## 3 Results
### Fully Distributed Tracking of Pressure Waves in Pipes
(**a**) Signal acquired at 0 bar overpressure with white points indicating automatically detected signal onset of the pressure wave (yellow points are discarded as they belong to signal features caused by acoustic waves in steel) and fitted slope (white solid line); (**b**) signal acquired at 2 bar initial overpressure with white points indicating automatically detected signal onset of the pressure wave and white solid and dashed lines indicating fitted and manually plotted slopes before and after deflagration-to-detonation transition (DDT), respectively. Yellow points and yellow solid line show the signal onset and the fitted slope of signal features due to acoustic wave propagation in the pipe material. Figure 9: Temporal excerpt of data acquired using a fully distributed fiber-optic sensor configuration after igniting a gas mixture of 4.2% vol. propane in air on one end of a closed pipe at 6 bar initial overpressure. (**a**) Raw data with manually plotted yellow solid and white dashed lines highlighting signal features due to acoustic wave propagation in the pipe material and pressure waves traveling in the medium after ignition, respectively; (**b**) data filtered by differentiation (temporal shift of 4) with white points indicating automatically detected signal onset of the pressure wave (yellow points are discarded as they belong to signal features caused by acoustic waves in steel) and fitted slope (white solid line). For ambient pressure, i.e., 0 bar initial overpressure, similar signal features are observed as in the experiments using propane in air as seen when comparing Figures 9b and 10a. Particularly, propagation of the pressure wave is linear and the same data evaluation procedure is used. The velocity is deduced as \((500\pm 10)\) m/s in accordance with the value determined by the point pressure sensors of 469 m/s. At 2 bar initial overpressure a more complex signal shape is observed (Figure 10b). The automatically detected signal onsets may be clearly attributed to two different signals. For fitting and determination of propagation velocity only those points are considered which may be assigned unambiguously to one of the two acoustic signatures. First, a higher acoustic intensity is noticed leading to much more pronounced signal features of the structure-borne waves (marked in yellow) in the filtered data so that automatic signal evaluation is possible--albeit with a rather high uncertainty. The deduced velocity amounts to \((4850\pm 1360)\) m/s in agreement with former results. Moreover, propagation of the pressure wave along the observed length of pipe is non-linear and may be roughly divided into two linear regions with different slopes. In the first region as seen from the place of ignition (on the right in Figure 10b, 197-247 bins) a velocity of \((590\pm 40\) m/s) is calculated from signal onsets (white points) and linear fitting (white solid line). The second region (on the left, 179-196 bins) exhibits a considerably smaller slope corresponding to a much higher propagation velocity of the pressure wave. As the number of position bins in the region is rather low, linear fitting is not applicable here. Since the local pressure transducers are positioned in the region of the pipe where faster propagation is observed fiber-optically they may be used as reference. The velocity deduced from the measured raise in pressure is 2080 m/s. A line with corresponding slope is overlaid in the plot of the fiber-optic data in the second region (white dashed line) and shows an overall agreement with the gradient of the onset points.