 Of all the mixed-mathematical disciplines in which Mersenne took an interest, music was undeniably the one to which he devoted the most effort and passion. His 1623 Quaestiones already included a long musicological digression on the therapeutic power attributed to the music of the Hebrews, and generally to the music of the Ancients, whose harmonics he dreamed of restoring. The idea was taken up and developed in the 1627 Traité de l’harmonie universelle, published under the transparent pseudonym of “Sieur de Sermes”. The science of intellectual music and the understanding of the proportions that govern nature, lost through negligence, must be restored and linked to the science of “sensual music”, which addresses our hearing but which, separated from the former, is no longer able to fulfill its true function: elevating the senses above material objects, rather than linking them together. Mersenne’s great work on musical theory, the eight books of Harmonie Universelle, was published from 1636 to 1637. It presents a great many editorial variants, as Mersenne never ceased to work on it, annotating his own copy. The important “Traité de la nature du son”, which constitutes the first part of the work, focuses on the study, undertaken in a mechanistic spirit, of acoustic quantities, their physical nature and their effects on physiology and passions. Mersenne, drawing on Isaac Beeckman, establishes experimentally the laws connecting the vibration, the length and tension of the strings, making an important contribution to acoustic science.