Studies on the sensitivity of man's hearing to the reception of amplitude modulation (AM) of the sound reveal statistically reliable threshold minimum in the region of 4-6 c. p. s. It is suggested that this phenomenon is evolutionary conditioned by tuning of human auditory system on the perception of speech, in which AM results from articulation of the syllables. The acoustical analysis of several speech extracts demonstrated statistically reliable maximum of AM of the sound flow just within 4-6 c. p. s. Therefore, the selective sensitivity of hearing to perception of these AM frequencies increases the safety and interference resistance of the system "speech-hearing".