Excerpt from Wizard: Nikola Tesla - The Life and Times of a Genius: Mach's experiments in Wave Mechanics

...in retrospect, it appears obvious that Stumpf's opposition did not stop Tesla from studying Mach's experiments in wave mechanics. Born in Moravia (now the Czech Republic) in 1838, Mach graduated from the University of Vienna in 1860. By 1864 he was a full professor at Graz, and by 1867 he was head of the department of experimental physics at Prague, with four books and sixty-two articles to his credit. Influenced by the research in psychophysics of Fechner in Graz and Ludwig von Helmholtz in Berlin, Mach studied the workings ofthe human eye, along with his Prague colleague "famed physiologist and philosopher" Jan Purkyne. Both the eye and ear collected information from the outside world, analyzed it, and transferred it, via electrical impulses in the nerves, to the respective processing centers in the brain. This traditional line of research had been taken by many other well-known scientists, including Isaac Newton, Johann von Goethe, and Herbert Spencer, all favorites of Tesla's.
In his laboratory, Mach had constructed a "famous instrument known as a wave machine. This device could make progressive [and standing] longitudinal [and] transverse waves..." Mach could display a number of mechanical effects with these acoustic waves and "demonstrate the analogy between acoustic and electromagnetic events." By this means, the "mechanical theory of the ether" could also be demonstrated.
By studying acoustical-wave motion in association with mechanical, electrical, and optical phenomena, Mach discovered that when the speed of sound was achieved, the nature of an air flow over an object changed dramatically. This threshold value became known as Mach I.
Mach also wrote on the structure of the ether and hypothesized that it was inherently linked to a gravitational attraction between all masses in the universe. Influenced overtly by Buddhist writings, which no doubt filtered down to esoteric discussions by the university students, Mach could hypothesize that no event in the universe was separate from any other. "The inertia of a system is reduced to a functional relationship between the system and the rest of the universe." This viewpoint was extended to the relationship of mental events to exterior influences. Like Stumpf, he agreed that every mental event had to have a corresponding physical action.
Since Mach's writings so closely parallel Tesla's later research and philosophical outlook, Mach seems a curious omission from Tesla's published writings.

Page 19 From Wizard- Nikola Tesla - the life and times of a genius