## Materials and Methods  
### Fiber-Optic Sensing System and Data Evaluation  
In all experiments a sensing system was used consisting of a standard single mode optical fiber (SMF-28e+, Corning) without jacketing and a commercial direct detection C-OTDR-interrogator (Helios DAS, Fotech Solutions Ltd., Hampshire, UK). This system is specified for a maximum range of 40 km with optical pulse lengths between 10 ns and 1 \(\upmu\)s and has a maximum pulse repetition rate of 100 kHz. The settings chosen in this study are specified for each experiment in the following paragraphs. For optimal signal transduction, the optical fiber was pre-strained and directly applied onto the surface of the pipe under investigation by helical wrapping using an inhouse designed wrapping device. Uniform and defined pre-strain was achieved by employing a loosely hanging fiber spool with an attached weight onto the sensor fiber during the wrapping process. Optical pulse lengths \(T_{\mathrm{P}}\) used to interrogate the fiber were optimized for each experiment. Longer \(T_{\mathrm{P}}\) entail higher sensitivity but lower spatial resolution \(r\cong cT_{\mathrm{P}}/(2n_{\mathrm{eff}})\), where \(c\) is the speed of light in vacuum and \(n_{\mathrm{eff}}\) is the effective refractive index of the optical fiber mode. Pulse lengths were chosen between 30 ns and 100 ns, corresponding to an expected spatial resolution of approximately 3 m to 10 m along a straight fiber stretch. With respect to localization along the pipe, spatial resolution is additionally increased by a factor given by the pitch of the helically wrapping of the fiber sensor and the outer diameter of the pipe. To capture transient processes on a short time scale with high precision the pulse repetition frequency \(f_{\mathrm{P}}\) was set to 80 kHz--lose to the maximum possible setting of the interrogator--for experiments targeting acoustic waves in the material and to 40 kHz for experiments targeting pressure waves in the medium. According to [PERSON]'s theorem, this allows for registering of signals up to 40 kHz or 20 kHz, respectively. It is noted that the maximum monitoring distance \(L_{\mathrm{m}}\) along the optical fiber inversely depends on the repetition frequency given by \(L_{\mathrm{m}}=c/\left(2n_{\mathrm{eff}}f_{\mathrm{P}}\right)\). This restricts the maximal monitoring distance to 1.25 km of optical fiber in this case. However, \(f_{\mathrm{P}}\) may be adjusted to lower values depending on the specific monitoring task and required bandwidth. Position along the fiber is usually given in bins, where one bin corresponds to the portion of fiber from which the interference pattern is analyzed within the interrogator. With the system used one bin represents either 0.57 m or 0.68 m of fiber depending on the digital sampling settings (180 or 150 mega samples per second) for the two sets of experiments, respectively. Raw data recorded by the C-OTDR system was further processed using a home-written Python program. Transient events propagating along the pipe appear as edges within the three-dimensional plot of signal intensity versus time and position along the fiber. For optimal detection of these edges, we employ edge filtering by numerical differentiation of raw data along the time axis. Step width for calculating estimated derivatives was optimized for each experiment. After preparing data in this way, edges were automatically detected by thresholding the signal intensity within a certain bin or summed up intensity within a range of bins with respect to the noise level. The threshold level was set as a multiple of the standard deviation of the noise. From the signal onsets detected in this way the propagation velocity and/or localization of events was deduced by either slope fitting in the case of a fully distributed setup, or by calculation of the runtime difference for the quasi-distributed setup.