Experimental Study Of The Nonlinear Properties Of Microinhomogeneous Solidmedia

The nonlinear acoustics is a branch of the acoustics, which is about the acoustic phenomenon of the finite amplitude or large amplitude acoustic wave. The nonlinear acoustic phenomenon is very common, such as the standing wave field in an acoustic resonator, explosion, and the intense sound filed around the engine. The cumulative character of nonlinear acoustic filed often leads to considerable distortions of the wave profile, and even shock waves in the end.The nonlinearity of various elastic media are very different. Some media have small nonlinearity, such as water, steel etc. On the other hand, for some other media such as air, biological soft material etc., their nonlinear coefficients are big. However, as for some ’dispersive’ media, like liquids with gas bubbles and micoinhomogeneous solid media,’anomalously’ large nonlinearity has been observed. In those dispersive media, even at relatively small amplitudes of the acoustic filed (in the sense of smallness of the acoustic Mach number), strong nonlinear phenomena still occur. Such rather new effects for acoustics as self focusing, self-induced transparency, wavefront reversal, etc. can be observed. These study objects of nonlinear acoustic seem to be becoming as various as those one used to see, for example, in plasma physics and nonlinear optics. The dispersive media bring new approach to make a acoustic rectifier. Just as its counterpart of electronic rectifier have brought revolutionary influence to electronics, the successful fabrication of acoustic rectifier will bring tremendous driving force to acoustics.In this work, we have investigated the nonlinearity of the microinhomogenous solid media experimentally. According to the analogy between the porous solid media and liquids with gas bubbles, it was considered that just as the resonance of the gas bubbles in the liquids, the resonance of the cavity in the solid media leads to the strong nonlinearity in the past studies. However, the theoretical results and the experimental results agree badly. In our experiments, the influence of the cavity is hard to be found. Though the mechanism are not altogether clear, we believe that the microstructure of the molecule, such as ring, chain and net (which are often composed of large amount of material and water molecules) are responsible for their strong acoustic nonlinearity.We have also measured the strength of the second harmonics after propagation through different material length. The nonlinear coefficient of the material is evaluated. The material shows strong nonlinearity at low frequency which is a bit above the audio frequency range. At high frequency range of several MHz, no obvious nonlinear phenomena can be observed.