Periodicity in ambient noise and variation based on different temporal units of analysis

The Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) has been continuously monitoring ambient sound levels in the Indian Ocean at Diego Garcia since 2000. Using different temporal units of analysis, the variation in the low-frequency ambient sound level is examined. Exploring the influence of the time unit of analysis is important because sound sources contribute to the sound level variability on different temporal scales. For example, variation in the sound level caused by the distant ship traffic may not be detectable over short periods of time, such as an hour or a minute. For daily averages examined over a year, the contribution of ship traffic in the overall sound level variance could be identified using an autocorrelation analysis. Such an analysis would exclude the possibility of detecting some shorter sound level variations such as those caused by vocalizations from marine mammals, which are more easily detected on a minute-to-minute scale. This study will examine different intervals of periodicity relating to each of the separate sources: wind, ice, animal activity, ship traffic, and other anthropogenic contributions. Determining the variability in ambient sound over multiple temporal periods is critical to understanding how sound sources contribute to the overall ambient sound level.