text: referring more particularly to fig1 - 4 , wherein like numbers refer to similar parts , a pedestrian 20 is shown being impacted by an automobile 22 which is equipped with the vehicle soft impact detection system 23 of this invention . in the event of such an impact , the pedestrian 20 normally impacts the bumper 24 first , followed by an impact with the car hood 25 . in order to allow timely activation of active devices 26 which form part of a safety system , and which are designed to prevent harm to the pedestrian , sensors are needed which indicate an impact with a pedestrian . the system 23 has a piezo film sensor 28 which can be used to differentiate between pedestrian impacts and other types of impacts . in particular , the piezo film sensor 28 is used to differentiate soft , low mass impacts , from crashes into a barrier or another vehicle , and also is used to differentiate soft low mass impacts from bumper strikes by rocks , ice or other road debris . it has been found experimentally that piezo film sensors are sensitive to low - frequency shock waves , as shown in fig3 , where a rubber hammer was used to create stress waves 30 in a metal plate ( not shown ). the resulting stress waves 30 have a frequency of about 500 - 600 hz and an amplitude of between 200 and 600 mv as detected by the piezo film sensor 28 . the table of fig4 shows the frequency and amplitude response , i . e . the stress waves 31 , of the piezo film sensor 28 responding to a ¼ inch steel ball - bearing dropped onto the metal plate from a height of about 1 m . the ball bearing produced a frequency of about 22 , 000 hz with an amplitude of about 200 mv . by using a ratio between the amplitude of the signal produced by the piezo film sensor 28 at two different frequencies , the impact of a pedestrian or animal can be differentiated from a stone strike , or an impact with a rigid object . for example , the output of the piezo film sensor 28 could be analyzed as a power spectral density function , which could then be correlated with the type of impact . a power spectral density function is defined as the fourier transform of the autocorrelation sequence of the time series , or the squared modulus of the fourier transform of the time series , scaled by a proper constant term . an alternative embodiment employs a terfenol - d type sensing element 32 such as disclosed in u . s . pat . no . 7 , 081 , 801 , the disclosure of which is incorporated by reference herein . the terfenol - d sensor 32 is a type of giant magnetostrictive sensing element with a composition of ( tb0 . 3 dy0 . 7 fe1 . 92 ). the terfenol - d sensor 32 shown in fig2 is located in substantially the same location on the bumper 24 as the piezo film sensor 28 , or may be spaced on the bumper from the piezo film sensor 28 . the terfenol - d sensor 32 has a frequency response that is substantially opposite the frequency response of the piezo film sensor 28 . as shown in fig3 , the terfenol - d sensor 32 responds to the rubber hammer blow with resulting stress waves 33 producing an output signal of about 50 mv , but , as shown in fig4 , the terfenol - d sensor responds to the ball - bearing hit and resulting stress waves 35 with an amplitude of over 1000 mv . thus it can be concluded that the output of both sensors 28 , 32 may be compared , so that when the amplitude of the piezo film sensor 28 exceeds the amplitude of the terfenol - d sensor 32 the automobile could be concluded to be impacting a soft body such as a pedestrian or an animal . in contrast , when the amplitude of the terfenol - d sensor 32 is greater than the amplitude of the piezo film sensor 28 , the automobile could be concluded to be impacting a rigid body . again , the power spectral density function of the output of each sensor 28 , 32 can be compared , or combined , and when frequencies below some cut off frequency , for example 1000 hz or 2000 hz , have a greater power density than the frequencies above the cut off frequency , then a soft body impact is taking place . when the situation is reversed a collision with a rigid body is taking place . the output of the piezo film sensor 28 or the piezo film sensor and the terfenol - d sensor 32 , can also be combined with the output of acceleration sensors mounted to the automobile as part of an overall crash detection system . the output of the acceleration sensors can provide a rough indication of the mass of the object with which the vehicle has collided . thus the mass of the object , and the power spectral density , can be used to decide the type of object that has been struck . other data available from the vehicle safety system or other vehicle sensors such as vehicle speed can also be combined , for example to differentiate between pedestrians which are typically struck at lower speeds , and animals which are typically struck at higher speeds . the sensors 28 , 32 , which are mounted directly to the bumper 24 , provide an early indication of the type of impact taking place , providing maximum time for deploying active systems , for example systems which elevate the automobile hood 25 either to prevent a pedestrian &# 39 ; s head from impacting rigid components under the hood 25 , or to allow the hood 25 to be raised sufficiently to prevent an animal with which the vehicle is colliding from breaking through the vehicle windshield 36 . piezo fluoropolymer film sensors are known and are , for example , supplied by measurement specialties , inc . the piezo film sensor might also be constructed from other film materials for example piezoelectric polymers that are known today include polyparaxylene , poly - bischloromethyuloxetane ( penton ), aromatic polyamides , polysulfone , polyvinyl fluoride , synthetic polypeptide and cyanoethul cellulose . the film sensor 28 may be bonded to a rigid support 34 , as shown in fig2 , which is riveted , screwed or bolted to the bumper 24 in a way similar to the terfenol - d type sensor shown in commonly owned u . s . patent application ser . no . 10 / 792 , 739 . alternatively , the piezo film sensor may be bonded directly to or formed with the bumper 24 . similarly , the terfenol - d type sensor may be mounted by screws , rivets or bolts to the bumper 24 . it is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described , but embraces all such modified forms thereof as come within the scope of the following claims .