| (< 5 ms) | us | ultrashort | Short syllables | Syllables < 91 kHz that are < 5 ms regardless of the shape | [39] |
| (< 10 ms) | s | short | Syllables < 91 kHz that are < 10 ms regardless of the shape | [26] |
 | f | flat | Simple syllables | Syllables < 91 kHz with < 5 kHz frequency modulation | [26] |
 | d | down | Syllables < 91 kHz that decreases in frequency for > 5 kHz | [26] |
 | up | up | Syllables < 91 kHz that increase in frequency for > 5 kHz | [26] |
 | u | u-shaped | Syllables < 91 kHz that first decrease, and then increase in frequency for > 5 kHz each | [26] |
 | ui | u-shaped inverted | Syllables < 91 kHz that first increase, then decrease in frequency for > 5 kHz each | [26] |
 | c | complex | Complex syllables | Syllables < 91 kHz that contain ≥2 directional changes in frequency and > 5 kHz modulation of frequency | [26] |
 | c2 | complex 2 | Syllables < 91 kHz consisting of 2 elements separated by 1 frequency-jump without time separation | [20] |
 | c3 | complex 3 | Syllables < 91 kHz consisting of 3 elements separated by 2 frequency-jumps without time separation | [20] |
 | c4 | complex 4 | Syllables < 91 kHz consisting of 4 elements separated by 3 frequency-jumps without time separation | Added category for our classification |
 | c5 | complex 5 | Syllables < 91 kHz consisting of ≥5 elements separated by ≥4 frequency jumps without time separation | Added category for our classification |
 | h | harmonic | Syllables < 91 kHz that have an harmonic element | [26] |
| (> 91 kHz) | uh | ultra high | All syllables > 91 kHz regardless of the shape | [23] |
 | uc | unclassified | Syllables that do not fit any other of the 14 categories due to background noise or that lack clearly defined spectrographic features (shape) | [40, 41] |
