Fractal acoustic metamaterials for transformer noise reduction

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. Published by AIP Publishing.