## 5 Conclusions  
The paper presents a sonic watermarking method developed based on the principles of the ENF criterion, a highly acclaimed audio authentication solution. Unfortunately, the new recording devices are built to reject the mains hum, the essential element needed for the ENF criterion to function. The proposed sonic watermark uses a signal that can exist in any room without drawing attention to it: the ticking of a clock. The method uses this signal as a masker for a variable number of chirp signals that encode the ENF information. Choosing the parameters of the chirp signals is a crucial task, and was done based on five rigorous principles, presented in the paper. The sonic watermark generator was thoroughly described and, to ease its reproduction by other researchers, all the mathematical operations were presented. Also, the integrity check methods are detailed. The method was comprehensively tested, using for each step of the experiments 100 watermarked vocal signals, considering also the signals' propagation through the room. The resources used in the experiments are available in referenced online databases, helping other researchers to re-implement for further study. The results show how the power ratio between the involved signals affect the performance of extracting the watermark from the recording, and the limit values are presented. A great importance is played by the investigated signal duration, the results showing that only 10 min of recording allow the perfect finding of the recording moment in a one year period. The performance of perfectly detecting the cut parts of a recording (moment and duration) is also impressive, with over 95% perfect detection. The main contribution of the method presented in this paper is that it extends the use of the ENF criterion to the recordings captured with newer devices. The other contributions of the paper are the proper design of the system to assure auditory masking of the chirp signals; the proper choice of the frequencies of the chirp signals to assure that they can be played using small enough speakers,allowing the system to be concealed and to maximize the watermark extraction performance; the encoding scheme of the ENF, assuring that the number of chirp signals that are used to encode the ENF value is kept to a minimum (if the ENF has the nominal value, only one chirp signal is played and the number of chirp signals increases with the deviation of the ENF from the nominal value); determining the values of the power ratios between the signals involved in the system that should be met to obtain good results, allowing a user to correctly set up the system; evaluating the impact of the acoustic environment on the proposed method; evaluating the perceptibility of the proposed sonic watermark through subjective testing. The limitations of the proposed method are represented by the minimum duration of the recording that would allow a correct identification of the recording moment (i.e., 10 min), the maximum required power ratios between the watermark components (i.e., 20 dB ticking sounds to chirp signals power ratio), and the maximum power ratio between the vocal signals and the sonic watermark (i.e., 6 dB). Better performance is obtained in less reverberant environments. It is true that if the recording device does not capture signals above 8 KHz, it will not capture the chirp signals, but it will still capture the time delayed ticking pattern, generated based on a pseudorandom sequence. It can be assumed that at least one of the meeting members has the intention to record the true conversation with good quality, which can be the reference recording. Because the sonic watermark is embedded into all the recordings made in the room, if anyone modified their recording, there will still be enough clues to demonstrate that theirs is a modified copy of the reference recording and not the other way around (e.g., the modified recording has a lower sample rate, has been filtered, has been cut or modified through other ways to remove information).