## 2 Materials and Methods
### Data Acquisition and Evaluation  
When recording the test data, several measurements were combined into one audio signal. This simplified the recording process, as it was not necessary to store each signal individually. The piezoelectric frequency of the print head can theoretically be set to a value of 30 kHz, but was kept constant at 10 kHz for all test series, as this is the usual printing frequency. Table 1 summarizes the most important parameters of the print head and the data acquisition.  
We attempted to determine whether a nozzle was functional or not on the basis of the recorded experimental vibration data. To determine the nozzle condition, the ejection of a droplet from the nozzle was utilized as the evaluation criterion. This means that if a droplet ejection was visible, the nozzle was classified as working correctly. If no droplet ejection could be detected when the nozzle was actuated, the nozzle was considered to be faulty  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Frequency of the piezoelectric actuator & f\({}_{\text{Piezo}}\) & 10 kHz \\ Number of nozzle shots & n\({}_{\text{shots}}\) & 10,000 \\ Sample rate of the measuring system & f\({}_{\text{sample}}\) & 48 kHz \\ Number of samples in the signal & n\({}_{\text{sample}}\) & 40,000 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Recording and print head settings for the experiments in this article.  
Figure 1: Mounting of the acceleration sensor on the print head. Highlighted are the accelerometer (1), the threaded nut for position assurance (2), the threaded nut for clamping (3), the spacer plate (4), and the surrounding steel plate (5).  
(clogged). The vibration data was recorded on four different days to study time-dependent influences. In addition, the times of the recordings were chosen such that the level of background noise was as low as possible in order to achieve a minimum of background noise. Therefore, the measurement recordings took place before 8:00 a.m. or after 5:00 p.m.  
The tested nozzle was located approximately in the center of the print head and represents the 70 th nozzle in the fourth module of the print head. To check the nozzle condition, a glass mirror was used, which was located below the piezoelectric modules during activation. After each nozzle activation, it was visually checked whether there was a drop on the mirror. If a drop was visible on the mirror, the mirror was cleaned and placed under the print head again for the next measurement. A total of 480 individual measurements were recorded in the course of this series of tests, which were recorded at four different dates, each with 120 individual measurements. At each measurement date, the measurements were again split into four data blocks. This means that in one day two data blocks with a functional nozzle and two data blocks with a clogged nozzle were recorded, each with 30 individual measurements of a nozzle actuation. The sequence of a data block of 30 individual measurements can be seen in the following figure.  
Figure 2 shows the recording of a data block with 30 nozzle shots. At the beginning, all nozzles of the print head were always cleaned. A standard cleaning and a purge process were carried out. Then, the measuring position was approached and the corresponding nozzle condition was checked. If the visual inspection showed that the state of the nozzle was as desired (functional or clogged), the nozzle could be triggered again. If the state was not as desired, the print head was cleaned to achieve a functional nozzle or the experiments were paused for 5 min to achieve a clogged nozzle. Thirty nozzle shots form a data block, with a consistent nozzle state in each block.