Dense vegetation barely reduces highway noise below 2000 Hz

Evidence

Trees, shrubs, and low, round hills often help only a little. Recently, the author made measurements near a 15-foot-thick, 10-foot-high, and 100-m-long, dense, neatly trimmed shrub with squared sides near a busy highway. There was only a few dB difference on the highway side versus the “protected” side. Despite what our eyes tell us, such a shrub acts like a continuum region with a somewhat slower wave speed, at least for sound below about 2000 Hz in frequency, since the objects doing the scattering are packed closer together than a wavelength. — Eric J. Heller, Why You Hear What You Hear- An…, 2603

Categorization

• XREF: Connects to the ‘Grafting & Growth: On Trees as Infrastructure’ piece’s premise about vegetation — here vegetation fails as an acoustic infrastructure buffer, a useful counterpoint. — connects to: Ground impedance, not foliage, governs low-band attenuation by vegetation. — Donald Aylor, Noise Reduction by Vegetation and Ground, 2; Ground and vegetation attenuate noise in complementary frequency ranges. — Donald Aylor, Noise Reduction by Vegetation and Ground, 122
• QUESTION: What planting density or species composition WOULD meaningfully attenuate traffic noise? The passage only establishes the null case.