We must address the important question of why maple is used for the back and spruce for the top plate. There is still no satisfactory explanation to this issue. Savart assumed that the low weight and density of spruce is paired with the flexibility of glass and steel, therefore, it works well as a violin top. Although it is clear why spruce is appropriate for making the top part of the violin, it is still unclear why maple is used for making the back of the violin instead of spruce. Apian- Bennewitz explains the necessity of using maple for the back plate at the place of the soundpost, as due to the "higher density of maple and the lack of tension, it seems to present the same vibrations when a flexible top is used with string tension and bridge pressure." I have already presented my views on the fact that in the case of the back there is no hub creation. In addition, as we have seen it, there is no lack of tension at the back part. The soundpost provides considerable tension by pressing the back part outwards; and some tension is provided by adjusting the strings at the back as well. I have demonstrated how pulling and pressure have a peculiarly modest effect on the tone of the violin. My explanation for the choice of the top and the back is as follows: when choosing the wood for the top, not only flexibility, but light weight is very important as well. The lighter the flexible weight is, the easier it is to create vibrations. As I have already mentioned, the vibration of a light weight easily fades away when the bow - that generated the vibration - stops. However, a vibrating structure is not only expected to react quickly when vibration is generated, but to stop vibrating when the bow is removed from the string as well, so that the instrument can react to the next touch. Spruce is the most appropriate material to fulfil this requirement. Savart's well-founded assertion about emphasising the low weight of spruce, however, does not take the weight of the glued plate into account. Whereas the two free plates on the completed violin are tuned to the same sound (system sounds), the better resonance base created at the top and the back by the different materials and various height of sounds cannot be explained. However, after gluing and with similar thickness, spruce - as we have seen it - creates lower sounds than maple. This apparent disadvantage is compensated for by its lighter weight and by its outstanding vibrating ability, like no other material. Maple, which is a denser and harder wood with somewhat poorer vibrating ability, still produces a higher sound when glued. Although spruce can create a back panel sound higher than the pitch when it is left thicker with a beam in the middle. However, if the sound post is placed, the beam sound would be too close to this back panel sound - or it would be the same - resulting in a useless vibration base. However, the vibration base can be perfectly formed for the back panel when maple is used: the lower tone of the top - as it is reflected well in the beam sound - has an easier contact with the system sound instead of the lower tones, and the back significantly improves the formation of higher tones by its high basic pitch and even higher back pitch. This is how the vibration base opens up.
This sound formation is also supported by the structure of the violin: the middle of the top is narrower than the back, so its basic sound gets lower and when the violin is ready for play, the beam sound and the back sound are clearly differentiated. High pitched basic tones can be made more favourable by forming a vibration surface (the sound of the top plate) by the sound holes, where the sound of this vibration area is connected to the similar pitch of the sound of the back plate by the sound post. This is the reason why no other material is considered to be used for the top plate; though another material can be considered for the back plate. Classical Italian violin makers actually experimented with other materials for the back panels, such as poplar, linden and pear trees, etc. However, experience has shown that maple rises above any other wood when used for the back panel, because, compared to linden or poplar, it can be crafted into a thin plate with good vibration ability resulting in a high pitched base sound. Due to the high pitched side sound - or as I named it, the high pitched back sound - the vibration base will be especially favourable for high tones and overtones.
Author: Christopher Grimes