Different from the early acoustic metamaterials, we draw on the application of the Hilbert curve in the radio frequency identification tag antenna, to make Hilbert fractal acoustic metamaterials (HFAMMs) with a self-similar fractal structure. This type of ventilated, sub-wavelength acoustic metamaterial has a relatively high reflection coefficient near the anti-resonance frequencies, resulting in high sound transmission loss (TL). The TL of the HFAMMs increases with the decrease in the waveguide width; similarly, the number of frequency bands of acoustic insulation increases and the frequency bands of sound insulation shift to low frequencies with the increase in the fractal order. The experimental results show that due to the viscothermal losses, the transmission coefficient at the resonant frequencies is greatly weakened, so that the HFAMM has broadband sound insulation performance. The TL of the third-order HFAMM can reach more than 30 dB at the main frequencies of the transformer noise. Furthermore, based on the method of selecting the optimal HFAMMs, we can easily design the HFAMMs suitable for the frequency bands of the noise need to be reduced; therefore, there is a broad application prospect in the field of noise reduction especially for low frequency noise.

In this paper, a broadband metamaterial absorber suitable for the S, C, and X bands is designed and manufactured. The absorber is made of FR-4 substrate, resonant metal structure, lumped resistance, and metal backplate, has a unit size of 0.11λL, and a total thickness of 0.084λL. The absorption principle of the absorber is analyzed using equivalent medium theory and parametric research. The calculation results show that the absorber achieves more than 90% broadband absorption (relative bandwidth of 130%) between 2.7 GHz and 12.7 GHz, has a good polarization angle and incidence angle insensitivity, uses electromagnetic resonance to explain its absorption characteristics, and experiments confirm that the absorber has good broadband absorption. The proposed absorber has higher absorption and simpler construction than the previously described broadband absorber, and it has the potential for practical applications in EMC, radar, and electromagnetic protection.

Lorentz force electrical impedance tomography (LFEIT) combines ultrasound stimulation and electromagnetic field detection with the goal of creating a high contrast and high resolution hybrid imaging modality. To reduce the peak stimulation power to the ultrasound transducer in LFEIT, linearly frequency-modulated ultrasound pulse was investigated in this work. First, the coherency between linearly frequency-modulated ultrasound excitation and the resulting local current density was established theoretically. Then, experiments were done using different agar phantoms of conductivity ranging from 0.2 S/m to 0.5 S/m. The results showed: (1) using electrical signal of peak instantaneous power of 39.54 dBm to the ultrasound transducer, which was 25.5 dB lower than the peak instantaneous power of the high-voltage narrow pulse adopted in traditional LFEIT (65.05 dBm), the linearly frequency-modulated ultrasound pulse based LFEIT can detect the electrical conductivity discontinuity positions precisely; (2) the reconstructed B-scan image of the electrical conductivity discontinuity distribution is comparable to that obtained through LFEIT with high-voltage narrow pulse; (3) axial resolution of 1 mm was achieved with the experimental setup. The method of linearly frequency-modulated ultrasound pulse stimulation and coherent detection initiates an alternative scheme towards the clinical application of LFEIT.

Magnetoacoustic tomography combines the advantages of electrical impedance tomography with sonography. It can reconstruct conductivity images of biological tissue with high spatial resolution and high contrast. In addition, magnetoacoustic tomography is a new nondestructive functional imaging technology, with which we can realize the early diagnosis of human diseases. In this paper, through theory analysis we deduced the analytical formula of acoustic pressure. Computer simulation and experimental studies on magnetoacoustic tomography with current injection (MAT-CI) were made, and a copper ring was used as the model for the simulation and experiment. Experimental setup and methods are introduced, and the acoustic signals and the image of a metal object by means of MAT-CI are presented. Results of the simulation and experiment were in a good agreement. The reconstructed image of an acoustic source can reflect the conductivity information of a sample. The conductivity distribution of a sample can be obtained by means of MAT-CI. The results provide foundations of study on a model with complex conductivity distribution and tissues for MAT-CI.

This paper presents new results on a novel coupled-physics modality called magneto-acousto-electrical tomography (MAET). With static magnetic field, a beam of ultrasound is applied to the sample to be imaged; the ultrasonic vibration can lead to the separation of the charges due to the Lorentz force and produces the current distributed in the object. In this paper, simulations were conducted on biological tissue model, and the electrical properties were reconstructed, which could reflect the state of physiological or pathological. We also conducted experiments to detect the voltage signal of microvolt level by the electrodes placed around the sample. The series of experiments were conducted on low-conductivity (0.2 S/m) animal gelatin phantom and real biological tissue, successively placed in a 260 mT magnetic field and sonicated with a plane ultrasonic transducer of central frequency 2.25 MHz. In this paper, we obtained axis resolution of 1 mm, much more significant improvement than the previous reports. More importantly, the experimental result does also demonstrate the feasibility of MAET to image the electrical properties of biological tissue.