**4. Experimental Verification**

To verify the feasibility of this method, the AE source location experiment is carried out in this paper. The acoustic wave is collected by a *DS5-16C Holographic Acoustic Emission Signal Analyzer* (made by Beijing SoftLand Scientific and Technology Co., Ltd., Beijing, China). AE sources are generated by pencil-lead breaks. The HB pencil lead is 0.5 mm in diameter, and the pencil lead is broken at 30◦ on the surface of the specimen, according to the requirements of *Metal Pressure Vessel Acoustic Emission Testing and Result Evaluation (GB/T18182*—*2000)*. The granite specimen that is 200 mm × 179 mm × 84 mm in size and an average velocity of 4600 m/s is used as a monitoring system. Sixteen RS-2A piezoelectric ceramic resonant sensors with the coordinates of (10, 10, 84), (190, 10, 84), (190, 170, 84), (12, 170, 84), (0, 80, 74), (110, 0, 74), (200, 80, 74), (90, 180, 74), (0, 170, 10), (0, 90, 10), (10, 0, 10), (100, 0, 10), (190, 0, 10), (200, 90, 10), (190, 180, 10), and (100, 180, 10) (in mm) respectively are mounted on the surface of the monitoring system. Sensors are numbered as *Si* (*i* = 0, 1, ···, 15)

according to the arrival orders of the AE signals, and the nearest sensor *S*<sup>0</sup> from the AE source is regarded as the reference sensor. Later, 15 TDOA measurements *ti* (*i* = 1, 2, ···, 15) between the sensor pairs of *S*<sup>0</sup> and *Si* are obtained. In addition, there are four AE sources generated by pencil-lead breaks, and their coordinates are *P* (80, 120, 84), *Q* (160, 60, 84), *R* (0, 120, 42), and *T* (120, 0, 42) respectively. Before signal acquisition, the voltage threshold of each channel is set as 10 mv, and the preamplifier gain is 40 dB.

Figure 4 shows the received AE waveforms by two sensors, in which *t*<sup>0</sup> and *t*<sup>2</sup> are the arrival times at sensor *S*<sup>0</sup> and sensor *S*2, respectively. Then, TDOA measurement is formed by using the arrival times at the sensor pair *S*<sup>0</sup> and *S*2, i.e., Δ*t*<sup>2</sup> = *t*<sup>2</sup> − *t*0. The arrival times of the acoustic waves are picked manually according to the waveform diagram to ensure a high picking quality.

**Figure 4.** Acoustic emission (AE) signals and arrival times at different sensors.
