Design and Sound Absorption Characteristics of a Broadband Acoustic Absorber for Transformer Noise Control Based on a Composite Structure of Traditional and Side-Slit Helmholtz Resonators.

This study proposes a targeted sound-absorption design strategy to address the challenge of broadband and low-frequency transformer noise. Based on field measurements, transformer noise is identified as being characterized by a dominant 100 Hz fundamental component, pronounced low-order harmonics in the range of 200-500 Hz, and an overall broadband distribution within 100-1000 Hz. To meet these noise-control requirements, a composite acoustic metamaterial based on a traditional Helmholtz resonator and a side-slit Helmholtz resonator (HRSS-HR) is proposed. An electro-acoustic analogy model is established to predict its acoustic performance and is validated through finite element simulation and impedance tube experiments, thereby revealing the underlying sound-absorption mechanisms. The proposed composite unit cell exhibits four characteristic absorption peaks over a broad frequency range, among which the peaks within 100-1000 Hz are directly relevant to transformer noise control. We systematically investigate the evolution of sound absorption from a single unit to coupled parallel structures and then to a 4 × 4 array, leveraging the tunability of the HRSS-HR structure. Parallel coupling broadens the effective absorption bandwidth but weakens the original impedance matching condition, whereas increasing the slit width and decreasing the neck height can effectively recover the low- and middle-frequency absorption performance. Finally, a 4 × 4 HRSS-HR composite metamaterial array is developed. Both simulation and experiment confirm that the optimized array maintains relatively high absorption levels near 100 Hz and the major harmonic bands of 200-500 Hz, while preserving good broadband absorption capability over 100-1000 Hz. This work validates the strong application potential of the proposed HRSS-HR design in transformer broadband noise control and offers a practical solution for low-frequency harmonic noise mitigation in transformers and similar power equipment.