In the voice of man two registers can be distinguished: (1) the modal voice, (2) the falsetto. The falsetto is characterized in such a way that the glottis does not close during phonation, with the vocal chords swinging only at the edges and thus reaching higher frequencies. During the change of register into falsetto a discontinued jump occurs to a fundamental frequency which, on average, lies markedly above the range of the modal voice. Typical examples are: yodels, ventriloquists, and singers (e.g. counter tenor). In primates, there is a relation between the "basic modal voice'' t= functionally not frequency modulated) and the body mass (Y (Hz) = 1443.02 * X^-0.62302) (X = kg). A positive correlation between frequency and intensity exists in the speaking voice, whereas in the singing voice parameters can be influenced separately. Homo sapiens is a primate: modal voice and falsetto arose during the process of evolution of the voice of mammals; the speaking voice evolved from the modal voice in the context of human evolution. Only in special cases is falsetto used in speech (e.g. ventriloquists). In particular situations of excitement the crying of babies changes into falsetto; the readiness for a change of register, however, differs considerably from one individual to another. The change into falsetto is very conspicuous for the receiver and enhances the readiness to react. Our comparative studies of mammals showed that the change of register occurs in various taxa and is widespread. Since falsetto has so far not been subject to investigation in bioacoustic research there are no studies dealing with the change of register.