## 2 Materials and Methods
### Overview of the Training Set  
With respect to the defined steps, it can be seen that the development of the predictive model should start with the consolidation and clearance of the modelling data. Although it was impossible to list the data individually, an overview is provided to facilitate the understanding. Each earphone design was characterized by eight variables, while the corresponding options can be seen in Table 1. Additionally, 388 designs were involved in the modelling, and the number of samples and the corresponding percentages can also be observed in the table. It can be seen that the training set covered all categories of design parameters with a roughly even distribution. Thus, the dataset was appropriate for the development of the predictive model. Figure 2: Research framework for the current study. Apart from the design parameters, the distribution of the quality outcomes of the sound of the design was included to enhance the comprehension of the data. Instead of the numerical values of the measurement, the sound quality indicators were classified with respect to levels so that a comparable scale could be deduced. The corresponding levels of sound performance and explanations can be observed in Table 2. The levels were assigned values to represent the design's excellence. For instance, the THD was divided into seven levels, while the assigned value's size indicated the performance scale's preference. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters of the Design \({}^{\star}\)** & **Possible Options** & **Assigned Values** & **N** & **\%** \\  & & & **(Total = 388)** & **(Total = 100\%)** \\ \hline Type & Dynamic Unit/Moving Coil & 1 & 196 & 50.52\% \\ (Primary Unit) & Balanced Armature Unit & 2 & 90 & 23.20\% \\  & Planar Magnetic Unit & 3 & 102 & 26.29\% \\ \hline Magnet Type & N35 Grade & 1 & 119 & 30.67\% \\ (Primary Unit) & N40 Grade & 2 & 114 & 29.38\% \\  & N45 Grade & 3 & 85 & 21.91\% \\  & N45 Grade & 4 & 70 & 18.04\% \\ \hline Voice Coils & Copper Wire & 1 & 161 & 41.49\% \\ (Primary Unit) & Aluminum Wire with Copper Covered & 2 & 106 & 27.32\% \\  & Silver Wire & 3 & 121 & 31.19\% \\ \hline \multirow{3}{*}{Diaphragm (Primary Unit)} & Polyethylene Terephthalate & 1 & 141 & 36.34\% \\  & Polyethene Napththolate & 2 & 98 & 25.26\% \\ (Primary Unit) & Polyetheretherketone (PEEK) & 3 & 77 & 19.85\% \\  & PEEK + Polyurethane & 4 & 72 & 18.56\% \\ \hline \multirow{3}{*}{Type (Secondary Unit)} & No Secondary Unit Included & 0 & 86 & 22.16\% \\  & Dynamic Unit/Moving Coil & 1 & 83 & 21.39\% \\ (Secondary Unit) & Balanced Armature Unit & 2 & 100 & 25.77\% \\  & Planar Magnetic Unit & 3 & 119 & 30.67\% \\ \hline \multirow{3}{*}{Magnet Type (Secondary Unit)} & No Secondary Unit Included & 0 & 86 & 22.16\% \\  & N35 Grade & 1 & 80 & 20.62\% \\ \cline{1-1}  & N40 Grade & 2 & 74 & 19.07\% \\ \cline{1-1}  & N45 Grade & 3 & 78 & 20.10\% \\ \cline{1-1}  & N45 Grade & 4 & 70 & 18.04\% \\ \hline \multirow{3}{*}{Voice Coils (Secondary Unit)} & No Secondary Unit Included & 0 & 86 & 22.16\% \\ \cline{1-1}  & Copper Wire & 1 & 127 & 32.73\% \\ (Secondary Unit) & Aluminum Wire with Copper Covered & 2 & 71 & 18.30\% \\ \cline{1-1}  & Silver Wire & 3 & 104 & 26.80\% \\ \hline \multirow{3}{*}{Diaphragm (Secondary Unit)} & No Secondary Unit Included & 0 & 86 & 22.16\% \\ \cline{1-1}  & Polyethylene Terephthalate & 1 & 83 & 21.39\% \\ \cline{1-1}  & Polyethene Naptholate & 2 & 75 & 19.33\% \\ \cline{1-1}  & Polyetheretherketone (PEEK) & 3 & 75 & 19.33\% \\ \cline{1-1}  & PEEK + Polyurethane & 4 & 69 & 17.78\% \\ \hline \hline \end{tabular} \({}^{\star}\) The earphone was assumed to have at most two drivers, while the options of the design were finite.  
\end{table} 
Table 1: Summary of the possible options of the design in the simplified formulation, while the assigned values in the proposed formulation are also included in order to highlight the correspondence between the options and the represented values in the formulation.