The Effect of Altitude on Absolute Hearing Threshold Levels

Abstract

The present study investigates the effect of hypobaric environments on absolute hearing thresholds for pure tones at octave frequencies between 250 Hz and 8 kHz. Absolute hearing thresholds were obtained for ten subjects at ambient pressure (732 mmHg) and at an altitude of 6500 feet (582 mmHg), relative to the laboratory. Subjects were decompressed in a hypobaric chamber at a rate of 500 ft./min. and were subjected to the low pressure environment for one hour before being compressed to a normobaric environment, at a rate of 300 ft./min. Absolute hearing threshold levels obtained at altitude, after the one hour exposure to the hypobaric ambient and on descent to normobaric conditions, were compared with scores obtained prior to the hypobaric exposure. In order to compare the frequency averaged threshold shifts at octave frequencies of 250 Hz, 500 Hz and 1 kHz, for the altitude and normobaric conditions, the audiometer was calibrated at ambient pressures of 732, 582 and 512 mmHg. The results of the study indicate a slight increase in sensitivity of the audiometer headphones with altitude and an insignificant difference between the mean frequency averaged threshold shifts for the pre-exposure test and the three experimental conditions (at the onset of hypobaria, after a one hour exposure to the altitude and on descent to surface).