Fig. 9
From: Monochromatic infrasound waves observed during the 2014–2015 eruption of Aso volcano, Japan
Results of FDTD numerical calculation. a Simulated waveforms to be observed at ACM without consideration of topography (see Fig. 8b–f). The results obtained using four different pulse widths of the source (Fig. 8a) and four conduit lengths are shown. Radius of the conduit is fixed as 25 m for all cases. b Simulated waveforms of the FDTD calculation including the topographic data of Aso volcano. c The peak frequency of the 10-s synthetic waveforms as shown in a for the no-topography calculation. The observed frequencies for the \(>10\,\text {m}\) conduit lengths are fit to theoretical values estimated using Eq. (3) (red line). d The frequencies of the waveforms in b using the Aso DEM are 15% smaller (red line; \(>30\,\text {m}\) length) than the theoretical expectation at a simple pipe with a flat topography (broken red line; same line in c)