Georgia Tech Audio Laboratory Loudspeaker System

W. Marshall Leach, Jr., Professor
Georgia Institute of Technology
School of Electrical and Computer Engineering
Atlanta, Georgia 30332-0250


These notes describe the construction of a vented-box loudspeaker system which has been developed over a period of several years in the Georgia Tech Audio Laboratory. Many students have participated in the design. Each time a system was tested in the lab, changes were made when required to improve the performance. In this way, the final system design has evolved. The system uses an enclosure with an internal volume of 2 cubic feet with inside dimension ratios of 0.6 by 1.0 by 1.6. These ratios minimize the effects of standing waves inside the box for better frequency response. The bass frequency response is characterized by a fourth-order high-pass transfer function which has a lower half-power cutoff frequency in the 30 to 40 Hz range and an electroacoustic efficiency of about 0.8%. The efficiency is quite high for home systems so that a high-power amplifier is not required. An amplifier rated at about 30 watts to no more than 100 watts per channel is adequate.

You can look at pictures that documents Elliott Jackson building his speakers here.

Source of Wood Materials

A woodworking shop that can supply the materials and give assistance in cutting and/or assembling the boxes is Wood Wonders, 1200 Foster St., NW, Atlanta, (355-4778). They are skilled at furniture grade construction and do high quality work. They have the speaker plans on hand and can supply the cut panels or assembled boxes made of pre-veneered plywood. I recommend them. Wood Wonders will not cut the holes for the drivers in the front panels and the holes for the terminal cups in the rear panels unless you mark off the holes for them.

Order of Construction

The box construction is illustrated in the assembly diagram. The boxes are made of 3/4" industrial grade plywood, fiber board, or particle board. Plywood is recommended. The boxes are braced internally with "two-by-two" fir or pine strips having the cross-section dimensions of 1.5" by 1.5". If hardwood or pre-veneered plywood is used, the cutting plans must be modified so that the joints between the tops and sides can be miter-cut at a 45 degree angle with a table or radial-arm saw, and the screws must be installed from inside the box (see end of plans). David Bramlett has built a beautiful set of speakers using this procedure. The cutting diagram which he included with his report can be seen here. The approximate order of assembly is as follows:

  1. Cut panels and bracing.
  2. Cut holes in front panel for drivers and port.
  3. Cut hole in rear panel for electrical terminals.
  4. Assemble top, bottom, sides, and internal bracing.
  5. Install front panel on box.
  6. Caulk all inside joints in box.
  7. Drill pilot holes for rear panel screws and install weather stripping.
  8. Install drivers and port on front panel and electrical terminals on rear panel.
  9. Install crossover network on bottom panel.
  10. Wire crossover network to drivers.
  11. Staple fiberglass to interior walls.
  12. Wire input terminals to crossover network and install rear panel.
  13. Finish box exterior and install grill.

A Recommended Method of Assembling the Box

The bracing for the front panel is recessed 3/4". The bracing for the rear panel is recessed 13/16". The extra 1/16" allows for the thickness of the weather-stripping seal for the rear panel. A simple procedure for assembling the top, bottom, and two sides is described below. The procedure assumes that the screws are installed from outside the box.

  1. Use 2 corner clamps (Craftsman 6666) to clamp the bottom and one side together at a 90 degree angle.
  2. Apply wood glue to the inside joint.
  3. Place one corner bracing strip in the joint. The two surfaces of the brace which contact the box panels should be coated with wood glue.
  4. Use two 3" C-clamps (Craftsman 66673) to clamp each end of the corner bracing strip to the bottom panel.
  5. Mark and drill three pilot holes through the side panel and into the corner bracing strip for three wood screws. A wood screw pilot bit for the particular size of screw to be used is recommended.
  6. Install screws into the three pilot holes. A drill-powered screwdriver or an automatic return screwdriver is recommended.
  7. Remove the two C-clamps and tighten the screws.
  8. Mark and drill two pilot holes through the bottom panel and into the corner bracing strip for two wood screws. The holes should be centered between the screws installed in the side panel.
  9. Install the screws into the bottom panel.
  10. Remove the two corner clamps and repeat the procedure until the top, bottom, and both sides are assembled. It is not necessary to wait for the glue to dry, for the screws will hold the joints together.
  11. Install the eight remaining bracing strips. Use the C-clamps to hold these for installation. Use wood glue between the bracing and panels. Use enough screws to ensure that the bracing fits flat against the panels. The screw holes can be filled with Elmer's Wood Putty.

The Front and Back Panels

The drivers mount to the front panel from the outside of the box. Do not mount the drivers from the inside of the box. Circular holes must be cut into the front panel for the three drivers and the port. In addition, the appropriate hole for the speaker input terminals must be cut into the back panel. A saber saw can be used to cut the holes. The circular holes may be easier to cut with a hole saw or with a circle cutter in a drill press. The drivers should be placed close together (about one panel thickness apart) on a vertical line with the tweeter at the top and the woofer at the bottom. The port or vent tube should be close to the woofer and away from the side panels. The port has an internal diameter of 3" and a length of about 4". It mounts flush with the outside edge of the front panel. If desired, a router can be used to recess the drivers so that they mount flush with the front panel. The drivers should be secured to the front panel with machine screws, flat washers, and nuts.

Caulking the Box

After the box (minus the rear panel) has been assembled, seal all inside joints with a permanent caulk. Clear silicone rubber caulk is the best, but it is also the most expensive. One 10 oz. tube (the type which requires a caulking gun) should be more than enough. Paintable silicone rubber caulk is not recommended. The inside joint between the front panel and the port should also be caulked.

The Crossover Network

The crossover network is installed on the inside bottom panel of the box. Several solder-lug terminal strips screwed the panel can be used for binding posts to wire the crossover network. The crossover network elements should be secured to the bottom panel with glue or caulk. Alternately, the crossover network can be assembled on "perf-board" or a similar product. Cable ties can be used to secure the components to the perf-board. To prevent magnetic coupling between inductors, do not mount any inductor on top of any other inductor. The Audio Lab of Georgia sells a printed circuit board which can be used for the crossover network.

All electrical connections should be soldered with rosin core solder only. Any corroded terminals should be cleaned with steel wool or a wire brush before soldering to them. #18 or #20 stranded wire is recommended for all wiring. Do not use single strand wire. Particular attention should be paid to the polarity of each driver connection to the crossover network. The positive reference input terminal to each driver is indicated by a red dot or plus sign. This terminal must be connected as specified on the circuit diagram for the crossover network. Do not overheat the tweeter terminals, for the internal connections can break. The inductors in the crossover network are wound of wire which has a reddish-brown shellac insulation. This must be scraped off with a knife before soldering.

Fiberglass Lining

All inside surfaces of the box except the front panel should be lined with 1" thick fiberglass. This can be purchased from Radio Shack (Cat. No. 42-1082). The fiberglass should be stapled to the box walls with a staple gun. To prevent the staples from cutting through the fiberglass, use a small rectangular strip cut from an index card and folded double between each staple and the fiberglass. Any scraps of fiberglass that are left over can be stapled to the from panel over the mid range and the tweeter. It should not cover the woofer or the port opening. If construction type fiberglass is used, the paper backing should be peeled off.

Weather-Strip Seal for Rear Panel

In order to provide access to the inside of the box in case repairs are needed, the rear panel is not glued in place. To ensure an air tight seal when the rear panel is screwed to the bracing, foam weather stripping is used between the rear panel and the bracing. The recommended weather stripping is 3/16" thick by 1-1/4" wide self-adhesive, closed-cell vinyl foam weatherstrip tape made by Macklanburg Duncan Co., Dennis, or ACE Hardware. (The ACE part number is 85017.) The bracing for the rear panel must be set into the box by an amount equal to the thickness of the panel (3/4") plus the thickness of the compressed foam tape (about 1/16"). The length of the front to rear corner bracing strips specified in the Wood Parts List has been reduced by 1/16" to allow for the thickness of the weather-stripping. When the screws for the rear panel are installed, they should be tightened just enough to compress the weather-stripping by 1/8" so that the rear panel will fit flush.

Electrical Input Terminals

The electrical input terminals on the rear panel can be either 5-way binding posts or those specifically designed for loudspeakers. If 5-way binding posts are used, they must be recessed. Otherwise, they are subject to breaking off if the loudspeakers are ever transported. Cheap 5-way binding posts such as those sold by Radio Shack break off easily. The better ones are made by the H. H. Smith Co. (model 899). Do not buy the gold plated ones, for they are expensive. A recessed "cup" type terminal with two integral 5-way binding posts that is specifically designed for loudspeakers is recommended.

Considerations for the Grill

The box design that is given has the front panel flush with the front edges of the top, bottom, and side panels. When the front grill frame is installed, it will cover these edges. A grill frame can be made with rectangular molding strips. The outer edges can be faced with quarter-round moulding to give a rounded edge if desired. The best grill material is black single-knit fabric that can be purchased at most cloth stores. This can be stretched around the grill frame and stapled to the back. The grill frame can be installed on the front panel with standard grill mounting hardware. Alternately, 4 small blocks of wood can be attached to the front panel over which the grill frame fits. An alternate box design would have the front panel and grill recessed so that the front edges of the top, bottom, and side panels are exposed. This design requires that these edges be finished with veneer tape when the final finish is applied. If this design is preferred, it will be necessary to increase the appropriate dimension of the top, bottom, and side panels so that the internal dimensions of the box will not be changed.

Drivers and Crossovers for a Stereo Pair

The woofer can be purchased at the Audio Lab of Georgia, 3611 Clearview Place, Atlanta, GA 30340 (770-455-0571). All other components may be purchased from the Audio Lab of Georgia or from Madisound Speaker Components, 8982 Table Bluff Rd., Madison, Wis. 53711 (608-831-3433). Madisound will accept credit card charge orders by telephone with a delivery time of about one week. A second supplier for all components other than the woofer is A&S Speakers, 3170 23rd St., San Francisco, Calif. 94110 (415-641-4573). Mylar capacitors are recommended if they can be found and/or afforded. To prevent breaking the leads of the capacitors, do not bend or flex the leads up close to the capacitor body, for they can break off and cannot be repaired. If necessary, capacitors can be paralleled to achieve the value specified. The better inductors are wound with a higher gauge wire and exhibit less series resistance, but they are more expensive. Do not substitute components.

List of Electrical Components

Do not use any substitute drivers or crossover network components. If it is not specified on this page, do not use it. Do not use the alternate midrange that Audio Labs of Georgia has listed for this system. Theirs is not the correct midrange. The correct midrange can be obtained from MadiSound.

The printed circuit board for the crossover network is Audio Lab of Georgia part number PCB133. These circuit boards have holes drilled for all leads except for the 15 ohm and 20 ohm resistors. These components can easily be added to the boards. The inductors should be tied down to the circuit boards. Plastic cable ties are best for this. It may be necessary to drill holes in the center of each inductor for the cable ties. Do not tie the inductors down with uninsulated wire.

An alternate tweeter is the Philips AD0163/T8. The only difference in this tweeter and the specified one is the case is round instead of square. An alternate midrange is the Philips AD50600/Sq8. This midrange does not give as good a frequency response as the Peerless unit, but it may be used if the Peerless unit is unavailable. The Peerless midrange used to have the model number K040MRF/8.

Wood Parts List for Two Enclosures

All enclosure panels are cut from a single sheet of 4' by 8' by 3/4" industrial grade plywood, fiber board, or particle board. Because it is not so heavy, the plywood is recommended.

The dimensions of the front and back panels have been reduced by 1/16" from the exact calculated value to ensure proper fit without force.

The following internal bracing strips should be cut from fir or pine "two-by-two" bracing. The cross section dimensions are 1.5" by 1.5". Do not use hardwood bracing for it is too difficult to screw into. The front panel bracing is recessed by 3/4". The back panel bracing is recessed by 13/16". The extra 1/16" at the back panel allows for the thickness of the weather-stripping seal. The lengths of the top-to-bottom and left-to-right bracing strips have been reduced by 1/16" to ensure proper fit without force. Use caulk to seal any gaps between the bracing.

The rectangular bases for the boxes are cut from "one-by-two" bracing which measures 3/4" thick by 1-1/2" wide. See the assembly diagram.

Miscellaneous Parts and Supplies

Wood glue. Wood screws (2" dry wall screws are recommended). Clear silicone rubber caulk in 10 oz tubes. Caulking gun. Unfaced fiberglass, 1" thick. #18 or #20 stranded hookup wire. (Don't buy Radio Shack hookup wire. Some of it does not solder properly, i.e. it does not make good electrical connection.) Solder-lug terminal strips for crossover network binding posts. Electrical binding posts for rear panel input terminals. Machine screws, flat washers, and nuts for mounting drivers. Two 3" internal diameter PVC tubes cut 4" long for the ports. (Audio Lab has the correct size ports.) Grill frame material (rectangular or quarter-round moulding painted black). Grill cloth material (black single knit fabric).

Alternate Box Plans

If veneered plywood is used for the enclosures, the panels should be miter cut at a 45 degree angle so that they fit together without any edges showing on the front, top, sides, and bottom. The panel edges then show only on the backs. (Wood Wonders can cut the boxes in this way. In addition, they will assemble the sides, top, bottom, and front of each box. However, they do not install the bracing.) The screws which are used to install the bracing should be put in from the inside of the box. Pilot holes for the screws should be drilled in the bracing before it is installed. With 1.5" by 1.5" bracing and a 3/4" panel thickness, 2" dry-wall screws are recommended. These are available at hardware stores. Do not forget to use wood glue between the bracing and the box panels.

Crossover Network

Cutting Diagram for Box

Assembly Diagram for Box

This page is not a publication of the Georgia Institute of Technology and the Georgia Institute of Technology has not edited or examined the content. The author of this page is solely responsible for the content.

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