a vehicle soft impact detection system uses a piezo film mounted to a front vehicle bumper . the piezo film is selected and mounted to have a greater sensitivity to low - frequency shock waves as compared to high - frequency shock waves , the frequency and amplitude is then compared to a selected range of frequencies and amplitudes that are indicative of a crash with a soft tissue body . in a further embodiment the output of the piezo film is compared with the output of a second sensor which is more sensitive to high - frequency shock waves than to low - frequency shock waves such as a terfenol - d based sensor .