The subject of soundboards is both straightforward and complex, largely like the soundboard itself. For an in-depth understanding of the function of piano soundboards, the book ‘Physics of the Piano’ by Nicholas J. Giordano is a good place to start. Electronic speckle interferometry has also been applied to visualize the activity of a soundboard under stimulation. All of these studies bring us to several conclusions regarding the optimum soundboard for our piano:
It should be a rigid (high Young’s Modulus) and low mass structure
It should have limited resonance nodes that are overly distinct, to avoid balance problems across the compass
It should be large enough to offer sufficient bass response to the lowest note on the instrument for a balanced sound
It should be stable to temperature and humidity
Of course we quickly find out that this describes an impossible object! Getting all of the objectives met is a material not yet discovered. Further, with respect to our early pianos, and particularly squares, the bass response of the soundboard is poor, and the treble can be limited also, simply due to shape and size and the configuration of the wrest plank/hitchpins.
That said, square pianos are capable of some surprising shadings of tone and dynamics, and a well made soundboard is essential to achieving this objective. Which leads us to ask what might a well made soundboard consist of?
We begin with the wood itself. Coniferous woods are well known for high stiffness and low mass, so they achieve the superior end of our first objective. Wood is not homogeneous though, and a quick examination will show you thinner dark lines and wider lighter colored lines, which correspond to the growth rings of the tree. Each spring as the weather warms the sap rises and the lighter colored sap wood begins to build, expanding quickly through the warmer growing season. As fall approaches the sap falls to the roots, and the growth slows to nearly a stop, only to repeat the cycle next Spring. The stiffness of the dark segments is far higher than the lighter colored wood. Further, ray tracheids may be present in the wood which further stiffens the resulting board. If our aim is to have a very stiff board, we want a lot of those darker lines in it. Woods that grow very quickly, such as short needle pine, may only have 4 growth rings in an inch, while a highland hemlock may boast over 25. Sitka Spruce as it is found today will usually have about 18 rings/inch in better boards, and occasionally as many as 20-24.
Our early pianos were built with woods harvested over 200 years ago, and may themselves have been growing 200-300 years! This puts many of them in their prime during the little ice age of the late medieval period that lasted into the 1600s, and boards are frequently found with as many as 45 rings per inch, where the tree grew less than a mm/year during the extremely short growing period. Such wood is highly prized for it’s tonal response, and old soundboards should be saved whenever possible.
Soundboards are always sawn on the quarter, meaning the growth rings are perpendicular to the surface. This gives the board maximum flexibility, but more importantly, leaves it less susceptible to humidity changes. The sap wood swells with water much more than the darker winter wood, and by using quarter sawn wood the best stability is achieved. The orientation of the growth rings (the grain of the wood) usually runs nearly parallel to the direction of the strings, though in early squares it is often simplified and runs from left to right as you face the piano. Sound travels 4 X faster along the grain than against it, and communicating the sound down the longest length of a soundboard gives the best response.
That said, soundboards, and the wood they are made of, have varied throughout history. Here is a short discussion from David Hackett regarding old and new wood for soundboards, and some useful observations!
Maximum flexibility though means the soundboard must be supported to sustain the downward pressure of the strings as they pass over the bridge. This support takes the shape of ribs and struts made of coniferous woods, glued to the underside of a soundboard. The strut is called the bass bar or cutoff bar, and typically runs the length of the soundboard largely against the grain. It is generally much wider and thicker than the ribs, which run in a ‘trunk and limb’ pattern from the bass bar. This is only a generalization though, and square pianos will exhibit a wide variety of ribbing patterns, with no two makers alike. Some examples of ribs and cutoff bars are shown below.
Frederick Beck 1778
As you can see, there are about as many ways to rib a soundboard as the geometry will allow, and all of these pianos are within a few decades at most of each other. With this in mind, we can expect some fairly distinctive sounds to come from each pattern.
As-found, English square piano soundboards may be unfinished, waxed only, glue size ( a thin hide glue wash applied) shellacked, or varnished. Generally, some finish is applied to help with soiling and to stabilize against moisture. There is no clear pattern that I have found to the finish, and I would enjoy input from others on this subject. It seems that shellac became more common after ~1800, and prior to this we find wax or a glue size, but I have seen exceptions for sure. A good heavy French polish would be historically inaccurate and do nothing for the sound, if not actively hurt it, so we can assume that any finish should be minimally intrusive to response and the better it can prevent moisture uptake the better it will function on the piano. I think a thin shellac and wax coat that is not too shiny and plastic looking has the best ‘look’ after stringing. Comments are invited!
To function properly, the edges of the soundboard MUST be well secured, and as with all elements we will ever discuss here, a good hide glue joint is required. The ribs and bass bar similarly must be well attached, and here is where we usually find trouble. Over the years, things have shifted under tension, and the ends of the ribbing have started to detach. When more serious structural failure has occurred, the soundboard may have a distinct dip and ribbing is actively loose. This cannot go uncorrected and expect a proper sound, so the soundboard must come out.
Removing a soundboard is in the category of ‘surgery’ and will constitute one of the more invasive steps in a full restoration job. It need not be a nightmare, but setting expectations for time involved and approach to use is important. Belowyou will find my version of a step by step how-to guide to removing soundboards from early square pianos. Please treat this as a reference only. Many competent restorers will have alternate takes on this task, and I would welcome additional thoughts on the subject.