Calculation of the physiologically developed nighttime noise index from existing noise indices

BACKGROUND: The effects of noise on sleep and health have been evaluated in earlier studies using noise indices chosen on the basis of practical considerations and not on the physiologic mechanisms of sleep disturbance due to noise exposure. We investigated the neurophysiologic mechanisms of sleep and found that the arithmetic mean of the sound levels above a threshold ([Image: see text]) may be used effectively to evaluate the effects of nighttime noise exposure. However, the efficacy of using [Image: see text] has only been confirmed in a specific setting; therefore, the reliability of [Image: see text] in other situations should be investigated by applying it in epidemiologic studies. In this study, we aimed to obtain an alternative equation for calculating [Image: see text] from existing noise indices, given that detailed information on fluctuations in sound levels, needed to calculate [Image: see text] according to the definition, is not readily available. MATERIALS AND METHODS: We examined statistical relationships among noise indices namely [Image: see text] , the number of noise events above 60 and 70 dB (N(60) and N(70)), and the night equivalent sound level (L(night)). The study area was around the Kadena military airfield, where the sound levels were recorded at noise monitoring stations. RESULTS: [Image: see text] showed a very strong correlation with N(60) and N(70) but not with L(night). Among regression equations, an equation representing N(60) and the interaction between N(60) and L(night), which is equivalent to the product of N(60) and a linear expression of L(night), showed the highest prediction capability. CONCLUSION: We obtained a regression equation to calculate [Image: see text] from N(60) and L(night). Although this alternative equation for [Image: see text] is only applicable for military aircraft noise, it will aid the re-analysis of existing epidemiologic studies as well as further investigations on the relationship between noise exposure and health effects.