## 5 Conclusions  
In this study, the temporal resolution of acoustic process emissions is addressed toward realizing real-time acoustic process monitoring and controlling systems for laser welding. Considering that the resolution varies depending on the specific information being sought, this study aims to investigate the relationship between the signal duration and their informativeness to identify and quantify joint gaps in laser butt welding. For this purpose, cluster analysis of signal segments was conducted, whose classification accuracy was used for evaluating the segment informativeness. The obtained results reveal that the signal segments can be shortened to a certain point while preserving their informativeness. Such a point, however, varies depending on the sensor type and the task at hand. With our specific configuration using the laser power of 3.5 kW at a welding speed of 12 m/min, structure-borne sensor signals can be shortened to 50 ms, yielding almost 95% gap detection accuracy. For the same task of gap detection, airborne sensor signals can be shortened further to 25 ms, maintaining 92.5% accuracy. When it comes to identifying the gap size, 25 ms suffices to maximize the segment informativeness regardless of sensor type. However, the highest achievable accuracy is lower than that of gap detection. These findings encourage the optimization of the segment duration in accordance with the task and the required accuracy. Furthermore, the airborne sensor in our study demonstrates more reliable detection and classification accuracy compared to its structure-borne counterparts. This results from a large variability in structure-borne sensor signals due to their susceptibility to renewal mounting. For maintaining signal quality, a reliable mounting technique needs to be established. Another remarkable finding is that large-sized gaps, specifically 0.3 mm asymmetric gaps, demonstrate the induction of unique characteristics in the acoustic process emissions, which is not a mere decrease in signal energy. This poses the question of what such unique characteristics look like and how they differ from those induced by small-sized gaps. The following are the possible research topics for enabling real-time acoustic monitoring systems based on the findings of this study. The obtained results demonstrate that the effect of noise increases when signal segments become shorter. This indicates that alleviating the effects of noise is beneficial for real-time applications. Sensor data fusion can also improve the temporal resolution because the kind and the availability of weld-relevant information seem to vary depending on the sensor type. To optimally place the sensors, the emission directivity needs to be identified. Considering the actual monitoring setups, the monitoring system also needs to cope with the transitions between a zero gap patch and a gap one, requiring further investigation. Because different welding configurations, such as welding speed or different materials, effect the signal characteristics of the acoustic emissions, such effects need to be studied as well. Another important aspect of the practice is compatibility with a crossjet, which produces substantial acoustic emissions in the same frequency range as that of the weld-relevant signals [13]. For realizing in-process monitoring systems, a measure to mitigate its effect is indispensable. Lastly, it is also worth paying attention to sensor integration. For achieving low latency, recordings need to be synchronized with the welding process. This requires identifying the exact time point when the welding process is started. For this purpose, this study employed a synchronization device, which was triggered by a high-speed camera serving as a reference. To ease the installation effort in practice, however, a synchronization measure based solely on acoustic sensors is beneficial.