Open grid bridge noise from grid and tire vibrations

The noise generated during the passage of vehicles on open grid bridge decks has a strong tonal component, which can be a source of annoyance to nearby residents. Potential sources of noise are the vibrations of the grid and the vibrations of the tire. In this paper, the relative contributions from the grid and the tire are investigated by using analytic modelling and field and laboratory measurements. It is shown that the contribution to the overall noise level measured at the curb from the vibrations of the grid is small compared to that from the vibrations of the tire. This is attributed to the fact that the grid has a poor acoustic radiation efficiency because of the large percentage of open area, and the individual grid elements are also very inefficient acoustic radiators in the frequency range of interest. Based on an estimate of the interaction force between the tire and the grid, the radiated noise from the tire alone accounts for the field measured noise level at the curb. To verify that the tire is the dominant source of noise, sound pressure level measurements were performed on a simulated track with the contribution from the grid vibrations removed. The measured sound pressure levels were similar to those measured at the curb. The significance of this result is that mitigation procedures that only reduce the vibration of the grid will not significantly reduce the overall radiated noise, and alternative mitigation procedures must be considered that attempt to reduce the excitation of the tire. (C) 1996 Academic Press Limited