# Synthesis of Non-Linguistic Utterances for Sound Design Support Using a Genetic Algorithm
## 4 Discussion  
This work introduced a novel method for creating non-speech sounds, for use in social robots or other applicable devices, using a genetic algorithm (GA), and validated by a random forest model, and corroborated through human ratings. The approach uniquely combines MIDI-encoded sound generation with machine learning to dynamically produce sounds that social robots and other devices can use to express a wide range of emotions effectively. Unlike previous methods, which largely relied on manual creation or were limited in emotional range, our GA-based system demonstrated the capability to explore a broad spectrum of valence and arousal, generating sounds that humans consistently rated as expressive of specific emotions. This method's ability to produce a wide range of sounds rapidly and its validation through a machine learning model and human subject ratings make it uniquely applicable to sound design for human-robot interaction. The clusters identified through the analysis exhibit a diverse emotional spectrum, spanning from Activated/Alert to Sadness, reflecting the GA's ability to capture complex emotional states through sound. These clusters were carefully examined for their distribution, and the application of [PERSON]'s Honest Significant Difference test provided insights into the significant differences between the clusters, further elucidating the method's capacity to distinguish between nuanced emotional states. * Activated/Alert (Cluster 1): Sounds in this cluster are characterized by higher arousal and moderate-to-high valence, indicating a state of alertness or activation. This \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Cluster (Colour)** & **Min Valence** & **Max Valence** & **Min Arousal** & **Max Arousal** & **Emotion Label** \\ \hline Cluster 1 (blue Xs) & 0.43 & 0.55 & 0.64 & 0.82 & Activated/Alert \\ Cluster 2 (Maroon Triangles) & 0.24 & 0.40 & 0.54 & 0.65 & Tense/Fearful/Angry \\ Cluster 3 (Green Dots) & 0.40 & 0.64 & 0.46 & 0.60 & Neutral/Calm \\ Cluster 4 (Pink Squares) & 0.52 & 0.81 & 0.61 & 0.85 & Happy/Excited \\ Cluster 5 (Beige Pentagons) & 0.25 & 0.33 & 0.32 & 0.52 & Sad \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Cluster analysis of emotion labels based on valence and arousal. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Statistic/k** & **Inv Error V** & **Inv Error A** \\ \hline Levene test stat & 1.220 & 1.586 \\ Levene \(p\)-value & 0.310 & 0.187 \\ ANOVA test stat & 17.719 & 7.329 \\ ANOVA \(p\)-value & \(2.81\times 10^{-10}\) & \(4.89\times 10^{-05}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistical tests summary. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **Activated/Alert** & **Tense/Fearful/Angry** & **Neutral/Calm** & **Happy/Excited** & **Sad** \\ \hline Activated/Alert & - & 1 & 0 & 0 & 1 \\ Tense/Fearful /Angry & 1 & - & 1 & 0 & 0 \\ Neutral/Calm & 0 & 1 & - & 0 & 1 \\ Happy/Excited & 0 & 1 & 0 & - & 1 \\ Sad & 1 & 0 & 1 & 1 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Relationship matrix of emotional states. cluster's sounds effectively evoke feelings of attentiveness or readiness, resembling emotions that may be used in scenarios requiring immediate attention or action from the robot or the user. * Tense/Fearful/Angry (Cluster 2): This cluster, with lower valence and medium arousal, captures emotions associated with tension, fear, or anger. The method's ability to generate sounds that distinctly convey these emotions is crucial for situations where a robot may need to express urgency, discomfort, or disapproval without resorting to language. * Neutral/Calm (Cluster 3): Representing a balanced emotional state with moderate valence and arousal, the sounds in this cluster can be seen as calm or neutral. This demonstrates the GA's capability to produce sounds for everyday interactions where a serene or neutral emotional tone is desired, facilitating a comfortable ambiance for interaction. * Happy/Excited (Cluster 4): High valence and arousal mark this cluster, indicating joy or excitement.