Unveiling the Secrets of the Piano’s Soundboard: From Spruce to Sound

As the sound source part of the piano, the main function of the piano soundboard is to convert the vibration energy of the strings into sound energy and spread it out. The soundboard plays a vital role in the quality of the piano’s sound. Piano vibration has different vibration characteristics from other musical instruments. Therefore, it is necessary to understand the impact of the vibration characteristics of the piano soundboard on the sound quality. This article mainly explains the impact of the vibration of the soundboard, the main sound transmission system of the piano, on the piano timbre and the vibration impact of the piano soundboard and the various components of the soundboard. If you want to understand the piano’s tone and sound transmission performance, you need to conduct in-depth research on the piano’s soundboard. Only by deeply understanding the vibration characteristics of the soundboard can we create a piano with more beautiful sound and bring beautiful sound to thousands of households.
1. The history of the piano soundboard

The piano has a wide sound range, and its vibration frequency ranges from 27.5Hz to 4186Hz. Therefore, it is difficult for a soundboard to give all the sounds the same radiation. Almost all soundboards lack sensitivity to sounds in the extremely high and extremely low areas to varying degrees, and even do not radiate the fundamental frequency.

In order to solve this problem, manufacturers in history have conducted a lot of experiments, including changes in materials and structures. In addition to the common wood we have now, various other materials have also been tried. For example, metal materials were used in the early days, but because the metal materials lacked a certain elasticity, parchment was later used, but it failed due to lack of hardness. Improvements are being made not only in single materials, but also in composites. Structurally, there are also double soundboards, separate soundboards, cylindrical resonators, box resonators, lever mechanisms and auxiliary soundboards. Through various experiments and explorations, around 1951, a fixed pattern emerged in the production of soundboards: spruce was used as the soundboard material. Spruce was proven to be the most ideal soundboard material, mainly because it has a high Spruce is light and elastic among woods. The entire soundboard is made of spliced ​​slats. The wood grain is in the same direction, which is roughly the same as the string direction.

In terms of acoustic effects, the ideal soundboard vibration should meet the following three points: 1. The soundboard should be able to transfer the energy it obtains from string vibration to the air with as little loss as possible; 2. The sound radiated by the soundboard should filter out more harmful overtones and redundant noise, so that it has a better overtone structure and sufficient loudness and persistence; 3. The soundboard should be able to evenly enhance all sounds in the entire range of the piano. Frequency of sound radiation.
2. Soundboard structure and soundboard vibration

The soundboard is composed of a resonance board, ribs, strings, soundboard frame, etc. The resonant board is like a thin sheet, with wooden strips arranged at a certain distance and direction attached to the bottom, that is, ribs. The edges of the resonant board are closely connected to the soundboard frame. On the resonant board, the mid-range, treble strings and bass strings are fixed respectively. The impact of soundboard structure on sound quality can usually be seen in loudness, pronunciation persistence and timbre. To enhance the loudness of the instrument, it is necessary to increase the elasticity of the soundboard; reduce the thickness of the resonant board (to a certain extent, subject to the occurrence of the formant frequency); increase the stability of the edge of the soundboard; increase the area of ​​the soundboard; Use tall, narrow bridges, with the ends of the bridge resting on the ribs. The reason why the pronunciation durability is reduced is because: the overall mass of the soundboard or other components increases; the edge of the soundboard is not stable; the elasticity of the soundboard is reduced; the frame of the soundboard is not heavy enough; there is significant glue on the soundboard. Piecing together; the supporting surface in contact between the string code and the string is too small, etc.

The timbre refers to the composition of harmonics within the frequency characteristic curve of the soundboard. The elasticity of the soundboard is high, and the harmonic composition in the frequency characteristic curve is more complete. Increasing the stiffness of the soundboard and the vibration of the soundboard, as well as being closer to its edges, will promote higher frequency radiation. If the middle of the soundboard vibrates farther away from the edges, the soundboard radiates low frequencies better. If the various parts of the soundboard are not firmly connected to each other, the acoustic quality will be greatly affected.

The shape and peripheral dimensions are basically determined by the established outer contour of the piano. Grand pianos have wing-shaped features, while upright pianos are rectangular. We should not think that the larger the peripheral size of the soundboard, the better the sound quality of the piano. It is not the entire area of ​​the soundboard that participates in the vibration, but only the basic part of its oval shape. If the dimensions of this major axis remain constant, enlarging any side of the soundboard is effectively useless. It can be seen that the ideal shape of the soundboard is an oval and extends in the direction of the treble strings. However, since it is technically inconvenient to directly make the soundboard into an oval shape, the soundboard is made into a rectangular shape and the excess is removed. corners, or use blocking wood to block the excess corners. The size of the angle of repose can be determined by the method in which the resonant plate forms an approximately elliptical structure, and the length of the major axis diameter of the ellipse is approximately equal to the length of the treble string code. Most of the soundboards of upright pianos have the lower corners cut off and the upper corners blocked with blocking wood.

The thickness of the soundboard resonant board is generally between 8 mm and 10 mm. If the quality is too thick, it will affect the vibration of the resonant plate and attenuate quickly; if it is too thin, the difference between the resonant peaks and valleys of the layout will be too large, destroying the coordination and uniformity of the sound. The thickness of the soundboard resonant board of a traditional upright piano is generally about 8.5 mm, and the thickness of the resonant board of a grand piano is generally about 9.5 mm.

Ribs are an integral part of the soundboard, which strengthen the resonant board and resist the pressure from the strings. Because sound waves propagate at different speeds when passing through growth rings in wood compared with horizontal growth rings, the sound wave vibrations vibrate through the string code. We can clearly see this through the experiments that follow.

The number of ribs arranged on the soundboard is designed based on the area of ​​the soundboard and the stress on the soundboard. Usually there are 9, 11, or 17 ribs arranged on each soundboard. A small piano has a small soundboard area and has fewer ribs; a large piano has a large soundboard area and has a large number of ribs. The spacing of the ribs on the resonant board is generally between 90 and 140 mm. There are also spacings smaller than this number. This is to make the sound waves propagate faster on the soundboard, and the soundboard can more effectively support the sound coming from the strings. force. Experience shows that if the spacing between ribs is too small, the soundboard will become too hard and good sound effects cannot be obtained; if the spacing between ribs is too large, problems such as premature deformation will occur due to insufficient support of the soundboard. On the same soundboard, the ribs are not arranged at exactly equal distances. Usually the ribs in the treble area are arranged at a smaller distance to make this area of ​​the soundboard harder and enhance the ability to transmit high frequencies. The distance arranged in the bass area is correspondingly larger, which is beneficial to the propagation of low frequencies.

If you look at the back of the new piano, you will find that the joint surface between the resonance board and the ribs is in the shape of a bow with both ends turned upward. This is because the ribs are made into an upward-warping bow shape before being combined with the resonance board. After the ribs are adhered to the resonance board, the soundboard takes on a dome shape with a bulge in the middle, which improves the soundboard’s elasticity, load capacity and resonance performance. The load of the ribs is maximum in the middle, gradually decreases toward both ends, and is the smallest near the ends. In order to enhance the elasticity of the soundboard, prevent the soundboard from being too “dead” or “hard”, and avoid poor vibration and a “hard” sound, the areas with smaller loads at both ends of the ribs are made into a transition pattern from high to low. Rounded slope shape. Since each of the ribs arranged at an angle is in a different position, and the required hardness of each area of ​​the soundboard is also different, the length of each rib on each soundboard and the size of the transition slopes at both ends are also different.

The cross-section shape of most ribs is a quadrilateral with an arc-shaped top surface. Some piano ribs have a quadrilateral cross-section with two corners shaved off the top surface. Although the ribs of a soundboard should only have one cross-sectional shape, the cross-sectional dimensions of each rib are different. This is deliberately arranged to ensure the elasticity and balance of the soundboard. The cross-sectional dimensions of the ribs should vary with their length. Generally, longer ribs have larger cross-sectional dimensions; shorter ribs have smaller cross-sections. Generally speaking, the width of the traditional soundboard of an upright piano is no less than 20 mm and the height is between 15 mm and 22 mm. Some multi-layer plywood composite soundboards have a harder resonant surface (compared with traditional soundboards), so the height of the ribs is reduced, or the thickness of the two transition slopes is thinned, or both methods are used simultaneously. In this way, the hardness of the resonance plate can be moderately reduced, the elasticity can be adjusted, and the sound attenuation rate can be increased.

The strings are the bridge that transfers the vibrating energy of the strings to the soundboard and provides a point of support for the effective string length. The upper fulcrum of the strings – the saddle is attached to the string frame. After the string frame is cast and molded, the position of the saddle cannot be changed. The effective string length can only be determined by the lower fulcrum of the string – the glued position of the string code. Sure. This is another function of the string code besides conducting sound.

Since the string strings with huge tension directly press on the strings, and many string nails need to be nailed on the strings, to ensure that the strings themselves are not deformed or damaged, and that the string nails do not become loose or dislodged, the strings should be made with higher Delicately structured wood with sufficient strength and nail-holding power, such as colored wood. The soundboard frame, like the ribs, is firmly attached to the back of the resonant board. The difference is that the soundboard frame is located around the resonant board. The soundboard frame is usually made of spruce or red pine. In order to ensure that the soundboard surface is arched upward and the shape is stable, the soundboard frame needs to be made into an arch shape with a slight protrusion in the middle. This will help the soundboard maintain a good working condition. The inclined frame introduced earlier to separate the quiet angle of the soundboard should be an integral part of the soundboard frame. The glue binding of the slanted frame should be consistent with the arched state and bevel of the four borders.

The ribs are firmly embedded in the resonance plate through the inclined frame and the slots opened under the frame. The soundboard components are tightly and firmly coupled with each other, which is one of the important conditions for the soundboard to obtain energy from the work of the strings and effectively exert it. For small upright pianos, in order to reduce the width of the piano body, the soundboard frame and the back frame are often integrated into one body, and a soundboard frame with a relatively large cross-sectional area is made of relatively hard and high-strength maple wood to replace the back frame. Frame, this kind of piano is commonly known as a backless frame piano. The soundboard frame of this structure can not only bond and stabilize the soundboard, but also play a role in supporting the piano body and jointly carrying the string tension with the string frame. The cross-section of the frame of this kind of soundboard is large, which affects the resonance area of ​​the soundboard (reducing the working area of ​​the soundboard). For soundboards with four sides glued to the soundboard frame, the resonant board and ribs are bonded in advance, and then the ends of the ribs are installed into the corresponding prefabricated grooves on the soundboard frame, and are bonded to the soundboard frame to form a soundboard assembly. After it is assembled on the piano back frame, it will bring convenience to the disassembly and replacement of the soundboard and other overhaul work. In another soundboard structure, the soundboard frame is pre-processed into an L-shaped interface, which is bonded to the backframe during the manufacturing process, forming an inseparable whole with the backframe, and is then glued to the backframe. The resonant board with the ribs connected is pasted on the L-shaped soundboard frame on all four sides, so that the ends of the ribs are sandwiched between the soundboard frame gap and the resonant board. When properly designed and manufactured, this fixed soundboard frame can partially increase the resonance area of ​​the soundboard, which is beneficial to improving the loudness of the soundboard. However, it will cause problems in the renovation, dismantling, and replacement of the soundboard. A certain amount of trouble. When you need to dismantle, repair or replace the soundboard, please pay attention to the connection method between the soundboard and the back frame before proceeding to avoid unnecessary losses.
3. Comparison of sound transmission performance between solid wood soundboards and composite soundboards

In a piano, the soundboard, string frame (iron plate), back frame and other main structural parts of the string tensioning system jointly carry the huge tension from the strings. When the strings are excited and vibrate, the soundboard is shaken through conduction through the string codes attached to the soundboard. The soundboard not only responds to the vibrations from the strings, but also resonates, enhancing the radiation of sound waves over a large area, thereby enhancing the volume. The soundboard is like a filter, which can modify the sound of the piano. This is everyone’s consensus on the main function of the piano soundboard. Because of this, when people talk about the quality of a piano’s acoustics, they usually directly associate it with the quality of its strings and soundboard.

In recent decades, composite soundboards have been widely used in ordinary and mid-range upright pianos, which account for the largest number of total piano production and sales, replacing many traditional structural lath-and-glue soundboards (hereinafter referred to as composite soundboards). traditional soundboard). Some piano enthusiasts and friends who care about piano manufacturing have questioned that the new structure of composite soundboard does not comply with the theory of sound wave conduction, which will weaken the volume of the piano and worsen the tone. Changing the traditional soundboard to a composite soundboard is a regression in piano quality. , is an undesirable move. Others have a different view. They believe that the transformation of piano soundboards from the traditional lath-glued structure to the composite soundboard with thin wood veneers on the surface is a reflection of people’s strengthening of piano manufacturing theory and process technology, and an improvement of the piano resonance system. A revolution. There are also some friends who saw that the new structure of piano composite soundboard has fundamentally solved the long-standing problem of soundboard cracking. A series of problems that affect the acoustic quality of the piano caused by soundboard cracking have also disappeared, so they are interested in composite soundboards. The soundboard is a popular alternative to traditional soundboards. Some friends believe that soundboard cracking is a normal phenomenon during piano use. If the soundboard is not severely cracked, it will generally not affect the sound of the piano. Therefore, it is pointed out that there is no need to change the traditional soundboard into a composite soundboard with complex technology and low economic benefits.

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