These spectrograms illustrate the most common blue whale song type for the year shown in the upper left corner of each at the labeled locations around the world. Some of the year to year differences seen in songs of the same type can be attributed to how clear a recording was available (signal to noise ratio).
Many baleen whale sounds are at frequencies below the optimal range of human hearing. For this reason and to better quantify differences in whale calls, spectrograms are used to visually display sounds. This visual display of sounds also allows recordings to be played back at many times normal speed, allowing an effective visual review of a year long recording with just a few weeks work.