## 4 Discussion  
The measurements presented above show that the AE technique can monitor melting processes of ice quantitatively as well as qualitatively. The acoustic activity of ice in an environment with a constantly increasing temperature starts when the temperature reaches the melting temperature of water. Independent of the amount of water, the acoustic activity is the highest at the beginning of the melting process where rates of up to 125 AE events per minute where measured with the described setup. This number is however strongly dependent on the physical measurement setup, used hardware and signal filters.  
[PERSON] et al. (2013) already reported a peak in the acoustic activity at the beginning of the melting process of ice. They identified the temperature gradient between the surrounding and the frozen sample as a root cause for crystal fractures that cause this peak. In the presented work, the very slow temperature increase (\(\Delta T_{\uparrow}=8\) K h\({}^{-1}\)) prevents high temperature gradients within the sample and crystal fractures should not be a relevant source of acoustic events.  
After this peak, the rate of the acoustic events decreases and a \"valley\" with significantly lower AE activity appears. For the measurements with amounts of water \(m_{ice}>2.0\) g, the acoustic activity again increases towards the end of the melting process.  
This behavior can have multiple reasons that include phenomena related to the intrinsic acoustic activity during the melting process or the acoustic coupling between source and sensor.  
In Figure 7 a schematic, simplified of the temporal histogram of AE events during the melting process is shown and divided into three parts (A, B, C) that might correspond to different situations inside the sample holder.  
Figure 6: The total amount of acoustic signals during the melting of ice plotted against the quantity of ice. The black and white data points show the complete sum of events and the number of events in the first peak respectively.  
During the first part of the melting (A), the sample is dominated by ice. At a certain point in time, the constantly increasing temperature results in the melting of ice and stress in the crystal due to the sudden change in volume during the melting. Therefore, a significant amount of acoustic energy is released in this initial stage. As seen in Figure 6, the number of acoustic events at the beginning of the melting process is dominated by the first peak and linear dependent on the amount of ice.  
After the first peak, the acoustic activity decreases and the second phase (B) with less acoustic activity sets in. Therefore, the coupling of the AE source (melting ice) and the sensor changes. An additional film of water between the sensor and the ice results in an additional surface that reflects a part of the acoustic energy.  
Finally, as described above and seen in all additional measurements, a remarkable increase of observed acoustic activity is measured at the end of the melting process. In this phase (C), only a little ice amount is left swimming at the surface of the water reservoir. This ice is therefore in contact with the air which is warmer than the water in the sample holder resulting in a higher temperature gradient. This could be one reason for a higher melting rate of the last bits of ice resulting in an increase of the acoustic activity.  
To investigate this effect, measurements with a bucket that was closed with a cover and some vacuum grease were done. In this case, the air inside the bucket was trapped and not interchangeable with the external air. Therefore, it can be assumed that the effect of the ice touching \"warm\" air was reduced. However, also these measurements show the three phases of melting including the second peak at the end of the melting phase which suggests that the ice touching the \"warm\" air has no or only a minor effect.