## 4 Conclusions  
There were two hundred and two samples of Norway spruce studied in this research. The relationships of structural factors and acoustic properties were analyzed.  
* The FEM analysis results displayed the range for setting up the experimental analysis of frequency for the resonance method.
* Important correlation coefficients between the latewood component and the density were obtained. Therefore, the %_LW_ showed strong correlations with all density dependent properties and most of all with the acoustic radiation. The \(R\) in bending displayed the maximum correlation index (\(r\)) value among %_LW_ and all \(R\)s. Consequently, the equation to calculate \(R_{FREQ}\) according to %_LW_ for resonance spruce was defined: \(R_{FREQ}=-0.1533\%_{LW}+15.58\).
* A strong correlation was also attained between L velocities defined by both methods and \(MOE_{FREQ}\). Thus, \(MOE\) in bending could be estimated according to L velocity: \(V_{FREQL}\); the equation is \(MOE_{FREQ}=5.564\)\(V_{FREQL}-17431\).
* The comparison of the two methods in L direction showed no significant differences in acoustic properties. Both methods were suitable to estimating the \(V_{L}\), dynamic \(MOE_{L}\), \(R_{L}\), and \(Z_{L}\) for resonance spruce for piano soundboard. A significant difference in L and bending directions was established. The dynamic Young's modulus from L direction to bending had a reduction of 5%, and the \(R\) from L to bending had a reduction of 3%.
* Differences between the L and the RT directions occurred. Therefore, \(V_{USRT}\), \(R_{USRT}\), and \(Z_{USRT}\) correspond to 1/5 of the equivalent properties of the L direction. The reduction was greater in the dynamic Young's moduli, reaching a reduction of 96% from L to RT.  
Figure 9: \(V_{\text{USL}}\) (direct transmission) \(\square\) and \(V_{\text{FREQL}}\) (frequency method) \(\blacksquare\) (y axis) according to density \(\rho\) (x axis).  
* [PERSON] highlighted that more than 1\({}^{\circ}\) of slope of grain produced an important reduction in L velocities. The slope of grain would never be more than 1\({}^{\circ}\).
* Another [PERSON] defined the limit of rings' angle in the cross section. Not to reduce the transversal velocity, the angle should be less than 15\({}^{\circ}\).
* Similar analysis, as mentioned above, was performed based on density. Generally, the higher the density, the greater increase in L velocity. The increase lessened when \(\rho\) was >440 kg m\({}^{-3}\).
* The average width of the growing rings should be less than 2.80 cm to maintain density and all density dependent properties constant.  
Measuring acoustic data, analyzing data, and writing the manuscript: G.M.; leading, supervising, and writing the manuscript: J.T.; analyzing acoustic data: P.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Internal Grant Agency Fund of Mendel University in Brno. Grant number LDF_VP_2019012.  
Not applicable.  
The data are not publicly available due to privacy restrictions. The data presented in this study are available on request from the corresponding author.  
The data were collected at the Research Center Josef Resel in Brno-Utechov, Mendel University in Brno. The research was supported by the Internal Grant Agency Fund of Mendel University in Brno, Faculty of Forestry and Wood Technology. Project No LDF_VP_2019012. Particular thanks go to the Petrof Ltd. company for dispending the material for the research.  
The authors declare no conflict of interest.