*3.1. Automatic Measurement of Relative Sensitivity and Frequency Response of Vibration Sensors Suitable for Working in the Low Magnetic Field Environment*

The preliminary measurements of the sensitivity and the frequency response of the vibration sensors were done using the vibration exciter ESE 201 on which the sensors were mounted. The Audio Precision System One (AP S1) with two programmable input and output channels was used as a signal generator for the exciter, as well as an input for simultaneous measurement of electrical signals from the sensors. The AP S1 output voltage for the exciter and the AP S1 input signal from the measured sensor were checked in parallel by the digital oscilloscope Rigol DS1102E. Three types of vibration sensors with the main element based on the piezoelectric principle were measured and tested:


The vibration sensors were measured and compared by two types of parameters:


Dependence of all four sensors' sensitivities on the excitation voltage can be seen in Figure 1a, and their frequency responses in [dB] in the range of 20 Hz to 2 kHz are depicted in Figure 1b.

**Figure 1.** Graphs of (**a**) measured sensors' sensitivities and (**b**) relative frequency responses [dB] in the range of 20 Hz to 2 kHz; *f*ref =125 Hz; *UexcBa0* =360 mV; *B*a0 = {12.9 (SB-1), 2.45 (HM692), 26.0 (SDT1)} mV/m·s<sup>−</sup>2.
