Fig. 2.

Examples of the different criteria for dividing a spectrogram into units. (A) Separating units by silent gaps is probably the most commonly used criterion. (B) An acoustic signal may change its properties at a certain time, without the presence of a silent “gap”, for instance becoming harmonic or noisy. (C) A series of similar sounds may be grouped together as a single unit, regardless of silent gaps between them; a chirp sequence is labelled as C. (D) A complex hierarchical structure to the sequence, combining sounds that might otherwise be considered fundamental units.