## 2 Materials and Methods  
### Subjective Preference Tests  
Since the guidelines [1] recommend a duration of 5 s or less for the sound signals, birdsongs and insect songs with one phrase less than 5 s were selected for subjective preference tests. We tested 18 types of birdsong that were recorded using a dummy-head microphone (KU100, Nuemann, Berlin, Germany) at a sampling rate of 48 kHz and a sampling resolution of 24 bits (WILD BIRDS, Victor, Tokyo, Japan). The birdsong stimuli are summarized in Table 1. Figures 1 and 2 show the temporal waveforms and spectrograms of the 18 birdsong stimuli. We tested 16 types of insect song that were recorded using a linear pulse code modulation recorder microphone (LS-10, OLYMPUS, Tokyo, Japan) at a sampling rate of 44.1 kHz and a sampling resolution of 16 bits (Japan NatureSounds Guide, UEDA NATURE SOUND, Tokyo, Japan). The insect song stimuli are summarized in Table 2. Figures 3 and 4 show the temporal waveforms and spectrograms of the 16 insect song stimuli. The duration of the stimuli was between 0.4 and 2.0 s. \begin{table}
\begin{tabular}{c c} \hline \hline
**Scientific Name** & **Order** \\ \hline _Halcyon coromanda_ (HC) & Coraciliformes \\ Latham (L) & Cuciliformes \\ _Cuculus caorras_ (CC) & Cuciliformes \\ _Cuculus saturatus_ (CS) & Cuciliformes \\ _Strix uralensis_ (SU) & Strigiformes \\ _Otus scops_ (OS) & Strigiformes \\ _Caprimulgus indicus_ (CI) & Caprimulgiforms \\ _Streptopia orientalis_ (SO) & Columbiformes \\ _Terpsiphone atrocaudata_ (TA) & Passeriforme \\ _Garrulus glandarius_ (GG) & Passeriforme \\ _Porzana fusca_ (PF) & Passeriforme \\ _Parus minor_ (PM) & Passeriforme \\ _Horornis diphone_ (HD) & Passeriforme \\ _Zosterops japonicus_ (ZJ) & Passeriforme \\ _Turdus sibiricus_ (TS) & Passeriforme \\ _Prunella rubida_ (PR) & Passeriforme \\ _Eophona personata_ (EP) & Passeriforme \\ _Emberiza coides_ (EC) & Passeriforme \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Birdsong stimuli used in this study. Figure 1: Temporal waveforms of the 18 birdsong stimuli used. Figure 2: Spectrograms of the 18 birdsong stimuli used. The experiments were conducted in a soundproof and comfortable thermal environment. The stimuli were presented to participants' ears through a headphone amplifier (HDVD800, Sennheiser, Wedemark, Germany) and headphones (HD800, Sennheiser, Wedemark, Germany). All stimuli were presented at an equivalent continuous A-weighted SPL (\(L_{Aeq}\)) of 70 dBA, measured over a time period \(T\), which was the stimulus duration in this experiment. This was because the guidelines [1] recommend that the \(L_{Aeq}\) of the sound signals be at least 10 dB greater than the \(L_{Aeq}\) of the background noise, which is approximately 60 dBA in the train stations of Japan [8]. \(L_{Aeq}\) was verified using a dummy head microphone (KU100, [PERSON], Berlin, Germany) and a sound calibrator (Type 4231, Bruel & Kjaer, Narum, Denmark). Figure 4: Spectrograms of the 16 insect song stimuli used. Figure 3: Temporal waveforms of the 16 insect song stimuli used. Fifteen (eleven males; four females) and sixteen (eleven males; five females) participants with normal hearing and an age range of between 20 and 41 years (median age of 24.5 and 24.4 years and standard deviation of 7.2 and 6.9) took part in the subjective preference for birdsongs and insect songs, respectively. It is considered, from our previous studies [9], that at least ten participants are necessary for the statistical power and generality of the results of these types of psychoacoustic experiments. Informed consent was obtained from each participant after the nature of the study was explained. The study was approved by the ethics committee of the National Institute of Advanced Industrial Science and Technology (AIST) of Japan. [PERSON]'s paired comparison methods [10] were conducted for all combinations of pairs (i.e., 153 pairs (N[N-1]/2, N = 18) for bird songs and 120 pairs (N[N-1]/2, N = 16) for insect songs) of stimuli by interchanging the order and presenting the pairs in a random order.