Ground effect for low-frequency noise

Low-frequency noise is an environmental concern, as it is poorly attenuated by buildings and atmospheric conditions, allowing it to propagate over long distances and affect human well-being. It is emitted by sources such as vehicles, wind turbines, and industry, and is often underestimated by standard A-weighting. Noise propagation in open space is significantly influenced by the ground effect. This phenomenon involves the interference between direct and ground-reflected sound waves, determined by the ground's finite acoustic impedance, characterized by the effective flow resistivity, . At very low frequencies, this interaction often results in constructive interference and increased sound pressure levels. This research aimed to verify a calculation model for accurately predicting low-frequency sound pressure levels, specifically considering the ground interaction. The study involved measuring single vehicle pass-by noise at two distances from a road: 7.5m and 50m. Sound power levels of the vehicles were determined from measurements taken closer to the road (7.5m). The effective flow resistivity, , of the ground was estimated by comparing maximum sound levels measured at the two points with model predictions for different  values. Using the estimated  value and the calculated sound power levels, noise exposure levels at the 50m point were predicted using the ground interaction model and compared with the measured values. The results showed good agreement between calculated and measured noise exposure levels, particularly for low frequencies. Larger differences observed at medium and high frequencies were attributed to atmospheric turbulence, which has a less significant impact on low frequencies due to their larger wavelengths. The study concluded that the utilized model allows for accurate prediction of noise levels at a greater distance from the source, especially in the low-frequency range, by correctly modelling the acoustic interaction with the ground surface.