1. Field of the Invention
This invention relates to acoustic absorption and damping materials, and more particularly, to acoustic absorption and damping materials that utilize a piezoelectric phenomenon to convert mechanical energy into electrical energy and to subsequently dissipate the converted energy as heat.
2. Description of Related Art
Absorbing or damping unwanted acoustic or vibrational energy involves converting that energy into another form, usually heat. At the molecular level, the only distinction between heat energy and acoustic or vibrational energy is the randomness of the vector directions of molecular displacements. Acoustic and vibrational energy is highly correlated with large numbers of molecules displacing at the same time and in the same direction. Heat in a particular object may well have the same or more energy than propagating acoustic or vibrational energy, but the motion of the molecules is random with the mean molecular displacement at any given location being near zero.
Two primary techniques are available for randomizing the vector directions of the molecules in a matrix material propagating acoustic or vibrational energy. Cushman, et al. (U.S. Pat. No. 5,400,296) teach the use of two or more species of particles with differing characteristic impedances in a matrix material to promote random internal reflections at boundaries within the matrix material and the subsequent increase in probability that phase cancellation at adjacent or nearby locales can take place. Single particle species may also be used in this manner, but with less effect. Phase cancellation effectively randomizes the vector direction of molecular movement where it occurs. A second approach involves the careful choice of materials that exhibit a high degree of internal hysteresis. This internal hysteresis is thought to be caused by metastable molecular energy levels within the material. Propagating acoustic or vibrational energy may boost a particular molecule into a higher energy level, thus subtracting that energy from propagating energy, where the molecule remains for some time before randomly returning to its original energy level. For a discussion of this effect see Hartmann and Jarzynski, "Ultrasonic hysteresis absorption in polymers," J. Appl. Phys., Vol. 43 , No. 11, November 1972, 4304-4312.
Instead of randomizing molecular displacements to dissipate propagating acoustic or vibrational energy, some of this energy can be removed by converting the mechanical energy of sound or vibration into electrical energy utilizing the piezoelectric effect. A piezoelectric material such as polyvinylidene fluoride (PVDF) may be polarized and a coating of a conductive material such as aluminum applied to produce a piezoelectric transducer that will convert acoustic energy into electric energy, thus facilitating removal of converted energy from the system. This approach is reported in a recent issue of the Japan New Materials Report (May-June, 1995, p 9). In this report acoustic energy reductions of up to 90% are claimed in material specimens only 10 to 30 microns thick. However, the need to polarize the material and apply conductive electrodes to tap off the electrical energy produced limits the usefulness of this technique.