Use of Ambient Noise: From Spectral Amplitude Variability to H/V Stability

The study of the variation over time of both spectral amplitudes and H/V curves, has been performed on three different sites, two close to cities and one in the countryside, during periods varying from week to over a month. It demonstrates the robustness of the H/V technique to give consistent peak frequency values. In particular, H/V peak frequencies, either fundamental (f0) or natural (fx, x•1), are not affected by weather nor the level of human activity. However, while fundamental H/V peak amplitudes are stable, they proved rather unstable for natural (secondary) peak. Spectral amplitude curves are very variable but follow human activity cycles from week-week end and day-night variations down to a very small scale, such as lunch breaks. Finally, the frequency limit between anthropic noise and natural noise, commonly taken at 1 Hz, is not straightforward and is varying from site to site from 0.7–0.8 Hz up to 2–3 Hz.