Sound absorption by tree bark

Scattering of sound waves by trunks is a main physical factor leading to sound pressure level reduction by tree belts, and it has been shown before that the absorbing properties of the trunks are relevant in this respect. However, detailed information on bark absorption is currently very scarce. Therefore, laboratory experiments were conducted with an impedance tube to measure the bark's sound absorption of various tree species, including characterizations of bark thickness, roughness, tree age and moss coverage. Preliminary measurements were made to come to a reproducible sample handling procedure. The measurements show that the absorption (at normal incidence) is generally below 0.1 for the species considered and rather frequency independent below 1 kHz. There are statistically significant differences in the averaged absorption between species. Overall, the barks of conifers absorb sound slightly better than in case of broadleaved species. The most relevant visual predictor for the sound absorption of bark is its roughness. Interestingly, moss grown barks provide a strong increase in absorption in the frequency range up to 800 Hz. Especially in dense tree belts, bark absorption might have an influence on the final noise shielding performance.