The Effects of High Intensity Noise on Human Equilibrium
Abstract : Five experiments were conducted on the effects of broadband, high intensity noise on human equilibrium. The ability of subject to balance on narrow rails was measured during exposure to the noise; and immediately after termination of the noise. Four different noise conditions were used in each experiment: control, 120, 130, and 140 dB (re. 0.0002 dyne/sq cm). In the first experiment subjects wore earmuffs and earplugs; in the second, only earplugs were worn; and in the third experiment, subjects wore earplugs and one earmuff to produce an asymmetrical exposure. At an ambient level of 140 dB, a detrimental effect was obtained in all three experiments. At lower intensities of noise, there were performance decrements only for the asymmetrical exposure. In the remaining two experiments, conducted after termination of the noise, detrimental effects were obtained for asymmetrical auditory exposure but not for equal auditory exposure. The results of these experiments are interpreted as a possible quantitative demonstration of the direct effect of high intensity noise on the vestibular system.
Vestibular Ataxia and its Measurement in Man
The designation of vestibular ataxia as a distinct phenomenon1 hinges upon reliable evidence of a person’s limited ability to coordinate voluntary muscular movements while walking or standing due to the absence, disturbance or loss of function of the vestibular apparatus per se. One probably distinct and well-known qualitative characteristic of such an ataxia is the difficulty experienced by individuals with extensive loss of labyrinthine functions in walking at night or in darkness. This particular form of vestibular ataxia can be recognized readily in the laboratory or clinic when such individuals attempt to walk tandemly (heel-to-toe) with eyes closed.
LXVIII Hazards of Intense Sound and Ultrasound
Ultrasonic vibration, usually shortened to "ultrasonics," refers to sound waves at frequencies above the upper limit of human hearing. The American Standards Association sets 15,000 cycles per second as the arbitrary dividing line. Sound waves at ultrasonic frequencies differ particularly from audible sound waves in their shorter wave length, in forming relatively sharp sound shadows, and in being attenuated more rapidly as they travel through air. Ultrasonics in nature are relatively weak. Familiar man-made sources are the jingling of small pieces of metal, such as a bunch of keys, and, of more practical importance, the escape of compressed air, the aerodynamic noise of aircraft and the exhausts of jet engines and rockets. It is with the development of the latter to their present tremendous power that ultrasonics have become a matter of practical concern to the otologist and the flight surgeon. Are these highfrequency high-intensity vibrations a threat to the ear, or to any other organ of the body? And if the ultrasonics are harmless, what about audible sound at the very high intensity levels of 150 db or higher that have been measured near certain modern power plants for airplanes? Fortunately, as far as general effects from ultrasonics in air are concerned, man has an important protection in the reflecting power of his skin. At 6000 cycles per second, for example, human skin absorbs only about 0.2% of the acoustic energy that falls on it. Absorption is even less at higher frequencies. These values have been determined experimentally for small areas of skirr' and certainly give the correct order of magnitude. Furthermore, they probably repre-