Fig. 2

Waveforms, spectrograms, and correlation coefficients. a Examples of the observed waveforms at 0.3–80 Hz (a1, a3) and 5–80 Hz (a2). The line colors indicate the microphones (blue, A1; red, A2; yellow, A3; purple, SI). The lower darker blue, red, and yellow lines in (a1–a3) show the waveforms recorded by the collocated MEMS microphones. b–f The waveforms at 0.3–80 Hz (b1, b2), the spectrograms (c, d), and the cross-correlation coefficients (CCs) (e, f) acquired during the array observation on the left (b1, c, e) and with the SI observation on the right (b2, d, f). The black dashed line in b1 indicates the start time for b2. The black triangles in b1 and b2 mark the origin times of a1, a2 and a3, respectively. The displayed spectrograms are of A1 (c) and SI (d), the array CC between A1 and A3 (f), and the SI-array CC between SI and A1 (e) using the waveforms at 5–80 Hz. The arrowheads on the right axis in e and f indicate the lag times between the microphones assuming the ISC source (red) and IWC source (green). The waveforms, spectrograms, and CCs for the other microphones are presented in Additional file 1: S4