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Hello I am an engineer working for a construction company in Turkey. By the end of June, I will be playing piano in front of a small group of audience (approx. 30 people) in a small conference room at our company. The piano we have is 60+ years old. The brand is "Zeist". It is approximately 135 cm high.

Despite its height, I find this piano extremely quiet. A local piano tuner here showed me the long cracks on the soundboard and recommended me to have the cracks repaired.

Do you think repairing the soundboard will have a considerable effect on its volume? Can I expect an increase of volume by at least 10-20%?

PS. I have two photos of the cracked soundboard, but I cannot attach them here.

Looking forward to your kind advice

There is a way to attach photos. You can do a search for discussion on that procedure.

Only if the soundboard is separating from the ribs will repairing the splits have any effect on the volume -- and then, only maybe. (Actually, you still don't have to repair the split, just re-glue the board to the ribs.) And, while checking on soundboard/rib separation, you should also look for separation of the soundboard from its attachment to the perimeter of the piano case, as well.

It sounds like this is a vertical piano, not a grand?

Hello sometime it is worth repairing an old piano soundboard cracks, but there is no direct relation with tone volume.

The problem with very tall uprights is that they have poor treble because of a too large and massive soundboard.
Then the volume of the tone is good as long as the panel have enough strencht to resist the strings pressure (building that resistance allows to install stress within the soundboard assembly and that allow the resonant fequencies of the assembly to raise, hence providing a better reproduction of the treble.

Under the tensile stress of the strings, the bridges tilt and the soundboard take a sort od S shape, with a concave portion in front of the bridge, and a convex one at the bottom.

Those 2 deformations are one of the cause for the most annoying soundboard cracks.
Those (front and back of the bridge) are generally signs that the soundboard is shot.
Whenever some pressure is possible on the bridge it apply then mostly at the back , where the backscale is, and the contact on the font of the bridge is poor.

Beside, old soundboard suffer from a lack of resistance, then the enegy input coming from the strings is dissipated in a short time, while the good soundboard keep some "tone alimentation" where the waves are allow to travel from bridge to agrafe and vice versa for a longer time , giving a fuller and longer tone.

Cracks are lso due to the differnce in tension between the external cambres part of the panel and the bottom.
Depending the way the panel is ribbed, the level of compression of the panel on the side where the ribs are glued is determining in future ability to adbsorb moisture changes, so some soundbopards will crack more easily than others.

creating and mastering that stress within the assembly is a very complicated matter. I suggest that it is understood better only recently. Many older constructions have very good quality wood but some used process that make the assembly sensitive to hygrometry a lot.

At last that is what I understood of my recent readings on the subject.

I hope that helps

PS a soundboard is to be seen as a drum skin , but the stress is created by different means, the strings pressure, the ribs glueing, the bridge. when the string plane is holding the panel and not the opposite, the dynamics of tone is rather poor

The simple answer is no. Repairing the cracks by gluing in a piece of wood or filling them in any other way will not have an appreciable effect on volume. The soundboard vibrations do not "stop" at a crack. The ribs carry the vibrations past cracks. Tests have shown that cracks are not noticeable to the ear.

If the whole piano is quiet, the hammers may be the problem. Maybe they are too soft an need to be shaped and voiced, perhaps hardened.

Bridge cap separation can cause lost volume as well. Baldwin Studio pianos are notorious for this where there will be an ever so slight gap between the bridge and soundboard from a twisted or loose cap. Those notes in that area will be muffled, Tell tale signs of repairs would be countersunk screws in the bridges between strings.

as unglueing of the ribs due to the cracks, generally all ungluing allow loss of volume
Certainly the hammers too may be too soft, if a plucked note ring clear and thicker than when hit by the hammer

Also check for carpeted floor, heavy drapes, soft furniture, and a low ceiling, all of which will reduce the perceived volume of the piano.

In the Turkish conference hall will be a lot of carpets on the walls. It's"taking away" all sound into itself

It has been proven experimentally that cracks in soundboards by themselves have no noticeable affect
on tone. Unless there is a rib buzz. However, cracks in the board are also often a symptom of a larger
issue, that of the board having lost crown. I suggest that you hold a thread to the back of the
soundboard between two long ribs, about a meter long, and see if there is a gap between the board
and the thread near the center. If the board is flat, there is probably also an accompanying loss of
bearing, which is the bend of the strings over the bridge. I suspect that the climate in Turkey is
pretty rough on pianos and that this is a soundboard structural matter that is not correctable.

But you should also follow Jurgen's suggestion and look into the hammers. If they are old and very worn, it
may be possible to reshape them and bring up the tone. You can have the tuner experiment on a few and see
what the result is. If it's not a structural problem, this is your next best bet.

From Baldwin:

Occasionally the question will arise concerning the proper way to measure crown on a soundboard. First, let’s differentiate between two related terms - crown and downbearing. Crown is the measurement of the spherical shape of the soundboard. The soundboard is often compared to the top of a violin in that the board is arched upward instead of having a flat shape. Downbearing is the measurement of the deflection of the strings as they cross the bridges.

Soundboards utilize a crown to help offset the downward pressure of the strings. Also, soundboards under tension will produce a fuller, more vibrant tone across the musical scale. The crown on a soundboard is normally designed to have approximately a 1/8” to 1/4” deflection in the middle of the board with the piano unstrung. However, once strung, the total downward pressure of the strings on the soundboard can result in a 400-1200 pound load that continually pushes the soundboard downward. It is the designed balance of the crown and downbearing that will enable a piano to resonate and produce tone to its fullest potential.

It must be stressed that crown cannot be measured on a tuned, strung piano. The downward pressure of the strings will prevent any accurate measurement of crown. Technicians will commonly attempt to measure crown with a string or straightedge along the underside of a grand parallel to a rib. This will only show crown in the loaded condition. The tone of the piano is a better assessment of the crown of the board. A full tone with adequate sustain is an effective indication of sufficient crown.