Acoustical performance of steel coil springs for vibration isolation

The acoustical performance of a set of commercially available steel coil springs was evaluated supporting a rigid structure above a slab on grade. The vibration isolation of this simple system is determined by exciting the on-grade slab with a tapping machine and measuring the resulting vibration transmitted into the isolated rigid structure above. Insertion loss of the system is determined by comparing the vibration level on the rigid structure with and without the vibration isolators under the same dynamic forces. The test results reveal the natural frequency, damping ratio, and insertion loss of the system as a function of the loading on the coil springs. The results also reveal the presence of surge frequencies (or wave resonances) that significantly degrade the system performance of at these specific frequencies, which are usually greater than 50 times the natural frequency of the loaded steel spring. Because of potential structure-borne noise transmission through steel coil springs into the supporting structure, designers should be cautioned when considering the use of steel springs to support equipment that can generate strong tonal vibrations in the surge frequency region of the springs.