## 2 Experimental Setup
### Data Acquisition and Processing  
A good review of AE measurements of chemical processes is given in [PERSON] and [PERSON] [PERSON] and [PERSON] (2017) where the general approach, usage and advantages of acoustic emission measurements are discussed. In addition, a general AE setup is described which usually contains an acoustic sensor, followed by a filtering and amplification of the signals and dedicated signal analysis by the CPU. In the following all three parts will be explained separately.  
The most common AE sensors are piezoelectric (PZT) sensors that use the piezoelectric effect which transforms elastic motions into voltages [PERSON] and [PERSON] (2017). In this work, the commercial broadband piezoelectric sensor \"_VS30-V\"_ from the Vallen Systeme GmbH with a flat frequency response between 25 to 80 kHz [PERSON] and [PERSON] (2017).  
The PZT sensor is connected to an AC coupled preamplifier \"_Model 5670_\" by Olympus NDT Inc. This broadband preamplifier provides a very-low-noise 40 dB amplification of ultrasonic signals with a flat (\(\pm\)1.5 dB) response in a frequency range of 50 kHz-10 MHz [PERSON] and [PERSON] (2017). The advantage of the bandwidth that does not match with the full operating frequency of the PZT is that frequencies below 50 kHz are not amplified which reduces the picked up ambient noise. Thereafter, a \"_PicoScope 2205A\"_ USB oscilloscope digitizes and saves the waveforms with a sampling frequency of \(f_{s}=1.565\) MHz. The oscilloscope operates in a simple edge threshold trigger mode with a trigger level of V\({}_{\text{THD}}=+10\) mV which was approximately two time the environmental noise at the beginning of the experiments. With every trigger, a time windows with a length of t = 2 ms (trigger point at \(t_{trigger}\) = 20% = 0.4 ms) is saved in a file that contains the time of occurrence.  
The settings of the oscilloscope define some additional parameters that are commonly used in AE:  
* hit definition time (HDT)
* hit lockout time (HLT)
* peak definition time (PDT)  
Classical AE systems allow the explicit setting of these parameters for a optimized operation in certain conditions. With the setup used in this study, these parameters are given by the operating mode of the oscilloscope and no additional (software implemented) realization of these parameters is implemented. The PDT = 1.6 ms is given by the recorded time window for each AE event. The HDT and HLT are not explicitly set but implicitly given by the oscilloscope settings. The recorded time window and the duration of the AE previous event determine the HDT such as D + HDT = 1.6 ms. Due to the short duration of the expected AE events (\(\overline{\text{D}}\approx\) 15% of the recording time window) and the low quantity of signals, no HLT is required and it can be assumed as zero.  
The post-processing of the recorded transients is done in real time in an additional module of the data acquisition software, which is coded in Python 3.7. Figure 1 shows a typical acoustic burst and some parameters that are also listed in Grosse and Ohtsu Grosse and Ohtsu (2017) and typically used for parameter-based analysis. The following parameters are calculated for each transient:  
* Amplitude (AMPL) in [mV]