Piano SoundboardWhat do Cuban rosewood, Bavarian spruce, Brazilian mahogany and Italian red spruce all have in common? (Hint: it's not that they're all trees). They are what are called "tone woods". A tone wood is a type of wood recognized for its acoustical properties – typically consistent tone when vibrated. And, as we all know, sound (i.e. music) is vibration. The choice of wood shapes the "voice" of the instrument from a Stradivarius violin (Italian spruce), to a Fender Telecaster guitar (white ash), an Indian tabla drum (rosewood or teak) and of course pianos.
It's amazing how wood can really make a difference. This is especially true for the soundboard of a piano. This is the "heart" of the instrument, responsible for amplifying the sound created by the hammer hitting the string. This is why a consistent tone is critical and also why piano builders have such a fixation with the wood the soundboard is made of.
Think of a soundboard as the diaphragm of a loudspeaker. The speaker takes an electrical signal from your stereo and converts it into movement or vibration, which is then amplified by the (usually) paper cone or diaphragm of your speaker, thus creating sound that we humans can hear. In the same way, the soundboard takes the vibration of the strings and pumps up the volume. Just as with your speakers, you don't want the soundboard distorting the tones you've worked so hard to play right. Ideally you want a clean, clear and sustained transmission of the notes you're playing without any deflection or distortion. An added complication for a piano is that a good soundboard must not only be flexible enough to vibrate but also strong enough to support the 200+ strings, which may put as much as a 1,000 pounds of downward pressure on the soundboard. Getting the balance right is why piano making is such an art.
So how do we get all that from our piano? It's all in the wood, not only the type of wood but where it was grown, how it was cut and if it was aged. Sounds picky and perhaps a little obsessive/compulsive, doesn't it? Well when you think that the sole purpose of a piano is to produce pleasant sounds like Debussy's piano works, attention to detail doesn't seem like such a bad thing.
Let's look at the wood first. We've already talked about the types of wood typically found in other instruments. Piano makers have largely settled on spruce as the wood of choice. Italian red spruce is the top of the line, generally considered the best amplifier. You'll find it in Fazioli, Grotrian and Shultz Pullman pianos. Arguably, Italian spruce is the most expensive, most desirable soundboard material in existence. The reason that so few manufacturers us it is partly due to the extremely high cost, but mainly because it is so hard to work with. Due to its extremely low specific gravity, it produces the cleanest, clearest most beautiful sound of any tonal wood, but very difficult to work with. Great lengths must be taken to carefully cure the wood and craft it in such a way that it will sustain the tension for the life of the piano. From those I have spoken to, who are industry experts on soundboards, only the Fazioli has proven itself over the past 25 years to build soundboards from this spruce that endures the harsh climates of North America and Russia. In his opinion, Fazioli soundboards are the most beautiful soundboards made in the world today. Unsurprisingly, Bösendorfers (a German design), use Bavarian or Carpathian spruce. Sitka spruce is a North American species (from British Columbia or Alaska) and is commonly found in many entry-level Chinese, Korean and Indonesian pianos; this wood often distorts sound. Sitka spruce was never made to produce clear undistorted sound, but is very inexpensive and extremely easy to work with, hence its common use. And the very cheapest pianos use plywood with spruce veneer – as terrible as that sounds it's nothing compared to the poor sound such a piano will produce.
In addition to the species of tree, what exactly are piano makers looking for when they choose a wood for the soundboard? The best way to answer is to tell a story. For centuries the violins made by Antonio Stradivari of Cremona, Italy have been considered the best – elegant craftsmanship with perfect tones. How did he do it? Was it in the design, the glue or maybe the shellac? It was in the wood. Scientists now know that in the time Stradivari was building his instruments, Europe was in the middle of a "little ice age". Trees grew more slowly. They were thus denser and the tree rings (created every year of a tree's growth) more consistent in size. This perfect tonal wood found its way into the famed Stradivarius violins.
Nowadays premium piano makers still look for the high quality tonal woods for their soundboards. The most premium makers use the finest spruce; older trees, growing at higher elevations in colder climates, give more dense wood with consistent growth rings. Cold weather makes trees grow more slowly so there are more rings, which you can see as wood grain when the tree is cut. Tighter, straighter wood grain is better for sound.
Piano makers will cut the wood is specific ways in an effort to unlock the purest of tones from it. Typically it's "quarter-sawn". Imagine you are looking straight on to the end of the log so that it forms a circle, like a pie. Now divide the wood into four equal pieces. Then cut those quarters into boards. That's quarter-sawn wood. The advantage is that it reveals the grain and enables more faithful tone. Then the wood is dried, either naturally or in a kiln. The finest manufacturers naturally dry the wood for years, and then very gently dry it in kilns that inject humidity in periodic bursts.
That's how wood contributes to the quality of the soundboard, but there's one more consideration – how the soundboard is fitted to the piano. The soundboard only touches the piano at two points, where the vibrations start (the bridge) and where they stop and reflect (where it fits against the rim). It's critical that the fit at these two points is good and the craftsmanship is high quality. If you look inside your piano you'll often find in premium brands that the soundboard tapers as it extends out. This affects the sound.
As with the rim, this is a lot of information to handle. You probably want to focus on piano playing, not botany so the best way to understand how soundboards affect the faithfulness of the instrument's sound is to visit a well-equipped showroom, look "under the hood" to see the different soundboards, ask questions about the choice of wood and then listen!