SOME TIME ago, it came to our notice that the airport personnel of jet-propelled planes ("Vampires") after a comparatively short time were affected with hearing disorders, such as impaired hearing, buzzing and diplacusis. These lesions seem to have arisen in spite of ear defenders introduced into the meatus and working on the principle of low-pass filters developed in our experiments on noise and report traumas. In order to study the special properties, if any, of jet-engine noise, sound analyses were performed on flying "Vampires" as early as the summer of 1946. A peculiar feature of those experiments was the impossibility of detecting the origin of the characteristic whistling sound of jet planes. It was first supposed to be formed subjectively within the ear, like the stroke sound of bells, only further measurements, undertaken in May, 1950, brought full explanation of the sound produced by "Vampire" planes and its cause. After