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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.
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.
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:
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
