Pulsed sounds are an interesting example of complex biological sounds, frequent in cetaceans' vocalizations. A two-group classification of these sounds is proposed: tonal or non-tonal. Through two simple mathematical models, it is shown that this classification can lead to better techniques for measuring the pulse rate. This classification is thus useful for improving measurement accuracy, but can also help in formulating hypotheses regarding mechanisms of sound production. This method of classification is applied to south Pacific blue whale vocalizations and it is found that the pulse rate corresponds to the fundamental frequency (not expressed in the spectrum) of the song. Thus, the hypothesis that the sound is produced by only one organ and then filtered by the body of the giant is reinforced.