Next experiment was to test built amplified sensor signal’s circuit. Results weren’t stunning. Seems like it’s precise calibration requires much more time. After it Furqan helped to test geophone’s sensitivity regarding to levels of low frequencies. Speaker was playing inaudible audio. The frequency was always the same (25hz). Even though we couldn’t hear anything it was obvious from led’s bar that decibels are going higher (test is on the video). p.s. It was 2nd test , not 46th!