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Article

Original Article for the American Woodworker
"SPECTACULAR SPEAKERS"
BUILDING SPEAKERS

by
Bob Wayland

Kit information

A good friend had just made a monitor speaker for his band and the thing really rocked and rolled with the most disturbing distortion. A cursory glance identified the all too-common problem of putting just any old driver (speaker) in any old enclosure and expecting it to work. You cannot do that (not if you want a quality system)! You must carefully match the drivers to the enclosure. Let me introduce speaker building in this article, illustrated by a high end system worth about $3000.

"...A cursory glance identified the all too-common problem of putting just any old driver (speaker) in any old enclosure and expecting it to work. You cannot do that (not if you want a quality system)!.."

The systems described were designed especially for this article by Nick Billeci and Brian Smith of A&S Speakers. (They are now out of business) with revisions by Frank Guerrero of Speaker City USA. They have taken the modern approach of using a two-stage three way crossover network. Just as we must match the drivers to the enclosure, so must the crossover be carefully designed to provide optimal performance of the driver/enclosure system. The signal from your amplifier is fed into the subwoofer, and the lower frequencies, usually those below about 100 Hz., to the subwoofer, and the higher frequencies to a separate speaker system, the satellite. In the satellite system, the frequencies are further divided, with the midrange (roughly those above approximately 100Hz. to many hundreds or a few thousand Hz.) to the mid-range driver. The remaining higher frequencies are sent to the tweeter. (There is a good introduction to crossover design in Ray Alden’s "Advanced Speaker Systems", available at Radio Shack. The best software design program, but also very expensive, available from LinearX Systems, 1556 S.W. Bridgeport Rd., Portland, OR 97224). In this way, the satellite can be used by itself, but with rather limited bass response. Then the subwoofer can be added later; you can build your system in pieces as your budget allows. Speaker City USA, 115 S. Victory Blvd., Burbank CA 91502 (818-846-9921) has agreed to make available a package that includes all necessary driver, assembled crossover and components at a special price for a high end system (about $425).

"...The dampened wall school suggests that a material be chosen which will have the least audible effect, and using braces to dampen wall vibrations. This currently means using thick (1 inch or thicker) medium density fiberboard (MDF)..."


Figure 1

There are two basic types of speaker enclosures: a sealed enclosure or a vented enclosure. The design of a sealed enclosure has been understood for a long time. In the early 1970’s, A.N. Thiele and R. Small developed mathematical techniques for designing vented enclosures that revolutionized the speaker building world. (In addition to these basic types, there are many alternate enclosure designs that you can check out in the reference given below.) Each drivers require a box that matches it electromechanical properties. I would suggest that you check out Vance Dickeson’s The Loudspeaker Design Cookbook for a in-depth overview (available from Old Colony Sound Laboratory, P.O.Box 243, Peterborough, NH 03458-0243). A good introduction can be found in Ray


Figure 2

Alden’s "Advance Speaker Systems", Master Publishing, Inc. There are also excellent computer design programs. The Cadillac of all computer programs is, again, Leap. A reasonably priced ($80) package is Top Box, available from Speaker City USA or ORCA Design, 1531 Lookout Dr., Agoura, CA 91301. Top Box is available in either IBM-compatible PC’s or Macintosh computers. The design has consequences that directly affect how you build your system, first seen at the elementary level of what material you do use. The choice of building material is centered upon the requirement that the enclosure should not itself vibrate, causing a delayed audible vibration. There are two current philosophies in building speaker enclosures, the sandwich and the dampened wall. In the sandwich school you try to make the panels as light as possible while keeping them very rigid. This is done by a composite of two walls with a dampening material between the walls. The dampened wall school suggests that a material be chosen which will have the least audible effect, and using braces to dampen wall vibrations. This currently means using thick (1 inch or thicker) medium density fiberboard (MDF), as well as carefully placed braces, and filling the enclosure volume with long fiber wool or fiberglass to absorb unwanted acoustic energy. The enclosure in this article follows the dampened wall school. I made the top, bottom and sides out of 5/4" black walnut, and the front, the baffle, and the back out of 1 1/8 inch MDF (See figures 1 and 2). The walnut is ancient, cut over 60 years ago.

 


Figure A

The system presented here is based upon the ideal of being as flexible as possible. By extending the frequency range covered by the satellite to below 100Hz., (there is useable response to 40Hz.) you can build a very good system that can be greatly improved by the addition of the subwoofer system (Fig. 2). The satellite shown here has a number of subtle features that present some interesting woodworking challenges. The use for a slot for venting, as compared to the more usual tube design, allows the vent not only to tune the enclosure, but also acts as a brace for the backs and sides of the enclosure. (See Fig. A in AW) This helps to control the vibrations of the sides and back panels. The dimensions of your enclosure can have a major influence on the quality of sound. Another consideration is the effect of diffraction when a sound wave encounters a sharp edge. For this reason, drivers are mounted flush to the front baffle, and if the baffle is thick, you should run over the inside edge of the driver mounting hole (Fig.C/AW). Corner braces, set at 45 degrees (see Fig B/AW), taking away the sharp interior corners, also increase the quality of sound. If the interior of the enclosure is filled (stuffed) with long-fiber wool or fiberglass, the internal vibrations will be dampened. This has the effect of increasing the apparent volume as much as 15% to 25%. The rigid amount and the specific packing of the dampening material are a art form. You can completely stuff the enclosure at a density of at least lb./cubic foot. However, be very careful not to overstuff. There is a interesting discussion on stuffing in the Loudspeaker Design Cookbook.


Figures B & C

Note the close spacing on the biscuits. This is very important to have airtight joints. Many of the joints that we normally use, i.e., dovetails, are not suitable unless you take great care that you insure all joint contact areas have more-than-adequate supply of glue (yes, messy but necessary). Of course, you should insure the integrity of the joints either with corner braces or by caulking the interior edges of the joints.

We changed the design of the enclosure for the AW article; I’m including my original design for variety. The bottom section of the back panel also houses the input terminal. At the midpoint, drill two 1-inch side-by side holes and remove the humps on the sides to make a smooth oblong opening. Then using a 3/4-inch roundover bit set to cut a 1/16-inch lip, route out the opening as shown in figure 3.


Figure 3

The first step is to assemble the port sides to the MDF of the upper and lower segments of the back panel (Photo 4/AW). MDF is mean stuff, so make sure you have a dust collector on your router. Once the excess glue is removed and you have sanded at least through 400 grit, it is a good idea to paint the inside surfaces of the slot. The normal practice is to use flat black, but you can jazz it up if you wish. (One other color combination that is very effective is a deep cobalt blue with walnut).

Begin assembling the enclosure with the front panel. For this enclosure, I have chosen to use biscuits, but other joints will work, i.e., spline joints. The joint pressure should be about 250-300 psi with a good PVA glue. If we use two or three I-beam clamps tightened to moderately high pressure, we should have a good joint. The top and the bottom are glued first; the side piece is not glued but held in place to insure the top and bottom are perpendicular to the front. The front edge of the top and bottom have been rounded before gluing up because of the side profile of this design would not allow them to be routed after


Figure 4

assembly. Next, glue on one side. The details for the biscuit arrangement are shown in Fig A/AW. Be sure you have cut all the biscuits’ slots that will be needed. Also, if you are going to mount the crossover to the side wall, now is a good time to drill pilot holes for the mounting screws.

It is a good idea to make up a piece that is the thickness of the slot. Then when assembling the back onto the glued-up side, you have a check to accurately establish the slot and provide a support when clamping across the slot (Photo 4/AW). Use a large deep-throated bar clamp to clamp the port sides. After the braces have set, you are ready to attach the remaining side. This is when many things come together simultaneously, making a dry clamping test especially important.

The closed box is now ready for cutting the driver openings. Before we can start the process of making the openings, it is necessary to have the back and front faces flat and smooth. Now is a good time to round over edges and form the detail of the upper and lower edges before we cut out the holes for the drivers; see Fig 4. For this edge treatment, it is easier if you have at least made the inside shaping before assembly. The final shaping of the edge at the corner on the sides is done by hand.


Figure 5

The driver holes for the front panel can be made with a template or jig (See Figure 5 and Photo 2/AW). The outer ring is cut first. I usually drill the pilots for the driver holes in the front panel before starting the gluing. After painting the routed rings, put a bead of silicone on the offset ring and let it dry. Using the wiring diagram supplied with the speaker system as a guide, attach wires of sufficient length to allow connection to the input terminal posts. The crossover is attached to one side panel with double-sided foam mounting pads, such as those used in mounting pictures to a wall and then is anchored with wood screws. The wires that go to the tweeters and mid-range drivers should be long enough to allow attachment to the drivers outside the enclosure. You will need to cut a notch in the through hole for the tweeter for its terminals. Mount the drivers.


Figure 6

The audiophile will tell you not to put a grill over the face of the enclosure. Nevertheless, for appearance and for casual listening, most of us want a grill over the speaker drivers, but the grill should be removed for serious listening. Normally the grill frame is made from particleboard because of its stability and strength. For our satellite use 1/2-inch particleboard or plywood cut to the shape shown in Figure 6. The edges can be left square; however, this makes for a very hard edge and a rather boxy appearance. I suggest that you either cut a 45 degree chamfer or a roundover for the outside edge. You also don’t want a sharp edge on the interior edges so they should also be rounded. Be careful, because the bottom edge is very close to the bottom of the mid-range cutout on the grill frame. The details for the friction fastener female sockets are determined using dowel points in the grill fastener holes. After you have drilled the holes for the friction grill fasteners, paint the grill frame the same color as the grill cloth. After installing the fasteners, the grill frame must be covered. Any loosely woven thin fabric will work, but you can purchase, from Speaker City, a special black cloth designed for use on speakers. The cloth can be attached with a good adhesive such as a good spray-on, or with my favorite, Goop.

Subwoofers

The subwoofer offers an entirely different approach to building speakers boxes. The top and sides are made from 1" MDF with massive 1 3/4-inch square edge braces. The design is for a down firing driver in a vented enclosure with a double-layer driver panel of two 1" MDF panels, as shown in figures 8 and 9. The MDF top is recessed to accept a slate top. The box is supported on special vibration isolation feet to insure a minimum coupling to the floor of your listening room.

After cutting two bottom panels, glue them together with a construction adhesive, being certain that there are no trapped air pockets. The easy way to make the mounting hole for the driver is to route a circular hole 1-inch smaller than the outside diameter of the driver. Then with a 1/2-inch rabbiting bit, rout the mounting edge deep enough to flush mount the woofer driver. [Watch out; this is MDF -- use good dust collection!] The inside of the woofer hole should be smoothed with a 3/4-inch roundover bit. Use the layout diagram of Figure 10 to locate the terminal box cutout and the vent hole. The terminal box should fit very snugly in its cutout and should be sealed with wood epoxy. To help reduce turbulence, the edge of the port tube to the outside of the box should be rounded over with a 3/4-inch roundover bit. The vent hole is actually two-stepped as shown in Figure 11. The tuning vent is a 4-inch section 2-inch ID black PVC pipe glued in place with polyurethane glue.

"...The dampened wall school suggests that a material be chosen which will have the least audible effect, and using braces to dampen wall vibrations. This currently means using thick (1 inch or thicker) medium density fiberboard (MDF)..."

It is simpler to glue the top and bottom 1 3/4 X 1 3/4-inch braces to the sides now. The top brace is chamfered as shown in Figure 9. When you cut the biscuit slots for the braces, put a 3/64" spacer under the biscuit cutter. (Of course, you can use another type of joint, e.g., a spline.) The sides are veneered 1 inch MDF and the matching slots are cut without the spacer. This creates a interesting outline with the braces acting as a frame, as well as protection for the veneered edges. The gap fillers should be glued in at this time (see Fig. 9). The corners posts are attached to the front and back panels. The 45 degree chamfers shown in Figure 12 are only cut in the post were they are within the enclosure volume (you can also cut with a chisel). Each end of the chamfer is tapered with a 45 degree bevel to match the other braces. By using post with this chamfer we have created a solid corner with a built-in brace. Be sure to cut the biscuit slots in the ends of the side panels. The top of the corner post extend 1/8" above the braces. A chamfer eases the extension (see Figure 12). After cutting the biscuit slots on both sides of the posts, glue them to the front and back panels.

You are now ready to start the main assembly of the box. The side panels are attached to the double-layered bottom and paralleled layer of biscuits. This helps to reinforce the dampening of the driver panel. The biscuits should be rather close together and the hole joint supplied with plenty of glue. Both the top and bottom are glued in place with the sides. Note that the two cross braces are also glued in at this time. Their locations are shown in Figures 13 and 14. These braces are to break up the vibrational modes of the panels. They should be placed in non-symmetrical positions to insure unequal sizes of free areas in each panel. This helps minimize same frequency vibrations. The braces are 1 1/4 inch hardwood dowels that are set 1/4 inch into each panel. Again, polyurethane glue is a good choice for anchoring the braces. The brace lengths are given in Table 1.

Table 1: BRACE LENGTHS
TOP/BOTTOM 1 1/4" X 8 1/2"
SIDE TO SIDE 1 1/4" X 21 5/32"
FRONT TO BACK 1 1/4" X 13 3/16

Next, glue the back onto this structure, with the brace in place.

Now is a good time to install the crossover network. Mount the terminal box with the posts pointed toward the front of the box. You can wire the terminal box to the crossover either before you mount them or after mounting the box and crossover. The network is mounted in the same way as described above for the satellite. Wire the network to the input to the set of terminals closest to the front. Be sure the leads going to the subwoofer are long enough to allow attachment to the unmounted driver. Completely seal around the terminal with wood epoxy. Before the front is attached, seal all interior joints with silicone or caulking. You are now ready to close the box by gluing the front on.

The speaker panel, the bottom, should be sanded at least through 400. After masking off the exposed walnut, paint the bottom; I usually apply three or more coats of epoxy enamel. The box should be stuffed with about one pound of long fiber wool. Keep the packing uniform except directly behind the driver and in front of the vent tube, where it should be a little less dense. Attach the leads from the crossover to the subwoofer driver, run a bead of silicone in the cutout for the driver and after the silicone has dried, mount the driver, you won’t have to have a grille. Personally, I feel safer with a grill. Speaker City sells a nice one.

The last thing you need to do before finishing the subwoofers is to attach the special vibration mounts. These are sold by Edmund Scientific (609-573-6250); 101 E. Gloucester Pk., Barrington, NJ 08007-1380). The Sorbothane mounts (part # C35, 264) cost $14.50 for four. The mount require an extension to the legs of the subwoofer cabinet as shown in Figure 15. To make the extensions, cut from scrap 8/4 walnut, disks the same diameter as the mounts. Place the disks on the legs in the position shown in Figure 15 and mark will you will cut them off. Note where there is a overlap of the sectors, so you will need to rotate the disks a bit for each marking. Cut the sectors off, sand flat the mounting side and glue in place, as shown in Figure 15.

I have found that polyurethane wood glue is the best way to attach the mounts to the bottom of the legs.

Now you are ready to finish the subwoofers. I would suggest that you choose a finish that provides good sealing. It is critical that the solid wood retain its integrity; no cracking is allowed. There are two final touches needed to complete the subwoofers. The first is the top. The plan presented here is for you to have any tile that fits your decorating scheme cut to fit the recessed area on top. Let the enclosure come to equilibrium with its environment and then grout the tiles in place. The second is to label the inputs at the terminal boxes.

You will have a excellent system if you follow these instructions. They are, of course, applicable to all speaker systems. One last point: move the speakers around in your listening area until they sound the best for you. I have found for this system that the satellites and subwoofers for each channel should be kept together. This is a personal choice and you might prefer them separated. The satellites should be at least at ear level when you are seated. Moving the subs close to the wall will increase the bass response.

 

Glossary
Crossover --
an electronic device that sends the right frequencies from your amplifier to the right drivers.
Driver -- the electromechanical device that most of us call a speaker.
Subwoofer -- the very low frequency driver, usually the largest driver.
Tweeter -- the driver for the highest frequencies: usually does not need an enclosure.

Bob Wayland Copyrights James Robert Wayland, 1996
500 Oakwood Pl. NE First Serial Rights
Albuquerque, NM 87123

 




    
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