## 4 Conclusions  
In the current study, preferable birdsong and insect song stimuli and the dominant physical parameters related to preferences were investigated. The results indicated that bird and insect songs that are commonly found in Japan, _Horornis diplone_ and _Teleogrylus emma_, were rated as the most preferable bird and insect song stimuli. Preferences for birdsong and insect song stimuli may differ in other countries. The maximum peak amplitude of the ACF, \(\varphi_{1}\), and loudness were significant predictive variables for both birdsong and insect song stimuli. Pitch variables related to the parameters of delay time of the maximum peak of the ACF, \(\tau_{1}\) for birdsong and the width of the first decay of the ACF, \(\mathrm{W}_{\varphi(0)}\), for insect song, were significant parameters. Spectral rolloff, which measured the skewness of the spectral shape, provided the largest contribution to subjective preference for insect songs. These findings gave insights into the future design of attention signals to utilize physical parameters, such as \(\varphi_{1}\), \(\tau_{1}\) and loudness. For example, attention signals with higher \(\varphi_{1}\) and lower \(\tau_{1}\) and loudness values are preferable in equal \(L_{Aeq}\) conditions.  
The current findings suggest that it may be useful for future research to develop attention signals that are preferable, even when presented at high SPLs. Current security guidelines in Japan recommend the use of simulated birdsong stimuli as attention signals rather than real birdsong stimuli. Based on the current results, combining the advantages of birdsong A and insect song B may enable the development of a more pleasing sound. In addition, the present findings need validation by participants who are visually impaired, because there were no visually impaired people among the participants.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Predictive Parameter** & **Standardized Partial Regression Coefficients** \\ \hline Birdsong & b\({}_{1}\), \(\varphi_{1}\)** & \(0.27\) \\  & b\({}_{2}\), \(\tau_{1}\)** & \(-0.23\) \\  & b\({}_{3}\), loudness ** & \(-0.45\) \\  & b\({}_{4}\), entropy ** & \(0.21\) \\ \hline Insect song & i\({}_{1}\), \(\varphi_{1}\)** & \(0.54\) \\  & i\({}_{2}\), \(\mathrm{W}_{\varphi(0)}\)** & \(-0.63\) \\  & i\({}_{3}\), \(\tau_{e}\)** & \(-0.09\) \\  & i\({}_{4}\), loudness ** & \(0.33\) \\  & i\({}_{5}\), spectral rolloff ** & \(-1.01\) \\ \hline \end{tabular}
\end{table}
Table 4: Significant predictive parameters and standardized partial regression coefficients in multiple linear regression analyses for birdsong and insect song. Asterisks indicate the level of significance, i.e., **\(p<0.01\), * \(p<0.05\).  
**Author Contributions:** Conceptualization, Y.S.; methodology, Y.S.; formal analysis, H.K.; investigation, Y.S.; resources, Y.S.; data curation, Y.S. and H.K; writing--original draft preparation, Y.S.; writing--review and editing, Y.S.; visualization, Y.S.; project administration, Y.S.; funding acquisition, Y.S. All authors have read and agree to the published version of the manuscript.  
**Funding:** This work was partly supported by a Grant-in-Aid for Scientific Research (B) (Grant No. 18H03324) from the Japan Society for the Promotion of Science.  
**Acknowledgments:** We thank [PERSON], MSc, from Edanz Group ([[https://en-author-services.edanzgroup.com/](https://en-author-services.edanzgroup.com/)]([https://en-author-services.edanzgroup.com/](https://en-author-services.edanzgroup.com/))) for editing a draft of this manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.