Sounds of blue whales were recorded from US Navy hydrophone arrays in the North
Atlantic. The most common signals were long, patterned sequences of very-low-frequency
sounds in the 15–20 Hz band. Sounds within a sequence were hierarchically organized into
phrases consisting of one or two different sound types. Sequences were typically composed
of two-part phrases repeated every 73 s: a constant-frequency tonal “A” part lasting
approximately 8 s, followed 5 s later by a frequency-modulated “B” part lasting …