AX020_025.pdf

The Table Tuba

speakers

This author’s latest horn

design offers subwoofer

capability, but also fits

nicely in the living room.

By Bill Fitzmaurice

T his project is in response to

PHOTO 1: The Table Tuba

as a coffee table.

numerous requests for a Tuba

folded horn for stereo and home

theater capable of reaching down

to 30Hz or lower. A 30Hz horn is easy;

having space left for anything else in

the room isn’t. Then there’s the matter

of the WAF: Wife Acceptance Factor.

The Table Tuba ( Photo 1 ) gives you the

sub that you want, cleverly disguised as

a living room table to keep your wife

happy. Or at least a little less unhappy

when she demands to know why you

need to have that “great big speaker

thing” in her home.

very high and can nearly double the

driver impedance ( Fig. 1 ). If you want a

4Ω load, you must use a pair of 4Ω driv-

ers parallel wired, or a dual 4Ω coil driv-

er with the coils parallel wired. However,

it is so efficient that you don’t need to

worry about maximizing your amp’s out-

put. An amp rated 100W into 4Ω will

put at least 50W into 8, and with the

sensitivity of this speaker that’s plenty

for most home-theater setups ( Fig. 2 ).

CONSTRUCTION

The entire cabinet is constructed from

½ ˝ material, either plywood, MDF, or

particleboard. If you plan on a natural

FIGURE 1: Impedance chart.

FEATURES

This design is very driver tolerant. I test-

ed it with a $35 MCM 55-2421 eight

( www.mcminone ), as well as an Eminence

HL10a ten that goes for four times that

price, and there was very little difference

between them. Target T/S specs are fs

30Hz, Qts 0.35, Xmax 8mm or more, Bl

at least 12, Vas 30-60l. Most drivers with

specs reasonably close to these will work

fine in this box.

If you wish, you can widen the cabinet

more than the original 15.75 ˝ . I chose

the dimensions so that there would be

minimal waste when building from

standard 5 × 5′ Baltic birch plywood.

Widening the cabinet will increase the

mouth area for higher SPL and deeper

extension, but not by much; you’d get

only an additional 3dB by doubling it.

I’d only widen it if I wanted to be able

to fit dual drivers for higher power han-

dling or lower impedance.

The throat impedance of this box is

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FIGURE 2: SPL response.

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finish, Baltic birch is the better choice.

With a veneer finish softwood plywood

is quite adequate as a substrate. Heavier

materials are not necessary, as this box

simply does not vibrate. You really can

use it as a table, and I can assure that a

glass will not scurry across it during the

chase scenes of Terminator 3 . Whether

you use 5′ × 5′ or 4′ × 8′ materials, you’ll

need two sheets.

There are many jointing methods

possible—from dado to biscuit to glue

and screw. If you’re an expert wood-

worker, I’ll leave the decision on which

jointing method to you. If you aren’t, you

can build your Table Tuba the same way

I made the original: with a hammer and

nails.

I used 1 ⁵⁄₈ ˝ ribbed panel nails, which

are intended for putting up pre-finished

wall paneling. They aren’t very strong,

but they don’t need to be. The only pur-

pose of the nails is to hold the parts in

place while the adhesive that keeps the

cabinet together cures.

Because the joints in the cab must

be airtight, I highly recommended that

you use a polyurethane-based adhesive,

which expands when curing to fill any

gaps. I use “PL Pro” construction adhe-

sive, applied with a caulking gun, which

works great and is inexpensive. Urethane

woodworking glues are fine as well, but

can be very pricey.

Construction starts by cutting out

the two sides, both

measuring 29.75 ˝

square. While the

“sides” may techni-

cally end up being

the top and bottom

if you set the cabi-

net up as a low-

boy table, I’ll refer

to the two largest

pieces as the sides,

and the cutaways

as side views. Draw

the locations of all

the joints onto one

side, as shown in

Fig. 3 . These di-

mensions show the

actual clearances of

the horn pathway,

surface to surface.

First draw the joints with the top,

back, bottom, and front. Then draw the

internal joints, starting at the mouth

of the horn, working backwards to the

driver chamber.

Figure 4 shows the angles of the

various intersections and the numerical

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FIGURE 3: Parts dimensions.

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FIGURE 4: Parts angles.

FIGURE 5: Approximate panel lengths.

designations of the internal panels. You

don’t need to utilize these angles in the

layout, but notice that where the angle

is 0°, such as at the junction of plates 7

and 8, the two parts are at right angles

to each other. You will want to refer to

these angles when cutting the panels,

so write them on the side and they’ll be

there when you need them.

Figure 5 shows the nominal lengths

of the internal panels. These are ap-

proximate dimensions only and depend

on the actual thickness of the material

you’re building with. You must confirm

all these sizes by measuring and/or dead

reckoning during the construction pro-

cess.

After you’ve drawn the joints on one

side, put the two sides back to back and

clamp them together. Drill pilot holes

through the middle of each joint, using

a drill bit the same diameter or slight-

ly smaller than that of the nails you’re

using. Do not drill closer than 2 ˝ or so

from the end of the joint, with the holes

spaced 6–8 ˝ apart.

Drill the holes entirely through both

sides, even if you plan on using screws

instead of nails. Unclamp the two sides

and put aside the second. Using either a

circular saw or saber saw, cut out a driver

access hole in the side as framed by pan-

els 1-4.

The remaining parts are all cut to the

same 14.75 ˝ width. If you’re using a table

saw, it’s best to cut them all to width at

the same time so that the rip fence can

stay in the same place and all the parts

will be identically cut.

drive some of the bolts primarily by feel.

Draw two intersecting lines connect-

ing opposing boltholes to determine the

center of the driver and use a compass

to draw the appropriate size hole on the

baffle. Then cut this hole with a saber

saw. Drive the T-nuts in place with a

hammer, ensuring that they are located

on the bottom of the baffle. Trial-fit the

driver to the baffle to be sure that all the

nuts are properly installed, then remove

it and put it aside.

Attach the baffle to the side. The best

ASSEMBLY

Cut out panel 1, which is also the driv-

er mounting baffle. Draw a line across

the baffle 1½ ˝ from one end. Place the

driver on the baffle, centered with one

edge on the line. Mark the bolt locations

through the frame holes. Put the driver

aside. Drill the baffle for T-nuts for the

driver mounting bolts. I suggest you use

³⁄₁₆ × 32 Allen head bolts, as the clear-

ance inside the box is tight and you must

PHOTO 2: Attaching panel 1.

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way to do this is to clamp a 2 ˝ × 2 ˝ guide

board along the joint edge, apply ad-

hesive to the joint, clamp the panel to

the guideboard, and when you’re sure

all is right nail it in place ( Photo 2 ).

To prevent splitting, drill through the

pre-drilled pilot holes about ½ ˝ or so

into the panel. Use a nail set to drive the

head of the nail below the face of the

side. If you choose to use screws, follow

the same steps, using a pilot/countersink

bit to prepare the hole before driving the

screw.

Double-check the length and cut

angles against the cabinet and cut out

panel two. Attach it to the side using

the same guideboard technique ( Photo

3 ). Use a clamp at the end of the joint

of the two panels and a scrap piece of

plywood to hold the parts together while

you first drill pilots and then drive two

or three nails or screws along the joint,

checking the alignment with a fram-

ing square to be sure that everything is

square. Using the same technique con-

tinue with panels 3, 4, 5, and 6, checking

alignment with a framing square as you

add each new panel, holding things to-

gether with clamps until after you have

driven the nails/screws ( Photo 4 ).

Starting with panel 7, bracing is added

between the panels ( Fig. 6 ). Note that

the braces don’t touch each other and

don’t need to extend fully across the en-

tire length of the panels. The braces are

spaced midway across the cabinet. You

figure their dimensions by dead reckon-

ing ( Photo 5 ). Place a piece of plywood

PHOTO 3: Attaching panel 2.

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PHOTO 4: Use clamps and a

framing square for perfect

alignment.

of sufficient size against the inner panel

to which it attaches, in this case panel 3.

Then set a straightedge across it where

the next panel will joint to it, in this case

panel 7, and draw the cut line.

After sawing, lay the brace in place

again to check the accuracy of its siz-

ing. Also draw on the panel where the

installed brace will join to it. After cut-

ting the brace that spans panels 3 and 7,

install panel 7. Liberally apply adhesive

to the edges of the brace and slide it

into place, pushing it far enough to just

be snug. Secure it in place with nails or

screws driven through panel 7.

Continue the assembly process. When

installing the braces joined to the front,

top, and rear panels, use a framing square

to check their alignment, pushing the

brace in only as far as required to square

the panels ( Photo 6 ). When you install

the cabinet bottom, do not install the

brace between it and panel 9.

After installing the bottom, lay the

cabinet on its side and apply a generous

bead of adhesive on all the panel edges.

Put the second side atop the assembly,

making sure that it is oriented so that

the pilot holes line up with the panels.

Nail or screw it into place. If required,

use a long pipe clamp to pull it into

alignment with the rest of the cabinet.

After installing the side, go back and

install the brace that goes between panel

9 and the bottom. Use a router with a

pattern following bit or a sander to make

flush the outer joints and those of the

panels surrounding the access hole. Fill

the holes over the nail or screw heads

before sanding and finishing the box.

After your finish is complete, install your

driver.

PHOTO 5: Dead-reckoning a

brace.

FIGURE 6: Braces (shaded).

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PHOTO 6: Keep it square.

SETUP

If you’re using a ten, things are rather

tight, so tight that reaching the far lower

corner bolt isn’t easy. Install that bolt

first ( Photo 7 ), with the driver offset

so that you can reach it, and then swing

the driver into place. Don’t fully tighten

any of the bolts until all are installed,

then go around the frame easing them

in until all are firmly set. This is a job

made much easier with a small ratchet

screwdriver.

Eight-inch drivers, on the other hand,

have room to spare, even when mount-

ed on a plywood reducer to fit over a

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