Fig. 3. Variation in magnitude and waveform in a click sequence recorded from an untagged whale. (A) Peak envelope level of each click. Clicks with a single pulse are indicated by open dots; solid dots indicate clicks with two or more pulses. (B) Absolute value of the normalized cross-correlation functions of an on-axis click with two clicks from the sequence (indicated in A by triangles). (C) Normalized cross-spectral magnitude of clicks in a second sequence. Each spectrum is the Fourier transform of a click matched-filtered with an on-axis click. The spectra are displayed in ascending order of peak envelope level, i.e. the strongest clicks in the sequence are shown to the left. The dots on the right-hand side have the same interpretation as for A. Note the smooth cross-spectra of the strong, mono-pulsed clicks while weaker multi-pulsed clicks have more variable spectra.