Conventional acoustic systems exhibit a difficulty in sensing weak acoustic fault signals in complex mechanical vibration environments. Therefore, it is necessary to develop an acoustic sensing mode and a corresponding functional device with pressure amplification. This paper proposes a three-dimensional device, coupling gradient acoustic metamaterials (GAM) with phononic crystals (GAM–PC). The strong wave compression effect coupled with the phononic crystal equivalent medium mechanism is utilized to achieve the enhancement of weak acoustic signal perception at the target frequency. The superior amplification capability of the GAM–PC structure for the amplitude of loud signals is verified by numerical simulations and experiments. Moreover, the GAM–PC structure has a narrower bandwidth per slit, making it more frequency selective. Furthermore, the structure can separate different frequency components. This work is expected to be applied to signal monitoring in environments with strong noise.

Acoustic sensing systems play a critical role in identifying and determining weak sound sources in various fields. In many fault warning and environmental monitoring processes, sound-based sensing techniques are highly valued for their information-rich and non-contact advantages. However, noise signals from the environment reduce the signal-to-noise ratio (SNR) of conventional acoustic sensing systems. Therefore, we proposed novel nonlinear gradient-coiling metamaterials (NGCMs) to sense weak effective signals from complex environments using the strong wave compression effect coupled with the equivalent medium mechanism. Theoretical derivations and finite element simulations of NGCMs were executed to verify the properties of the designed metamaterials. Compared with nonlinear gradient acoustic metamaterials (Nonlinear-GAMs) without coiling structures, NGCMs exhibit far superior performance in terms of acoustic enhancement, and the structures capture lower frequencies and possess a wider angle acoustic response. Additionally, experiments were constructed and conducted using set Gaussian pulse and harmonic acoustic signals as emission sources to simulate real application scenarios. It is unanimously shown that NGCMs have unique advantages and broad application prospects in the application of weak acoustic signal sensing, enhancement and localization.

In the field of industrial structure detection, acoustic signals have been pivotal. A cost-effective and highly sensitive acoustic monitoring system that can enhance weak acoustic signals has always been an interesting topic in many research fields. However, environmental noise signals have consistently hindered the improvement of the signal-to-noise ratio (SNR) of traditional acoustic systems. In this work, we propose a structure (PC-Mie) that couples phononic crystal (PC) point defects and Mie resonance structures (Mies) to enhance weak effective signals from complex environments. Numerical simulations have confirmed that the PC-Mie exhibits superior sound pressure enhancement performance compared to each individual PC point defect and Mies. Moreover, the capability to amplify the sound pressure amplitude is related to the angle and position of the Mies at the center position. Simultaneously, the PC-Mie has a narrower bandwidth, giving the structure stronger frequency selectivity. Finally, the experiment proves that PC-Mie can function as an enhanced acoustic device or sensor to detect harmonic signals, verifying the validity of the PC-Mie structure for acoustically enhanced perception. Both numerical and experimental studies demonstrate that the PC-Mie can effectively enhance the energy of specific sound frequencies in complex air environments, making it suitable for collecting high-sensitivity acoustic signals. This research has significant implications for the development of weak acoustic signal detection technology and the application of self-powered sensors.

In this study, we introduce a physical model of a three-dimensional (3D) guided wave sensor called 3D-CMUT, which is based on capacitive micro-machined ultrasonic transducers (CMUTs). This 3D-CMUT sensor is designed to effectively and simultaneously obtain 3D vibration information about ultrasonic guided waves in the out-of-plane (z-direction) and in-plane (x and y-directions). The basic unit of the 3D-CMUT is much smaller than the wavelength of the guided waves and consists of two orthogonal comb-like CMUT cells and one piston-type CMUT cell. These cells are used to sense displacement signals in the x, y, and z-directions. To ensure proper functioning of the 3D-CMUT unit, the resonant frequencies of the three composed cells are set to be identical by adjusting the microstructural parameters appropriately. Moreover, the same sensitivity in the x, y, and z-directions is theoretically achieved by tuning the amplification parameters in the external circuit. We establish a transient analysis model of the 3D-CMUT using COMSOL finite element simulation software to confirm its ability to sense multimode ultrasonic guided waves, including A₀, S₀, and SH₀ modes. Additionally, we simulate the ball drop impact acoustic emission signal on a plate to demonstrate that the 3D-CMUT can not only utilize in-plane information for positioning but also out-of-plane information. The proposed 3D-CMUT holds significant potential for applications in the field of structural health monitoring (SHM).

Acoustic sensing technology has attracted significant attention across various fields, including mechanical fault early warning and wireless communication, due to its high information density and advantages in remote wireless applications. However, environmental noise reduces the signal-to-noise ratio (SNR) in traditional acoustic systems. In response, this article proposes a novel Z-shaped channel gradient metamaterial (ZCGM) that leverages strong wave compression effects coupled with effective medium theory to detect weak signals in complex environments. The properties of the designed metamaterials were verified by theoretical derivation and finite element simulation of the model. Compared to conventional linear gradient acoustic metamaterials (GAMs), ZCGM demonstrates significantly superior performance in acoustic enhancement, with a lower capture frequency. Furthermore, the structure exhibits flexible tunability in its profile. In addition, the center frequency of each actual air gap is determined in this paper based on the swept frequency signal test. Based on this center frequency, a preset specific harmonic acoustic signal is used as an emission source to simulate the actual application scenario, and experiments are constructed and conducted to verify the performance of the designed metamaterials. The results consistently show that ZCGM has distinct advantages and promising application prospects in the detection, enhancement, and localization of weak acoustic signals.

The detection and extraction of weak signals are crucial in various engineering and scientific fields, yet current acoustic sensing technologies are restricted by fundamental pressure detection methods. This paper proposes gradient-equivalent medium-coupled metamaterials (GEMCMs) utilizing strong wave compression and an equivalent medium mechanism to capture weak signals in complex environments and enhance target acoustic signals. Overcoming shape and impedance mismatch limitations of traditional gradient structures, GEMCMs significantly improve control performance. Experimental and numerical simulations indicate that GEMCMs can effectively enhance specific frequency components in acoustic signals, outperforming traditional gradient structures. This enhancement of specific frequency components relies on the resonance effect of the unit cell structure. By introducing acoustic resonance within a spatially wound acoustic channel, a significant amplification of weak acoustic signals is achieved. This provides a new research direction for acoustic wave manipulation and enhancement, and holds significant importance in fields such as mechanical fault diagnosis and medical diagnostics.