Speech intelligibility in aircraft noise as a function of altitude

Few studies have addressed physiological effects of altitude and noise combined, although these two factors are inherent parts of all aviation. Altitude effects on hearing function have mainly been studied using gas mixtures with reduced oxygen content. The results have been inconclusive. HYPOTHESIS: The present study was designed to assess the effect of altitude on speech intelligibility in aircraft noise. The primary hypothesis was that exposure to altitude would produce a detrimental, hypoxic effect on speech intelligibility in noise. METHODS: Eight male subjects with normal hearing were fitted with an aviation headset specially adapted for use with the audiometer. Pure-tone audiometry, as well as speech audiometry in noise, were performed at 0, 10,000, 13,000, and 16,000 ft simulated altitudes in a hypobaric chamber. The four test altitudes were performed double blind with respect to audiometry operator and test subject. Arterial blood gases where measured using an intra-arterial catheter. Tympanometric measurements verified full middle ear equilibration. Noise levels where monitored and logged throughout all experiments. RESULTS AND CONCLUSION: A substantial increase in speech intelligibility in noise due to altitude was evident in our study. The physical effect of barometric pressure on noise causing an increased signal-to-noise ratio was found to greatly outweigh any possible hypoxic detrimental effect.