An investigation into the sensitivity of human hearing to perception of amplitude modulation of sound discloses a statistically reliable threshold minimum for modulation frequencies in the 4-6 Hz band. It is hypothesized that this phenomenon is evolutionarily caused by the adjustment of the human hearing system to the perception of speech, the amplitude modulation of which is the result of the articulation of syllables. An acoustic analysis carried out of the speech of different speakers confirmed the presence of a statistically reliable maximum in the amplitude modulation of the audio flow precisely in the 4-6 Hz band. Thus selectively increased sensitivity of hearing to the perception of these amplitude modulation frequencies increases the reliability and noise stability of the speech hearing system.