Chapter
Five

Plain
and Fancy Soundboards

Along with the string set, the material
and construction of the soundboard and string ribs are the primary determinants
of a harp’s sound.  I have
used a variety of materials for soundboards including Sitka spruce, aircraft birch plywood,
redwood, Western Red Cedar, and Douglas fir.   Debates have raged among builders
on the relative merits of solid boards vs. plywood and on the merits of various
species of wood.

Common Sound Board Woods

Density

Stiffness

Hardness

Strength

Douglas Fir

0.48

1.95

710

12,400

Sitka Spruce

0.4

1.57

510

10,200

Spanish Cedar

0.4

1.44

600

11,500

OG Redwood

0.4

1.34

480

10,000

Western Red Cedar

0.32

1.11

350

7,500

Eastern White Pine

0.35

1.24

380

8,600

Density = Specific Gravity

Stiffness = Modulus of elasticity,
Million lbf/square inch

Hardness = Side Hardness, lbf

Strength = Rupture Bending Strength,
psi

Sourced from Chapter Four of the US Forest Products Laboratory Wood Handbook
and http://woodworkerssource.net/onlinewoods/

ß Above – The back side of a Redwood
soundboard.  Below - A soundboard
veneered with a striking curly cherry veneer.

With some experimentation and
adjustments, each of these materials can be used to make a good sounding
harp.   Most of my clients
specify a traditional Sitka
spruce boards or a composite board of Sitka Spruce and Western Red Cedar.   Being able to source wood that is
strong and stiff is important, but developing a good understanding of how
different species can be applied to the specific structural and acoustic
demands of a specific design is equally important too.

ß
Proper grain orientation for a ply board - the outer plies should run across
the width of the sound box

Plywood boards

Many first time builders start using
thin birch (3-5mm aircraft) plywood for sound boards.   There are nice sounding 4 octave
harps that use a plywood board, but the design demands low tension or a
carefully sized string rib that can take a fair amount of the tension from the
bass strings.

When I tried plywood on one of my early
harps, I eventually had to replace the board because the grain was not oriented
correctly.   I have also
replaced soundboards on two other harps (different makers) for the same
reason.   I have had to repair
L&H troubadours because of the same plywood board grain orientation
problems.

If you are going to use three-ply 3mm
birch plywood for your soundboards, orient the grain across the width of the
sound box.    If you run
the grain down the length of the harp, that thin, 1mm center ply ends up having
to bear practically all of the string tension and will eventually fail, usually
by separating from the bottom ply where it is glued down to the side of the
sound box.

Making a Solid Sound Board

When
I started to build harps, I worried too much about the time/effort and specialized
equipment a solid board would require.
After I made my first few solid boards, I was surprised at how easy it
was.   I am convinced that
making a soundboard from solid wood is something every harp builder should try
early on.

If
you have trouble finding suitable wood for soundboards, see Sourcing woods
for soundboards in the appendix.

Photo – The bass end
of a soundboard with the planks glued up and wedged in compression.  You can see the frame, ring clamps and
wedges.  The frame is propped above
the workbench with 2/4’s.
The hand spring clamp on the end of the frame board holds a small piece
of scrap wood to keep the wedges from falling through the frame

Build the Frame

I
glue and shape the soundboard on a simple trapezoidal frame.  The runners of the frame are built of
two long pine 2x2’s.  The
cross pieces are made of 1x2 and they are secured with four drywall
screws.   The frame needs to be
2-3” longer than the finished sound board, and about an inch wider.

Packing
tape on the two long sticks of the frame to will keep the soundboards from
being glued on to the frame.
I make two long wedges from ¾” stock - 12” long that
taper from ¼” to ¾”.

Cut the boards to length

Next
cut the planks for the sound board to length, starting from the bottom.

I
arrange the boards on top of the sound box and cut them so they are at least an
inch longer than they need to be:

I
place the planks on the frame, and move the cross brace at the top up or down
so that the two wedges can be slid into place.   Check the seams between the planks
and make sure they are tight.
Fix any gaps with a jointer.    At this point I place the
sound box on top of the planks to make sure I have allowance all around.   I also strike a light pencil line
from the treble to the bass end and use it to align the planks during the glue
up.

Time saving trick –
if you have a thickness planer

You can do a lot of the longitudinal
tapering with a thickness planer.
Just leave the top three or four boards (that boards near the treble
end) in a longer plank, 12-20" long.
Most thickness planers will eat short thin boards, so I don't cut them
to length till I'm ready to glue.

Stack the boards in order and take them to
the thickness planer.   The
soundboard should taper from ¼ " thick at the bottom, to 1/8 "
thick at the top, so plane all the boards down to 5/16” inch, and take
out the bottom (bass board) and set it aside.  Crank the planer cutting head down
1/64" and plane all the remaining boards, setting aside the next to the
bottom board.  Crank down; plane all
the boards, setting the lowest board aside on each pass.   The last board or two should be
~5/16" thick.
Then, cut the last board into the pieces that will go at the treble end
of the harp.

This way you can do the bulk of the
longitudinal (treble to bass) tapering with a power tool before the boards are
even glued together.

Once
I'm happy with the fit of the frame, wedges and boards, I'll get ready to glue.

I use yellow
carpenter’s glue.  To spread
the glue quickly and efficiently, I "collate" the boards together in
an even stack.  I use a spring clamp
to hold them while I spread the glue on eight or nine edges all at once.  After taking off the top and bottom
boards (they only need glue on one edge) I flip the stack over, re-clamp and
spread glue on the opposite edges.

The
top board is only five inches long, so it only needs a pound or two of clamping
pressure.  Place all the boards on
the frame and push the wedges gently in by hand.  Next place two slit PVC pipe ring clamps
on each seam, one at each end.  The
ring clamps hold the boards to the frame and even with each other.  They also keep the boards from
"buckling up" as pressure is applied.

I start placing the ring
clamps at the top (treble) end of the sound board.  After I have put on the first six clamps
or so, I stop and tap the wedges lightly to generate a little more pressure.  When I've placed about 2/3 of the ring
clamps I'll tap the wedges a second time to increase the pressure between
boards.  Too much pressure will
starve the glue joint.  The glue
seam at the bottom is 16” long and has a lot more surface area than the
joint at the top, so the third function of the ring clamps is to maintain a
pressure gradient - light pressure at the top, more at the bottom.

PVC Ring Clamps

Ring clamps can be made by sawing a 4" PVC
pipe into one to two inch wide rings and then slitting the rings.  Cheap and effective.  They look like a box full of white
C’s.  Actually the internal
stress on the plastic makes them always want to close so they are more like
white plastic O’s.  With blue
lettering on the side.  Four  inch PVC pipe.  Just one more thing to look for on those
late night dumpster dives at local construction sites.

I make it easier to put the ring clamps on
the soundboard by raising the frame up off the work bench with scrap pieces of
2x4 placed underneath the frame.

The
PVC ring clamps ensure the boards are even on the edges, but boards may bow up
or down in the middle.   Run
your fingers across the center of the seams to ensure they are aligned fairly
well.  If one of the boards is high,
you can usually push it up (and the other down) in the first 2-3 minutes before
the glue has begun to grab.

After
the glue dries, you should have a soundboard that is dead flat on one
side.   Take the clamps off and
lift the soundboard off of the frame.
At this point the soundboard will be alarmingly floppy, so handle it
gingerly until the string rib has been glued on.

Tapering
the Soundboard

Turn
the frame over and screw it to the work bench.  I use hand spring clamps to hold the
soundboard down so that I can use a random orbital sander to take the glue off
and level one side of the sound board.

If
you will be doing most of the tapering with a hand plane, I suggest gluing the
string rib to the first side of the board after you have sanded it smooth.   Even with a newly sharpened plane,
I have to apply enough pressure that the board needs to be reinforced to keep
it from buckling and breaking, especially at the thin end.

Flip
the board over and taper the other side.   I do the rough tapering with a
hand held power planer.  These power
planers can eat a lot of material quickly, so you may want to use a fairly
shallow blade set (<0.4mm), practice on scrap and stop short of your final
dimensions.   I plane the
longitudinal taper first then thin the edges to give it the second (double
taper).   After planing, the
board may look rough and have some ledges where the edge of the plane blade dug
in.    I sand these out
with 60 grit disc mounted on a random orbital sander.    By the time I built my third
board, I could taper the board to its final dimensions with the power planer
and sander in less than an hour.

Tip;
at 2mm thick, solid boards are going to be awful fragile at the top end.   If you have a lot of trouble breaking
boards, pick the smoothest side and sand it flat, then  glue the string rib on to reinforce the
board before you do the final profiling.  After the first 30 harps, I started
doing this, using my vacuum sanding table to hold the board firmly or veneering
the front of the board before profiling boards down to 2mm at the treble
end.

If you used a thickness planer to pre-taper
the board:

I use a slightly different process for
tapering.   The top side of the
board will have a series of 1/64 steps as the board gets progressively
thinner.  The other side should be
pretty close to flat.

The first few times I used a power thickness
planer to pre-taper the blanks before the glue up, I flipped the board over and
tried to sand the flat side smooth.
When I pressed down too hard with the sander one board cracked along one
of the stepped ledges.

When I used the hand power planer, I found
the sound board was pretty thin (5/64”) at the treble end.  The power planer aggressively eats
stock.  At 5/64” I had to be reallllly
careful trimming the board at the narrow end.

Through trial and error, I developed a
process that allowed me to taper the board more efficiently and with less
risk.   First, leave the
stepped side of the sound board up, and use a belt sander (80 grit) to
smooth out the 1/64” steps.  I
keep the sander pointed to the treble end (it puts the board in compression),
use very little down pressure, keep the sander moving around and  stop frequently to checking
thickness.  After I have removed 90%
of the step corners, I start making the cross taper with the random orbital
sander (by thinning the edges of the board.  I usually leave the board a bit thick,
as a certain amount of material will be sanded off when I flip the board to
smooth the “flat” side and remove the glue.

If I use a light touch with the belt and
random orbital sander, I can usually do all of the shaping before gluing on the
string rib.

Dimensions:

The
optimal thickness of the soundboard is going to depend on

-String
tension,                                       -the
quality of the wood,

-the
width of the board, and               -how
long the builder wants it to last.

-how
thick the ribs will be                   -How
stiff the sound box sides are

A
thinner board will generally be more responsive, but is more fragile.

Here
are the scantlings and specifications for a five octave I build:

-16”
wide at the base end          -Tension
is 1300 lbs.

-Clear
Sitka Spruce board          -Inner
and outer String ribs

Sound
Board:

String
Ribs:

Treble
end center

5/32

Width
Treble End

1/2

Treble
end edge

3/32

Thickness
Treble End

1/8

Bass
end center

5/16

Width
Bass End

1-1/2

Bass
end edge

3/16

Thickness
Bass End

1/4

Like
a topographic map, these contour lines portray the shape of a double tapered
soundboard with a linear longitudinal taper.

The
flat plates below the contour drawing are enlarged cross sections of the board
at the bass and treble end.   I
use a linear taper along the length of the board – midway up the board -
the center thickness should be 15/64”.   The string ribs have straight line
tapers too.   The side to side
taper on the soundboard is a gentle arc.

Guitar
and Violin makers seem to be much more scientific about the profile of their
soundboards than most of the harp makers I have met.  If you want to go in that direction, you
will need a deep throat caliper to monitor your tapering as you shape it to a
specific sound board profile.  You
may also want to measure and record sound board scantlings so that you can
figure out how it relates to the sound of each instrument.

Left
– A simple caliper made from plywood scraps and a $10 dial indicator.
Even with a domed anvil, the reading will be thrown off if the sound board is
canted one way or the other.
To find a true reading, I rock the board back and forth to where the
needle registers the thinnest measurement.

Vacuum Veneering
a Sound Board

Many of most expensive harps cover the
soundboards with a fancy veneer.
This has several advantages:

-The hardwood is easier to finish than
Sitka Spruce or pine

-Fancy veneers are very striking (a
good selling point)

-The soundboard harder and more ding
resistant

-It forestalls the formation of cracks
in the soundboard core, and covers them up when they eventually form.

-The board is easier to work on.  I flatten one side of a solid board
blank and veneer it.  With the
veneer it is much, much stronger and easy to profile.

Orthodox traditionalists argue that
adding a veneer can muffle the sound of good board.  I think it depends a lot on the string
tension of the harp.  Getting good
volume and a lively response from a lightly strung harp requires a thin,
carefully profiled board, and adding a veneer seems to detract from that.   A veneered board does not seem to
adversely affect harps with medium to high tension, especially if you profile
the board so it is as thin as what is specified for a solid board.

The technique I use for veneering a soundboard
is identical to the one I use for veneering a plywood back panel for square
back sound box.

In conventional vacuum veneering, a
caul is used on top of the veneer to ensure it comes out flat.   In this stack, the veneer is
usually a bit smaller than the substrate and the caul is a bit smaller than the
veneer to prevent it from driving all of the glue out from under the edges of
the veneer as the bag is evacuated.

A barrier layer, used to keep from
gluing the veneer and substrate to vacuum bag, is typically plastic sheeting or
wax paper.   The breather layer
provides vacuum channels to the bag port, and can be made from bubble wrap,
carpet padding felt, or (my favorite) nylon window screening.

The vacuum bag needs to be about
4” wider and longer than the backer board.   If you plan to do vacuum formed
round backs too, you will want an even larger bag.  My all-purpose bag is 44” wide at
the mouth, tapering 84 inches to the other end which is 22 inches wide.

By
reversing the conventional stack and placing
the veneer between the backer and the substrate, the need for a carefully sized
caul is eliminated and the board comes out of the press with a top that is as
flat as the Formica it was molded on.    I use the flattest, shiniest
Formica I can get and cover it with a coat of car wax (that is then thoroughly
buffed) to prevent any glue that seeps though the veneer from sticking, gluing
the sound board to backer board.

I use a backer board made from plywood
that is at least 3/8” thick and is an inch or two wider than the
soundboard.   One side is
covered with Formica, and the back is scored with a hatch work of saw kerf
every 2-3 inches.   These kerf
s help ensure vacuum pressure is distributed across the entire board.   I preserve the longevity of my bag
by rounding the corners of the backer board to a ¼” radius.

Here are the basic steps:

1.
Prepare the veneer and place it on a
waxed backer board.  Tape two
opposing corners so it stays in place (the tape only needs to overlap the
veneer by ¼ inch or so).

2.
Gather the breather, barrier sheets;
make sure you have the closure for the bag ready.

3.
Mix about 90 ml of DAP Weldwood
Powdered Resin to cover a 16 x 48 x 4” sound board.    Dump the glue onto the sound
board, and distribute it evenly with a 4” foam roller.

4.
Let the mixed resin set for about 5
minutes so the adhesive begins to gel a bit

5.
Place the soundboard glue side down on
the veneer and tape the soundboard in place so it does not shift around

6.
Add the barrier layer (I slit a long
strips of wax paper in half and tape them over the edges of the sound board
where it is likely to seep out)

7.
Add the breather layer and tape
itsleading edge to the backer board so it stays in place as it is loaded into
the vacuum bag.

8.
Slide the stack into the bag, seal the
closure

9.
Apply a vacuum and wait for the
adhesive to cure (6-8 hours)

When the glue has cured, I remove the
stack and re-wax the Formica on the backing board so it will be ready the next
time I need to use it.

Three
Tips for working on veneer

I
usually cut two pieces of veneers from adjacent flitches and book match them
along the center line.   I cut
each half so they will be about ¼” larger than the soundboard
blank.   I join them with
veneer tape.

Many
veneer vendors ship their veneers rolled into a roll that is 18” in
diameter or so.   For two years
I wrestled with veneer that persistently tried to roll itself up.   After my second or third shipment,
I unrolled the veneer, cut it to the lengths I would need and stored it
flat.  When the veneer is stored
flat, it stays flat which makes it much easier to cut to size and join with
veneer tape.  A big flat box under
the sofa in the living room makes a great storage spot.

For
several years I cut veneers with a utility knife (with a new blade), then
dressed the edges square by clamping the veneers between two pieces of freshly
joined plywood and sanding them with a sanding board.    A veneer saw can be quicker
and more accurate.   It also
does not veer off or follow the grain the way a utility knife does.   Make sure the saw is sharp.   It only takes a few minutes to
learn how to use it.

I
veneer with DAP Weldwood Powdered Resin, and place the tape on top.  The resin will not penetrate through the
paper veneer tape, so the veneer is only weakly bonded to the substrate along
the center seam.   The tape is
easily removed be dampening it with water then scraping it with a newly
burnished scraper.  Don’t
sweat imperfect seams.  Most if it
will be covered up with the outer string rib.

Copyright ©
2007, Rick Kemper

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