Sound Board construction and function:
Check out a grand
rebuild involving a new sound board.
Check out an upright rebuild involving a
new sound board
It is my opinion that when replacing any part of a piano that the best materials available in the world should be used. Of course there are many products on the market of varying quality and price. When choosing a rebuilding plan with a customer I always explore with them all of the different materials they can select. We weigh the benefits of higher quality materials against the cost of lesser materials and the customer's expectations. However, the selection of sound board woods is one area that I don't like to offer options. In my opinion, there is no point in replacing a sound board with anything but the best woods. If a money saving option is desired, then rebuilding the original board is the money saving option I like to recommend. In fact, the difference in cost between average quality sound board material and high quality material is not that great. I personally am recommending Bolduc sound board materials at this time.
It is interesting to note that at one time Steinway and Sons of New York was making their sound boards out of wood that had been harvested as much as 100 years prior to its use in a sound board. This allowed the wood all that time to season before it was put into use. The result of this is that those boards lasted beautifully. It is not uncommon to find a Steinway piano built in the 1920's, for instance, which has a sound board that is still a marvelous tone producer. In fact, I feel it is often a crime to replace such a sound board unless your going to use the very best materials and very best building techniques to do so.. Many rebuilders will replace a sound board like this using new wood that is actually inferior to the original board and will build the sound board using techniques that leave the new board operating worse than the original.. Far better to reuse the original sound board that was made of wonderful wood (provided that wood is still able to take the strain) than to replace it with something new that hasn't been selected with anything like the care and quality of the original material. However, if the best quality wood is used and the best techniques for making the board are used, the results should prove worth the expense since the new wood will have more resiliance than the old wood had. Also, the glue joints holding the sound board together will be far stronger with a new sound board than the original board whose glue may be drying out and loosing its strength.
The function of the sound board in a piano is to amplify and modify the vibrations initiated by the strings. In so doing, there are certain aspects of the shape, installation and materials of a sound board that enhance its ability to do this:
1) All sound boards have crown: If you take a string and stretch it along the under side of a piano's sound board you will discover that the board is arched upwards slightly. This helps the board to resist the downward pressure of the strings as well as distributing that pressure.
2) It is my belief that the highest part of the "arch" in a sound board should always correspond to the position of the treble bridge as closely as possible.
3) The ribs of the sound board should be shaped so as to reflect the same arch that is in the board. Many manufacturers will make their ribs perfectly straight. Then, when they glue the sound board together, the sound board "warps" itself into a crowned position as it acquires moisture from the air. This technique invariably leads to lesser tone, more compression ridges and cracks than other techniques and reduced longevity. When selecting a piano rebuilder, it is best to make sure which method they use in constructing their sound board ribs and what glue they are using. It should also be noted that the long bridge in a piano acts as one of the most important ribs. It, too, should be shaped in the correct arch and it should be glued to the sound board in such a way that this arch is preserved.
4) The sound board should be "fitted" to the rim or case of the piano so that the wood to wood joints are as perfect as possible. Again, many manufacturers and rebuilders don't try for a good tight fit here. The purpose of this joint between sound board and rim is to reflect the vibrations of the sound board back into the board itself and to keep the board from loosing crown by spreading. If there are major gaps in the fit of the sound board to the rim, then much of this vibration can escape from the sound board and if the glue "creeps" the board could spread out into the void, thus loosing some of its crown.
5) While the sound board is being constructed and before it is installed into the rim of the piano, it is carefully dried. The reason for this lies in the fact that wood shrinks when it is dry and expands when it is moist. The sound board is glued into the piano while it is dry and therefore small. Once the board is glued securely in place, the moisture content in it is allowed to slowly and evenly rise. This causes the board to expand into the piano and become tense and resilient. This is how the board is finally turned into a device capable of amplifying and modifying musical tones. It should be noted, however, that excessive dryness at this stage can cause the board to become too tense in the piano and for excessive compression ridges and cracks to eventually form in the board. I like to glue my boards into the pianos at a humidity level of 30%. Of course, a much lower setting is used during the building and fitting process. However, I modify this setting depending on where the piano is expected to live. I will dry a sound board more for a piano going to Phoenix than I will for one that will live on Long Island Sound.
Another consideration to weigh is the glue used for making the sound board. Many builders will use "Aliphatic Resin" glue in making a sound board. This is more commonly known as "Carpenter's Wood Glue" or "Yellow Glue." This glue is characterized by a slightly rubbery consistancy when it is dry. This means that it is totally inappropriate for sound board construction. Rubbery glues can "creep" over time. This means that the surfaces under tension can slide over time. Also, this glue acts like a vibration shock absorber and therefore absorbs tone from the sound board rather than transmitting and reflecting it. Another glue that is commonly used is Resorcinol. (Forgive the spelling. I've only been taught about it and have never used it so am not sure how they actually spell the name.) This is a modern glue that is excellent at vibration transmission as well as reflection. However, if the glue is not fresh, it will only work well for a short time. Some months after the piano is done and back in your home, the sound board could fall apart if the glue was off. It is impossible to tell if the glue is bad until it fails. I've heard horror stories and so I avoid this glue. Another common glue to use is liquid hide glue. I admit that this has often been my glue of choice. However, it has similar problems to the high tech glue in that it has a limited shelf life. If the glue is a little old and you don't realize it, it can fail unexpectedly at a later date. For this reason, I have often gone to using nothing but absolutely fresh hot hide glue that I mix in the shop myself. I retard the set time on the glue with potassium chloride mixed into the glue. This chemical is mixed into liquid hide glue in large quanitites. The more you add to the glue the weaker it will become. Therefore, it is important to use as little as possible. I only add enough to keep the glue "open" for the time I need it and no longer. This method yields the strongest glue that money can buy for making sound boards. It transmits the vibrations wonderfully and is totally reversible in the future when another new sound board is required. Also, the warmed hide glue has the added benefit of mixing well with any small amount of glue residue that may be left on the rastin (the shelf the sound board glues onto in the rim of the piano.) The original glue used in antique pianos was hide glue so using the same glue in a warmed form makes a perfect mix to avoid any problems with the glue joint if every scrap of original hide glue isn't removed.
Since this article was originally written, I have found another glue that I favor. It is provided to us via Bolduc Piano and is simply referred to as Bolduc Acoustic Glue. I suspect that it is similar to a glue used in Europe under the name of Humbrol. It bonds well with hide glue residue, forms very hard acoustically sound joints and is totally reversible just like hide glue. Better yet, it stays open longer without the need of adding anything to it. Beside freshly mixed hot hide glue, this is the best glue to use in sound board construction.
Of course this is only a brief explanation of a very complex and even mysterious subject but it will hopefully serve to help you understand what to search for when seeking out a rebuilder of your own.
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