## 4 Results and Discussion
### Watermark Extraction Performance  
The flow diagram showing the processes of the experiments done to characterize the watermark extraction performance is illustrated in Figure 15. Two types of experiments were conducted, depending on the power ratios between the signals in the recorded mixture. We remind here that there are two main components in the recorded signal: the speech signal and the sonic watermark that is composed of the clock ticking sounds and the chirp signals. A database containing the ENF value for each second over one year available from [6] was used. The experiments were run 100 times for each power ratio, for each of the 100 speech signals from [43]. For each speech signal, a random part of the ENF variation was selected, according to its duration. Then, the sonic watermark was generated as described in Section 3.2, using the imposed power ratio between the watermark's components. After this, the sonic watermark is mixed with the speech signal at the required power ratio, then the obtained mixture is filtered using the acoustic impulse response of the environment (meeting room, lecture room). The method described in Section 3.3 is used to extract the ENF information from the recorded signal. The extracted sequence is compared with the reference one. The preferred evaluation was harsh, the result being either success (100% recovery) or failure (even in the case of 99% the recovery, for example). Even if one ENF value from the whole sequence was not extracted correctly, it is still considered a failure. Since the experiment is run 100 times for each power ratio, the number of successful runs, in which the ENF sequence was perfectly recovered, gives the probability of correct extraction.  
In the first experiment, the power ratio between the speech signal and the sonic watermark was varied between 6 dB and 20 dB, while holding constant the power ratio between the components of the sonic watermark (ticking sounds to chirp signals) at 20 dB. A total of 3000 runs were done to obtain the variation (100 for each power ratio, for the two acoustic environments). The results are illustrated in Figure 16a). In the second experiment, the power ratio between the speech signal and the sonic watermark was held constant to 6 dB, while the power ratio between the components of the sonic watermark (ticking sounds to chirp signals) was varied between 20 dB and 30 dB. A total of 2200 runs were done to obtain the variation. The results are shown in Figure 16b.  
From the results it can be observed that the method gives the best results when used in a meeting room. This represents a favorable result because the most important discussions are done in such environment. Another observation is that the degradation given by the PR between the speech signal and the sonic watermark is more pronounced than the one given by the PR between the components of the sonic watermark. For best results, care should be taken to set up the system in a way that the PR  
Figure 16: (**a**) Probability of correct watermark detection depending on the PR between the vocal signal and the sonic watermark (ticking sound to chirp signals PR was constant, equal to 20 dB); (**b**) Probability of correct watermark detection depending on the PR between the ticking sound and the chirp signals (vocal signal to sonic watermark PR was constant, equal to 6 dB).  
Figure 15: The flow diagram describing the processes of the experiment done to characterize the watermark extraction performance.  
between the vocal signals and the sonic watermark is 7 dB or less, while the PR between the ticking sounds and the chirp signals should be not more than 24 dB.