Structural vibration noise from open grid bridge decks

The contribution to the overall tonal component of the noise from the vibration of the grid section of an open grid bridge deck due to excitation by the interaction with the vehicle tires is investigated. Using vibration field measurements of the grid and laboratory measurements of the acoustic radiation efficiency, it is shown that the contribution to the overall noise level measured at the curb from the vibrations of the grid is insignificant. The main reason for this result is the poor acoustic radiation efficiency of the grid. Because of the large percentage of open area, the plate-like modes of the grid deck have very poor radiation efficiency. Similarly, the individual grid elements, based on their size and the frequency range where the predominant excitation from the tire takes place, are also very inefficient acoustic radiators. 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. [Work sponsored by FDOT.]