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.