Volumetric Density Governs Traffic Noise Attenuation in Urban Greenbelts: Evidence from Cross-Seasonal Monitoring Along a Subtropical Expressway

Urban traffic noise poses a critical environmental health burden; however, its mitigation via urban greenery is constrained by the limitations of conventional two-dimensional (2D) metrics. To address this gap, the Landscape Forest Space Density (LFSD), as a novel volumetric index quantifying the ratio of acoustically effective biomass to total canopy volume via geometric crown approximation, is introduced in this study. Field investigations were conducted along a 30 m transect perpendicular to a six-lane expressway in Nanjing, China. A-weighted sound pressure levels were recorded at 18 nodes (3 forest types × 4 distances, plus controls) under controlled meteorological conditions, with seasonal replication during spring (full-leaf) and autumn (leaf-off). Results indicate that deciduous coniferous forests delivered superior noise attenuation, achieving a peak insertion loss of 27.16 dB at 20 m depth in spring—exceeding evergreen broadleaf configurations by over 10 dB. Noise reduction followed a logarithmic growth pattern relative to roadside greenbelt width, reaching saturation beyond 20 m. Regression analysis identified LFSD as the dominant predictor of insertion loss (R2 = 0.86, N = 864), substantially outperforming traditional 2D metrics such as canopy coverage (R2 = 0.15) and optical porosity (R2 = 0.42). While a bivariate model incorporating both LFSD and porosity yielded an adjusted R2 of 0.89–0.94, the marginal incremental gain confirms that volumetric biomass packing, rather than planar void fraction, governs acoustic attenuation. Notably, even during defoliation, high-LFSD conifers maintained robust sound scattering through their fractal branch architectures. On the whole, a quantitative design threshold of LFSD ≥ 0.4 is recommended to achieve >15 dB attenuation in spatially constrained urban environments.