Soundboards-Their Function, Design & Construction ((C) 1995)
excerpted from PianoTalk, a newsletter by Del Fandrich
NorthWest PianoBuilders
2999 John Stevens Way, Hoquiam WA 98550 USA
ph (360) 532-6688 fax (360) 532-6582
Netiquette constrains inclusion of extensive sales info that would normally
follow this info.
MYTHOLOGY AND ANCIENT SCIENCE
When I began to investigate the different design elements that enabled
pianos to sound the way they do, one of the first components I looked at was
the soundboard. I discovered there was an almost mystical aura surrounding its
design and its function. The basic, and still mostly secret, principles had
been worked out decades earlier by the masters, who for the most part had all
taken their secrets with them to their graves. About the only clues were found
in books such as Piano Tone Building and a Treatise On The Art Of Pianoforte
Construction-both originally compiled or written before 1920. Alas, much of
the information contained in these references, while state of the art in the
1920s, was somewhat dated when I began my study in 1980.
I would read passages such as the following and scratch my head in
wonderment! Referring to a so-called "cup-shaped disc" found in the wall of
tracheids-a type of wood cell-an old master wrote, "In its center is a small
membrane septum, like the vibrator on a phonograph reproducer with a somewhat
thickened disc in its center...It is this vibrating mechanism that gives tonal
value to the woods derived from the spruce...If a log of spruce or pine is not
properly cared for after felling, by being promptly barked, or if the lumber
is subjected to careless kiln drying instead of being air seasoned, these
delicate membranes will be ruptured, thus giving rise to inharmonics or
destroying tonal value. All the crowning or pressure put on a soundboard of
this material will not improve the tonal effect."
Now, wait a minute here! Tracheids are wood cells that, depending on
the species, are about 20 to 60 microns in diameter (1 micron equals 0.001 mm
or 0.000039"). These vibrating membranes are found in a cup-shaped disc, which
itself is found in the wall of a cell of wood that is about 0.04 mm, (0.0016")
in diameter, and we're supposed to be able to hear these things vibrate? I
think not.
THE MISUNDERSTOOD SOUNDBOARD
Much of the misinformation about piano sound that we treasure still
comes from the incomplete understanding these early builders had of basic
engineering and physical principles. Wrong ideas such as the following die
hard:
* Only wood selected as "tone wood" can be used in soundboards.
* The best soundboard wood is grown on the north side of the mountain
(or the south, or east, or west-take your pick, I've heard them all).
* Kiln drying kills the "tone" of the soundboard wood.
* The best soundboard wood has 18 to 20 grains per inch.
* Old soundboards sound better than new soundboards.
* Soundboards should "resonate."
* Soundbaords are "amplifiers."
* Larger soundboards are better than smaller soundboards.
And the list goes on.
SO WHAT ARE SOUNDBOARDS?
When you strip away all of the mysticism, the soundboard assembly is a
sound-producing system that uses one of the most incredible engineering
materials ever-wood. Soundboard design and the type and grade of wood has an
important effect the piano's sound. In fact, the soundboard assembly and its
mounting system are the primary factors in determining power (loudness),
sustain, and the quality of the tone.
Soundboards are frequently, though incorrectly, called a piano's
amplifier. In fact they do not amplify anything. In engineering terms,
soundboards are transducers-a critical distinction. Amplifiers are devices
that take small signals and make them larger by adding energy to the original
signal. Transducers move energy from one system to another or change energy
from one form to another; this is what piano soundboards do. Energy is
"transduced," or changed, from mechanical wave energy in the string into sound
energy in the air surrounding the piano. No energy is added or created.
When a piano string is struck by a hammer, energy is transferred from
the hammer to the string, causing a vibrating wave motion to be set up within,
or along, the string. When this wave motion reaches the soundboard bridge, a
certain amount of energy is transferred through the bridge to the soundboard
causing it to move. The vibrating motion of the soundboard compresses and
refracts the air adjacent to the soundboard, creating sound energy which our
ears pick up and identify as piano sound. This cycle is repeated until all the
wave energy in the string is dissipated.
There is another thing that soundboards are not. They are not
"resonators." That is, they are not designed to resonate at any specific
frequency. Many bellymen will thump on a soundboard with their fists or
knuckle, listen for the resulting "boom" and pronounce a soundboard either
good or not so good. This test is somewhat misleading and is really a subject
for another paper. Suffice it to say that a "resonant" soundboard is a voicing
problem waiting to be discovered.
SOUNDBOARDS-THE MOST IMPORTANT ASPECT OF TONE
The tonal potential and sound quality of any given piano is determined
by a number of mechanical and design characteristics. The piano technician
and/or rebuilder can exercise varying degrees of control over some of these,
but not all.
* the overall design and construction of the frame and supporting
structure of the piano-the plate, rime bracing, belly rail, etc.
* the soundboard scale-its design and construction, the material used,
how it is mounted to the rim, its condition, etc.
* the stringing scale used-string lengths, diameters, tensions, etc.
* the type and condition of hammers and action
We'll limit our discussion to the soundboard.
The transfer of energy from the string to the surrounding air takes
place fairly rapidly and in a more or less controlled fashion. The speed of
the energy transfer varies with the soundboard's design and the frequency of
vibration in the string. Low frequency wave energy will transfer at a
different rate than high frequency wave energy. This means the fundamental and
the various partials will have different decay rates. The rate at which energy
is transferred from the string to the soundboard during the first few
milliseconds following hammer impact-and finally to the air as sound energy-
determines the piano's initial, or impact, sound envelope. It is this impact
sound that determines our impression of a piano's volume and tone quality. How
the soundboard system responds to the remaining wave energy in the string
determines the sustain.
EFFICIENT CONVERSION OF ENERGY
To do all this efficiently requires a soundboard with a unique and
predictable set of mechanical characteristics. From an engineering
perspective, the piano soundboard is a two-dimensional edge-supported
vibrating plate with clamped boundaries. It is not freely vibrating, but
instead is a driven plate in which the vibrating characteristics are carefully
controlled by the mechanical design.
All practical vibrating plates have mass, stiffness, and a certain
degree of internal friction. It is the relationship between these
characteristics, particularly the ratio of stiffness (elasticity) to mass,
that determines how a soundboard assembly will respond to the wave energy
being presented to it from the strings. When the bridge moves it
disperses energy over a fairly broad area of the soundboard. Energy from one
unison does not travel through the bridge to drive the soundboard at a point
source. It is actually spread over a considerable length of the bridge before
reaching the soundboard. With any luck (and a correctly designed soundboard)
it will then be dispersed over a fairly broad area. The soundboard should act
as a diaphragm, not a flexible membrane propagating wave motion. The
soundboard panel should move as a single unit, not breaking up into small
vibrational modes. That this is not actually possible in the real world should
not prevent us from trying to make it so.
CHARACTERISTICS OF SPRUCE INFLUENCES DESIGN
To achieve such optimal soundboard behavior requires a flat, very
stiff yet light-weight panel (we'll leave crown and string loading for another
time). Traditionally, this panel has been made out of wood, generally one of
several species of spruce.
Unfortunately, wood is anisotropic-that is, it does not have uniform
strength or stiffness in all directions. It is stiffer along its grain than
across its grain. Consequently, the panel must have a system of stiffening
ribs along one (or both) sides that cross the grain of the panel at
approximately 90 degrees in order to give it approximately the same stiffness
in all directions.
Since not even "old growth" Sitka spruce trees grew large enough to
cut soundboards from a single plank, the panels are built up using a number of
narrow boards selected for their uniformity of color and certain specific
mechanical characteristics. For a variety of reasons it's best not to have too
many glue joints in the panel, so the individual boards are typically between
75 and 125 mm (3 to 5 inches) in width. Less than 75 mm and the panel begins
to look "choppy." More than 125 mm and each board becomes too susceptible to
changes in moisture content (not to mention that spruce lumber meeting our
specifications is nearly impossible to find in widths greater than 125 mm.).
In the interest of better moisture stability, these boards are always
quarter-sawn.
WHY SPRUCE?
Over the years different woods have been used for soundboards, but an
overwhelming majority of piano builders use spruce, of which there are several
species. There are a number of very good reasons for this:
* Spruce is lightweight. In general, piano soundboards should be as
light as possible, consistent with strength requirements.
* Spruce is quite stiff for its weight. Other types of wood are as light
or lighter than spruce, but not as stiff. To make a panel with the required
stiffness from these woods would result in a much heavier panel.
* Spruce has good damping characteristics. It has a good balance between
stiffness, mass, and internal friction.
* Spruce has consistent characteristics from board to board.
* Spruce is structurally durable. It will withstand a high level of
abuse. (And the wood in piano soundboards is, by design, sorely abused.)
Other types of wood possess some of these characteristics; indeed,
some actually out-perform spruce in one way or another. Only spruce has it
all.
WHICH SPRUCE?
Normally, three species of spruce are used for soundboards. They are
Eastern white, Englemann, and Sitka. The following chart compares these three
species by examining three of the most critical factors pertaining to piano
soundboard design. (Sugar pine is not normally used for soundboards, but it is
included because several manufacturers have used it for ribs.)
species/spec grav/mod. of elasticity/comp perp. to grain
Eastern white/0.36/1.43-1.45/430
Englemann/0.36/1.30-1.55/410
Sitka/0.38/1.60-1.63/580
Sugar pine/0.36/1.18-1.20/500
Specific gravity is the average weight of the wood compared to water.
In general, for both soundboards and ribs, the lighter the better. Specific
gravity varies with grain density. Boards with a high "grains per inch" count
are more dense than boards with a low "grains per inch" count. They are also
stiffer-it's a trade-off, you can't have both.
You'll notice that all species of spruce have approximately the same
specific gravity. The actual weight of any soundboard and rib assembly will
vary with moisture content.
The modulus of elasticity (MOE) is the ratio of stress to strain
within the elastic limit of the wood sample. Stress is force (or a load)
acting on a unit area. Strain is unit deformation, or the bending resulting
from a given load acting on the wood sample. For soundboard wood, the higher
the MOE the better. Wood with a high MOE will resist bending better that wood
with a low MOE. In other words, it will be stiffer and better able to support
the strings' downbearing force. Sitka spruce has a much higher MOE than other
spruces. In fact, Sitka spruce has one of the highest stiffness-to-weight
ratios of any readily-available wood.
Compression perpendicular to grain is a measure of the ability of wood
to resist compression 90* to the grain up to its proportional limit, i.e.,
before fiber failure.
Wood is hygroscopic. As it absorbs and desorbs moisture, it will
expand and shrink if it can. Once a soundboard is installed in a piano,
though, its ability to expand is severely limited, so the swelling wood cells
create internal compression instead. Woods with higher compression
perpendicular to grain ratings will resist fiber damage resulting from
internal compression better than woods with low ratings. Of all the spruces,
Sitka spruce has the best compression perpendicular to grain rating, meaning
it has a greater ability to resist failure due to fiber crushing.
SITKA IS THE ONE
Studying the chart above indicates that there is one type of spruce
that stands out from the others in two of the three important parameters-Sitka
spruce. It has the highest modulus of elasticity and the highest compression
perpendicular to grain rating.
There are other characteristics of Sitka spruce that single it out as
one the world's best woods for piano soundboards. Its evenness and uniformity
of grain and its warm tan color are unsurpassed for beauty. Its low internal
friction provides near-perfect damping qualities. It can be cut into
defect-free boards wide enough for soundboard panel construction, requiring
little, if any, patching.
Besides making fantastic piano soundboards, Sitka spruce is used for
furniture making, mill products, interior house trim and millwork, and window
blinds. Some spruce is still being used in light airplane construction,
especially in home-built and kit-built craft. Many sailboat masts and spars
are still made of Sitka spruce.
While Sitka spruce has not disappeared from our forests, it is
becoming an endangered species. Lumber of the quality needed for musical
instruments is getting harder to find. For a long time I thought this was
because Sitka spruce trees were not being planted in any quantity. It turns
out they are-by the millions. However, they grow like weeds unless they are
planted in areas where they are heavily shaded, the water supply is limited,
etc. With "good" growing conditions they grow very fast which, of course,
means wide grain lines and low grain per inch counts. These fast-grown trees
make good studs for house building and good chips for fiber, particle boards
and other man-made products, but they're not much good for piano soundboards.
High quality pianos need wood from trees grown in old-growth forests
under difficult conditions, heavily shaded by a surrounding canopy of existing
trees and with limited moisture available (a condition hard to find in a rain
forest-well, they tell me it's the ground water that counts). These conditions
are hard to find and even harder to sustain under present-day economic
conditions.
Let's hope someone sees the light and begins planting trees soon for
sustainable yield of musical instrument grade lumber. It's not being done
today. But we can hope.
For further info on Del's own Clarity soundboards now for sale, contact him at
the above addresses.
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