Fig. 11

Length dependence on the fundamental resonant frequency of a pipe. a Relationship between pipe length (L) and resonant frequency (\(f_0\)) with variable sound speed inside the pipe (\(c_{\mathrm{air}}\); 100–600 m/s) calculated using Eq. (3) with a fixed radius of the pipe (\(a = 25\,\text {m}\)). Red solid and dashed lines show the velocity of 338 m/s used in our FDTD calculations (without and with topographic data, respectively). The peak frequency of the observed infrasound records is shown by a gray horizontal bar (0.51–0.52 Hz at 22:23:00–22:23:10 on March 25, 2015). b Dependency of variable size of pipe radius (a). The red line is the same as that in a: \(a= 25\,\text {m}\), \(c_{\mathrm{air}}= 338\,\text {m/s}\), and without topographic conditions. c Effect of pipe geometry from a simple straight pipe to a conical pipe. The bottom radius of the pipe changes from 25 m (red line) to 5 m, whereas the radius of the other side (top portion) is fixed at 25 m (a)