But the quality of continuity that sound has with respect to noise, which seems instead fragmentary and irregular, is not an element sufficient to make a sharp distinction between sound and noise. We know that the production of sound requires not only that a body vibrate regularly but also that these vibrations persist in the auditory nerve until the following vibration has arrived, so that the periodic vibrations blend to form a continuous musical sound. At least sixteen vibrations per second are needed for this. Now, if I succeed in producing a noise with this speed. I will get a sound made up of the totality of so many noises--or better, noise whose successive repetitions will be sufficiently rapid to give a sensation of continuity like that of sound.
Source: Russolo. English trans. Barclay Brown (1986: 37).