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

Abstract

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 () may be used effectively to evaluate the effects of nighttime noise exposure. However, the efficacy of using has only been confirmed in a specific setting; therefore, the reliability of in other situations should be investigated by applying it in epidemiologic studies. In this study, we aimed to obtain an alternative equation for calculating from existing noise indices, given that detailed information on fluctuations in sound levels, needed to calculate according to the definition, is not readily available.

Materials and Methods:: We examined statistical relationships among noise indices namely , the number of noise events above 60 and 70 dB (N₆₀ and N₇₀), 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:: showed a very strong correlation with N₆₀ and N₇₀ but not with L_night. Among regression equations, an equation representing N₆₀ and the interaction between N₆₀ and L_night, which is equivalent to the product of N₆₀ and a linear expression of L_night, showed the highest prediction capability.

Conclusion:: We obtained a regression equation to calculate from N₆₀ and L_night. Although this alternative equation for 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.

Keywords: Aircraft noise; neurophysiology; night equivalent sound level; nighttime noise; number of noise events.

Figure 1

A map around the Kadena military airfield. Among 21 noise monitoring stations that have been set up by the local governments, we employed 10 stations (white circles) near the extended lines of the two runways to investigate night-time aircraft noise exposure

Figure 2

Correlations of noise indices (N₆₀, N₇₀, L_night, and calculated from the sound level recorded at the 10 monitoring stations around the Kadena military airfield. The correlation coefficients (r) were also shown

Figure 3

Fitting results of the multivariate regression equation of with N₆₀ and interaction between N₆₀ and L_night (Eq. 3). The correlation coefficient (r) was also shown

Figure 4

The relative risk of the onset of motility due to night-time noise estimated from L_night and N₆₀. N₆₀ was set to 10, 25, 50, 100, and 150. The relative risk in L_night ≤ 43.2 dB was shown as 1.