Abstract: Conventional acoustic structures struggle to effectively suppress noise in the extremely low frequency range. This paper describes different gradient methods to determine the structural parameters for a non-uniform, segmented composite structure composed of five sub-units. Finite element simulations were performed to analyze the influence of sub-unit structural parameters on sound absorption characteristics by changing the structural parameters of the sub-unit. The sound absorption mechanism of the sub-unit was also explained. The research shows that the sound absorption performance of this structure is comparable to that of the unfolded structure, and better than that of the folded connected structure and the uniform segmented structure. The simulation results are consistent with the experimental results. The five sub-unit structures designed at a thickness of 78 mm can achieve effective sound absorption in the frequency range of 83–102 Hz, which provides a solution for ultra-low frequency noise control at sub-wavelength scale.