these and then do some experimentation with a sweep generator and a laboratory analyzer in an anechoic chamber to get them dialed right in. 

I did most of the preliminary testing in my backyard, as far away from any surfaces as possible, and then took the unit into an anechoic chamber for final verification. 

The crossover shown is constructed on a military terminal board; the illustration enclosed shows how it was put together. If you prefer you can construct it on perfboard, even on a screw terminal strip. Or you can purchase the pre-made crossover from Radian as part number 420-1250 (although their crossover uses a 1.4 mH choke where I am using a 1.5, this doesn’t seem to make much audible difference in the midrange and I personally prefer the 1.5).

The only important matters are that (a) the crossover design follow the schematic and (b) that the two chokes be as far apart as possible and mounted perpendicular to one another to reduce the coupling between them. The schematic shows an “optional resistor” on the horn network. This is normally left as a straight wire connection, but if you find the top end a little too hot (and these speakers do have a very hot top end, which can be a problem in bright rooms), replacing it with a 10W 0.5 ohm or even a 1 ohm resistor will provide a bit of a treble cut. 

You can also biamp this system, and frankly I think it sounds a lot better in the midrange when biamped. Start out with a fourth-order active crossover with the turnover point around 1250 Hz. These drivers have some weirdness around the crossover region, so you may find slight changes in the exact crossover frequency to improve things depending on the room.

If you want to biamp, just skip the whole crossover and wire the four pins of the Speakon connector (see parts list). Do NOT omit the 5A fuse, though, and be sure to indicate on the back of the device that it is wired for biamping. 

Assembly

Assembling should be done with white glue or other modern synthetic glue. Don’t use hide glue, which is a lot more elastic and less brittle. That elasticity can be a good thing when building a chair that has to hold up with varying stresses, but it’s a bad thing when building a speaker cabinet that is supposed to be as rigid as possible. 

After the cabinet itself is assembled with all corner braces glued into place on all corners per the illustration, the countersunk holes should be plugged, the plugs sanded down, and the whole thing sanded, stained, and a final finish put on. I tend to think a more flexible finish has less of a sonic effect overall; while I like Tung oil finishes a lot, I went with a phenolic spar varnish because it looks a lot less ugly than urethane, holds up well, and seems to have minimal sonic effect.

Two coats with a little sanding with #220 sandpaper after each coat gives a clean but not glossy look and a nice smooth feel. In actuality the birch cabinet doesn’t look bad with a light color stain to it, though a darker stain really looks poor on birch. A lot of folks will prefer covering it with Tolex, paint, or speaker carpet.

A thick coat of matte finish latex paint does not seem to affect the sound particularly, although the speaker carpet may well affect it (possibly for the better) by reducing edge diffraction problems. If you use the carpet or thick Tolex you will want to use a different model corner, as the corners specified won’t fit well on a rounded carpet covered edge. 

The inside of the cabinet should be coated with a mixture of three parts roofing compound to two parts sand or cat litter. This gives you a nice elastic inside coat that will reduce cabinet body resonances. Wait at least a week after coating this stuff for it to out-gas and dry out. I know it’s difficult to wait, but you don’t want the solvents in the roofing compound to damage anything.

Dickason’s Loudspeaker Cookbook mentions that you can use automotive undercoating instead, but that it quo;s full of nasty solvents. He’s right, but as far as I can see the solvents aren’t any worse than those in the roofing compound, though it can take a bit longer to set. The real disadvantage of the undercoating is that it costs a lot more money and the spray can actually makes it harder to apply carefully. 

While waiting for this to set, though, put the driver into position in the 11.25" hole and drill all the mounting holes out with a 1/4" bit. Take the driver out, put it away, and then drill those holes out in size to 5/16", which should be just large enough for you to insert the T-nuts from the rear. Do so, then hammer them into place. 

I suggest putting speaker carpet or thick felt on the front because it seems to reduce diffraction problems on the top end a little bit. A little 3M #77 adhesive goes a long way; stretch the material out flat and cement it down, then trim the holes with a single-edged razor blade. 

The fancy venting gadget comes in three pieces, separated by attachment rings. Screw the front flange (the one with the screw holes) into the smaller hole in the front panel with #6 screws. Put a little silicone RTV on the back before screwing it in to seal it.

Cut the center pipe down to three inches in total length. It’s a huge long thing, and you only need a small piece. Attach the center pipe and the rear flange, cementing them into place with PVC pipe cement.

Screw the legs on with #8 woodscrews and use #6 screws to attach the handles. The mounting cup on the bottom should be secured with T-nuts and #10 machine screws. Place the mounting cup into position, hold it there with a clamp or adhesive, and drill all four holes out with a small drill bit.

Then remove the cup and drill the holes out completely with a 1/4" bit, and insert the T-nuts from the inside, screwing the base onto the bottom. Don’t put the rear dish on yet.

At some point you will want to line the entire inside of the cabinet with the 3" thick R-13 insulation. (If you can only get R-11 it will do, and if you can only get the kraft-paper faced material, just tear the paper off). Wear rubber gloves and long sleeve shirts when working with fiberglass or you’ll be itching for days afterward. 

I actually did the lining before finally bolting the things together on most of the units, but I tried installing the lining through the hole cut for the driver on one unit and it worked well. Line the sides thoroughly, the back as well as you can while still leaving access to the crossover and rear dish hole.

Don’t bother lining the front since so much of it is cut out already, but if you want to try to line the bottom section it will do some good. I used 3M Super 77 adhesive to make sure it stuck well, even though the roofing compound was pretty sticky itself.

As it comes, the rear dish has two holes in it, neither of which is large enough. You want to punch one of them out to 1" with a Greenlee punch to accept the Speakon connector.

If you’re using the large round Speakon connector instead of the small square one (the NL4MPR), file down the top and bottom of the flange on the connector from the rear so it will fit into the gentle slope of the dish. Drill 1/8" holes for the mounting screws using the connector as a guide, and drill the righthand hole out to size to fit the fuse holder that you are using. 

With the cabinet itself completed, the crossover is screwed into place. If you’re building your own crossover with a terminal board rather than using the commercial one, you’ll have two large chokes hanging off the board, which you should glue down to the cabinet with some silicone sealant.

Solder the input leads to the connector on the dish, then wire the fuse on the dish up between the horn driver and the LF- output of the crossover. If you use the premade Radian crossover, just solder 18 gauge leads to it. Put the female disconnects on the end of the tweeter leads, and just tin the end of the woofer leads. Screw the dish into the rear panel hole with #6 screws and try to keep the leads as short as possible. 

After the various adhesives have set (say a week after applying the roofing compound), put the driver in. You should already have the holes lined up and T-nuts in place. The LF- wire goes to the black terminal on the front of the driver, the LF+ one to the red one. The two female disconnects in back are the same, with the HF+ to the red terminal and the HF- to the unmarked one.

Note here that the ground lead is connected to HF+ and to LF- because the crossover phase shift is significant enough that the two drivers are almost 180° out of phase in the crossover region. We reverse the wiring to compensate for this.

With the driver installed, hook up a signal source and give it a try! You’ll find it takes a couple days of use for the bass response to settle down as the driver breaks in, but it is quite the fine speaker system.

After checking the thing out, put the grille on over the driver and screw it in with a #10 wood screw and a #10 flat washer in each corner. You will want to drill pilot holes.

The grille size should actually be around 11 7/8" inside, but I have found that ordering the grilles 12 1/4" square gives you enough additional space to compensate for the inevitable problems that seem to occur. It’s better to have a little extra. 

The last thing to do is add the corners. If you prefer you can use rubber feet as well, but frankly I find them more cumbersome and the plastic corners do an excellent job of protecting the monitor. 

Acknowledgements

Many thanks to Dr. Denis Orton for the use of his wood shop and skills, to Dick Pierce for putting up with silly questions, to Scott Norwood for putting holes in things, and to Boots Coleman and the crew for being willing to try out something new. Thanks also to Allen Gean at Yukon Lumber for general advice.

Scott Dorsey may be reached via talkback@recordingmag.com

Parts

Quantity / Item / Supplier and Part

1 Sheet 5X5 15 ply Birch Use local supplier

2 Handles TCH 500-9234800

1 1 3/8" Mount TCH 513-7013900

1 Recessed Dish TCH 514-496900

1 Round Speakon Jack TCH 513-8318855 (Neutrik NL4MPR)

1 Speakon plug (for cable) TCH 513-8322855

8 1/4-20 by 1.25" panhead bolts TCH 512-6024900

8 1/4-20 by 1/2" T-nuts TCH 512-6001800

4 10-32 by 1" panhead bolts TCH 512-6014900

4 10-32 by 1/2" T-nuts TCH 512-6002800

28 #8-1-1/2 flathead wood screws

8 Reinforced Corners TCH 502-1404900

1 12 1/4" x 12 1/4" grille w/ 3/4 lip 

TCH 513-7301900 1 Panel-mount Fuseholder Local Supplier

1 5A fuse Local Supplier

1 3" Tapered Port Madisound 3FLARE

1 15" roll R13 insulation, 40 sqft. Local Supplier (enough for 4 spkrs)

1 Radian 5012 Coaxial Driver Image Communications

You can use:

1 Radian Crossover 420-1250 Image Communications

Or buy:

Parts for Crossover:

2 10 uF Mylar film capacitors Madisound 10MFD

1 8 uF Mylar film capacitor Madisound

1 3 uF Mylar film capacitor Madisound 3MFDT

1 1.5 mH 16ga air-core choke Madisound SW1.5

1 0.1 mH 16ga air-core choke Madisound SW.1

1 15 ohm 10W sandcast resistor

1 military-style terminal board

18AWG wire, preferably Teflon

Suppliers:

TCH Cabinet Hardware 1-800-465-6281

Image Communications (Radian Dealer) 1-800-552-1639

Madisound Speaker Components 608-831-3433




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