Table 2. Definitions of the 15 selected acoustic parameters.
| Parameters | Definitions |
|---|---|
| CALL LEVEL | |
| Scream duration (s) | Duration of one scream, described as a continuous vocal unit along a time axis on the spectrogram that is not interrupted by more than 0.03s of silence |
| Peak frequency (kHz) | Frequency taken from the spectral slice at which maximum acoustic energy occurs in the entire scream |
| Coefficient of frequency variation * | Coefficient of frequency variation representing the range of frequency variation around the mean |
| Coefficient of frequency modulation | Coefficient of frequency modulation representing frequency changes over time |
| Absolute transition onset (Hz) | Frequency at which maximum acoustic energy occurs at the middle of the scream minus the one occurring at the beginning of the scream |
| Absolute transition offset (Hz) | Frequency at which maximum acoustic energy occurs at the end of the scream minus the one occurring at the middle of the scream |
| Frequency quartile 50 (Hz) | Frequency quartile that divides the scream into two frequency intervals of equal energy (corresponding to 50–50%) |
| Inter-quartile range (Hz) | Inter-quartile range representing the difference of the frequency quartile of 25% and 75% (corresponding to Q25-Q75) |
| Shannon entropy | Measure of the uniformity of the power spectrum, with white noise having an entropy value of 1 and pure tone having an entropy value of 0 |
| BOUT LEVEL | |
| Bout duration (s) * | Duration of one bout, described as a specific stage of a conflict in which the social role of signallers and the conflict severity is stable |
| Number of screams | Total number of screams emitted by the signaller within a single bout |
| Average scream duration (s) | Average duration of all screams produced by one individual within a bout |
| Scream intervals (s) * | Average duration of the intervals between the screams produced by one individual within a bout |
| Scream rate (number of screams/s) | Rate of screams produced by one individual within a bout delivered per time unit |
| Percentage of screams with NLP (%) | Percentage of screams emitted in a bout containing at least one of the four following forms of NLP: frequency jumps, sub-harmonic segments, bi-phonation or chaotic segments |
* Parameters excluded due to high correlations or failure to reach symmetrical distribution, leading to analyses using 12 acoustic parameters (eight at the call level and four at the bout level; S2 Appendix)