Abstract: For low-frequency noise reduction, this article investigates composite sound absorption structures based on Helmholtz resonators with extended necks, micro-perforated panels, and porous materials. First, based on transfer matrix theory, a theoretical sound absorption model is established for composite structures comprising both single-unit and multi-unit cell configurations. The accuracy of the unit-cell theoretical model is validated using the acoustic finite element method, and the influence of key structural parameters on the sound absorption coefficient is systematically analyzed. Next, for the target frequency band of 50–250 Hz, a multi-unit cell composite structure is optimized and designed; its sound absorption performance is then verified via numerical simulation. Finally, optimized structural prototypes are fabricated, and their sound absorption coefficients are measured using an impedance tube. Results show that the experimentally measured sound absorption coefficient agrees well with both the theoretical predictions and simulation results, confirming the validity of the proposed theoretical model and optimization methodology. The test sample achieves a sound absorption coefficient greater than 0.7 over the frequency range of 43–270 Hz, with an average coefficient of 0.83 across 50–250 Hz. Moreover, the total thickness of the absorber is only 1/20 of the wavelength corresponding to the lowest operating frequency (50 Hz), and the sound absorption curve remains smooth within the target band