Application Cases

Experiment Name: Acoustic Wave Focusing Test in Liquid-Solid Weakly Coupled Structures

Research Direction:
Acoustic waves are an important medium that humans can manipulate to effectively transmit energy or information in the ocean. Therefore, underwater acoustic detection technology plays a significant role in fields such as marine resource exploration and seabed topographic mapping. However, the energy of acoustic waves gradually attenuates during propagation, reducing the sensitivity of underwater acoustic detection and severely constraining the development of marine resource exploration and seabed topographic mapping. Acoustic wave focusing technology employs various methods to concentrate acoustic waves within a relatively small spatial region with higher energy density. When receiving transducers are placed in this region, the detected acoustic signals are significantly enhanced, improving the sensitivity of underwater acoustic detection systems.

In recent years, the rapid development of acoustic metamaterials has provided new approaches to traditional acoustic wave focusing problems, promising to achieve acoustic focusing with lower cost, smaller size, and simpler structures [4]. Acoustic metamaterials are artificially manufactured composite structures made of conventional materials [5-11]. Unlike traditional materials, their structural unit sizes are comparable to or even much smaller than the acoustic wavelength (phononic crystals and acoustic metamaterials), exhibiting unique acoustic properties not found in conventional materials at the macroscopic level, such as negative effective density/modulus, graded refractive index, and ultra-high refractive index, which enable acoustic wave focusing.

Experiment Objective:
To validate the construction of liquid-solid weakly coupled conditions using a thick double-plate-single-slit structure, study the acoustic wave focusing characteristics of the water cavity slit, analyze factors influencing coupling strength, determine the materials and dimensions of the double-plate-single-slit structure, and experimentally verify its acoustic wave focusing effect.

Testing Equipment:
Acoustic pressure acquisition module, power signal source (ATG-2031), underwater acoustic transducer, double-plate-single-slit structure + microphone, data acquisition card.

Experimental Procedure:
The experimental system block diagram is shown in Figure 2-17. The host computer generates a modulated sinusoidal pulse train signal, which is sent to the power amplifier to drive the transducer in the water to excite the acoustic field. The acoustic pressure of the double-plate-single-slit structure is measured using a microphone, and the data is transmitted back to the host computer via the acquisition card for subsequent processing. The acoustic pressure acquisition module, shown in Figure 2-16(b), is installed in the slit of the structure, with the microphone positioned at the center of the slit. The transducer and the double-plate-single-slit structure are arranged at a certain distance, with the transducer facing the plate surface, as shown in Figure 2-18(a). The acoustic pressure-frequency curve is measured with the double-plate-single-slit structure, and then the same acoustic pressure acquisition module is used to measure the acoustic pressure-frequency curve without the structure at the same microphone position. Experiments are conducted in both a water tank (1.5 m × 1.5 m × 2 m) and a water pool (30 m × 27.5 m × 3 m), with field setups shown in Figures 2-18(a) and (b).

An automatic frequency-sweeping program with integrated transmission and reception is used to complete the entire experiment, as shown in Figure 2-19. After initializing the starting frequency, a sinusoidal modulated pulse signal is transmitted while triggering the acquisition process. After a frequency-sweep time interval, data acquisition is stopped, and a check is performed to determine if the upper frequency limit of the sweep has been reached. If reached, the program stops; otherwise, the current frequency is incremented by one step, and the signal transmission and acquisition process is repeated until the upper limit is reached.

Experimental Results:

1. Water Tank Experiment Analysis
   During the water tank experiment, the distance between the double-plate-single-slit structure and the sound source is 0.6 m. The collected time-domain signals are converted into voltage (acoustic pressure)-frequency curves via FFT, as shown in Figure 2-20(a), which represents two measurements without the structure using the same microphone. The voltage-frequency curves in the 10–12 kHz range show good consistency, while significant differences are observed in the 12–20 kHz range. Figure 2-20(b) shows the voltage-frequency curve measured with the double-plate-single-slit structure using the microphone from Figure 2-20(a). The curve shape is similar to that without the structure, with no distinct peaks.

   
   

   Averaging two measurements under different conditions yields the curve shown in Figure 2-21(a). The curve with the double-plate-single-slit structure shows little difference from the curve without it, and no significant acoustic wave focusing amplification is observed. The ratio of the voltage with the structure to that without it is calculated to obtain the amplification factor-frequency curve, as shown in Figure 2-21(b). The amplification factor is less than 2.5, with multiple peaks. Due to the small absolute signal values, fluctuations caused by measurement errors can exceed a factor of 2. Both acoustic wave focusing and experimental measurement errors could lead to the peaks observed in Figure 2-21(b), making it impossible to verify the acoustic wave focusing effect of the double-plate-single-slit structure in the water tank.

   
   

   Analysis indicates that this phenomenon occurs due to severe acoustic reflections in the water tank, resulting in an extremely uneven acoustic field distribution. Minor positional deviations of the microphone can cause significant changes in the measured signals. This experiment requires measuring voltage (acoustic pressure) signals with and without the double-plate-single-slit structure, inevitably involving two microphone placements. This introduces positional deviations between the two conditions, leading to significant changes in the measured signals and rendering the calculated amplification factors meaningless. Therefore, the experiment must be conducted in an open water environment.

2. Water Pool Experiment Analysis
   During the water pool experiment, the distance between the double-plate-single-slit structure and the sound source is 1 m. The voltage-frequency curve obtained after averaging four measurements is shown in Figure 2-22.

   
   

   Analysis of Figure 2-22 reveals that after adding the double-plate-single-slit structure, the signal at the slit center significantly strengthens at 13.06 kHz, indicating acoustic wave focusing and amplification at this frequency. The focusing frequency obtained from COMSOL simulation is 14.14 kHz, which deviates slightly from the experimental result of 13.06 kHz. This discrepancy is attributed to the unsmooth surfaces of the fabricated stainless steel plates, as well as the presence of fixed support structures and through-holes on the plates, which influence the focusing frequency of the double-plate-single-slit structure.