As it was shown earlier, normal hearing in man is characterized by selective sensitivity to amplitude modulation (AM) of sound within the frequency region of 4--8 cps, which corresponds to the most probably AM frequency of the speech signal. Since this phenomenon is presumably due to a constant training for speech perception, it has been investigated in ontogenesis, i.e. starting from 7--8-year children, in whom the process of speech learning is especially intensive. The results obtained indicate that the auditory threshold in children at the age of 7--8, 9--10 and 13--14 years exhibits evident preferable sensitivity in the region of 4--8 cps. Besides, it was found that although the thresholds to 1--2 and 32--64 cps frequencies decrease with ageing, no decrease of the sensitivity takes place within the region of 4--8 cps. Similar data were obtained during special or professional (musical) training of the subjects.