## 3 Experiment  
To verify the above calculation results, an experimental measurement study relating to the vertical reflection coefficient of acoustic waves was carried out, and the experimental setup is shown in Figure 7. A tank of 90 \(\times\) 55 \(\times\) 40 cm is filled with water that has been rested for more than 24 h and degassed. Two acrylic plates of 0.3 \(\times\) 50 \(\times\) 40 cm are placed in the middle of the tank with a spacing of 4 cm. The bubbles are generated by electrolytic water. Copper and graphite electrodes connected to the DC power are placed at the bottom of the tank, in the middle of two acrylic plates, thus forming a bubbly liquid layer of 4 cm thickness [29]. The sound waves are generated and received vertically by a cylindrical transducer placed 35 cm in front of the bubble layer. The transducer is connected to both a signal generator and an oscilloscope. The signal generator emits a pulse-modulated sinusoidal signal for excitation with a central frequency of 175 kHz, and the echo signal received by the transducer is sent to a digital oscilloscope for storage and display. First, the reflection signal of vertical incidence at the water-steel plate interface was measured, and the spectrum of the reflection signal was obtained to analyze the emission-reception spectrum of the detection transducer. And the reflection signal at the water-steel plate interface was used as the reference signal for calculating the relative reflection coefficient in subsequent experiments. The reflection coefficient in the case of pure water was obtained by comparing the FFT-transformed signal with the spectrum of the reference  
Figure 6: Comparison of frequency response curves of reflection coefficients at different layer thicknesses and different sound velocities. signal, and the results are shown in Figure 8. The results show that the theoretical reflection coefficients match well with the experimental measurements. The frequency spacing of the valleys is around 19 kHz, which is a reasonable verification of the reliability of the experiment. However, it should be noted that, due to the limitation of the transducer bandwidth, the results are reliable only near the center frequency of the transducer. The experimental measurements far from the center frequency have a low signal-to-noise ratio, and the difference with the theoretical calculation results increases. After the DC power supply is energized, bubbles are generated from the electrodes at the bottom of the water and move upward until escaping from the water surface. The content of bubbles is related to the voltage and current of energization. Due to the large resistance, the sensitivity of adjusting the current is low when the energizing current is small (<0.04 A), so the bubble content is changed by adjusting the voltage to a small degree. The reflection signal of the bubbly liquid layer was measured after 5 min of energization. The time-domain waveforms of the reflection signal at different energization voltages were obtained, as shown in Figure 9. The difference in the reflection signal at different gas contents can be seen in the figure. Figure 8: Comparison of experimental measurements and theoretical reflection coefficient when the middle layer is pure water. Figure 7: Experimental setup diagram. The received time domain signal is processed by FFT and then divided by the reference signal to obtain the reflection coefficient. The reflection coefficient at different voltages in Figure 9 is shown in Figure 10. As can be seen from the figure, the presence of bubbles causes a significant change in the spectrum of the reflected signal. The maximum value of the reflection coefficient decreases as the energizing voltage increases. This is caused by the attenuation of sound waves due to the bubbles, and the more bubbles there are, the more significant the attenuation is. However, the minimum value of the reflection coefficient increases with the increase of the energizing voltage.