The soundboard is a large flat wooden panel located inside the instrument's case and is responsible for amplifying the sound of the strings when struck by the hammers. Typically, the soundboard is made of red spruce, a type of wood known for its acoustic properties and strength.

The piano soundboard is built with great precision. Its curvature, wood thickness, underlying ribs, and bed are calculated with extreme precision. Each manufacturer follows their own procedures, such as some soundboards having variable curvature to evenly distribute the load, while others have uniform thickness throughout their surface. Additionally, the placement of the ribs underneath the soundboard is highly individual, as is the design and shape of the ribs themselves. Even the perimeter and shape of the soundboard's edge can vary greatly from one brand to another. However, accuracy and precision in the procedure do not always guarantee a good result due to the unique nature of the raw material used. This means that no two instruments will sound exactly the same.

Red spruce has been used for centuries in the construction of bowed instruments, guitars, harpsichords, pianos, and other musical instruments due to its acoustic properties. This is due to the high speed of sound transmission in red spruce. As is known, sound travels much faster in solid materials than in air, and in red spruce, this speed is particularly high. In addition, red spruce is very elastic, has great torsional strength, and low specific weight when dry. Only red spruce possesses these four qualities. As a conifer, red spruce has a different cell structure compared to other types of wood, such as hardwoods. The cell walls of red spruce are relatively thick and follow the wood grain throughout its length. If red spruce wood breaks, an irregular fracture with long splinters is noticeable. Thanks to its constitution, red spruce is a very suitable material as a sound carrier in musical instruments. Currently, there is a considerable shortage of wood for soundboards, and it becomes difficult for instrument makers to obtain good material. Due to this scarcity, the price is high.

The vibrations in a soundboard are largely determined by the properties of the material. Some of these properties include the weight of the material, the stiffness of the material, and the internal friction of the material. Red spruce is the most common material for soundboards, although sometimes other materials such as Douglas fir or plywood are used.

Red spruce remains the best material, however. It is light and durable, with a specific weight of 450 kg/m³. The thickness of the soundboard and the way it is joined to the inner rim are very important factors in the vibratory mode of the soundboard itself. The most common method is to firmly anchor the soundboard along the entire perimeter of the inner rim. In a few old instruments, the soundboard can be found suspended freely in some stretches. Red spruce is much more resistant or difficult to bend along the direction of the fibers than transversely to it (i.e., perpendicular to the fibers rather than parallel to them).

If we try to bend two rectangular boards of the same shape and thickness, cut with the grain respectively according to the longer and shorter side, we will easily notice that the modulus of elasticity is about 25 times higher in the first case. Furthermore, the internal friction of red spruce is slightly lower than other types of wood, but still about 1.3 times higher along the grain than transversely to it, while it is substantially equal in all directions.

When the pianist presses a key, the hammer is accelerated towards the string and strikes it, transferring kinetic energy and setting it in vibration. The string in turn induces a deformation that propagates towards the bridge and towards the nut, where it is reflected and returns to the point of percussion. However, the transverse vibration is relatively weak.

Every time the wave is reflected against the bridge, it loses energy on the soundboard, which acts as a reflective membrane, transferring over and under pressures to the air, which we perceive as sound. The soundboard is necessary because the string has a very small contact surface with the air, and to obtain a full and "carried" sound, the vibration of the string must be transferred to a larger surface capable of moving in unison and amplifying the vibrations. The stronger the hammer strike on the string, the greater the energy transferred and the number of partials present in the sound.

The soundboard in a vertical or grand piano should not have resonances but react uniformly to all frequencies between 27.5 and 4,186.01 Hz, only relating to the fundamentals, and similar to the frequencies of all other partials.

The cutting of the wood boards for the soundboard is called "quarter cut" because the trunk is divided into four with cuts through the pith, from which boards with vertical grain are obtained. This method reduces stress in the wood and the risk of cracking due to drying, making the soundboard stronger and resistant to the pressure exerted by the strings. Red spruce for the soundboard is cut into boards with a thickness of 12 mm, then dried and thinned to 7-9 mm with a width of no less than 70 mm. By creating special joints along the board's joining edges, the soundboard can be further reinforced. Finally, the boards are glued together to form a rectangular panel with the grain direction arranged diagonally and parallel to the bridge.