Study On Acoustic Energy Transmission In Periodic Composite Media And Its Application In Muscle Fatigue Assessment

When the elastic wave transmit in the complex structure which is consisted of different elastic material arranged periodically, the phenomenon of the pass-band, the forbidden band, defect mode and so on would arise same as the effects in the photonic crystal. Then the concept of the phononic crystal was introduced to that complex structure.It makes senses to explore the characteristics about the band gap for both the theoretical and practical filed of phononic crystal. The investigation about the change of energy during the wave propagation along the crystal helps to study the interaction between the elastic wave and the internal crystal structure, and it is also meaningful for the application of the crystal such as the vibration and noise reducing, the acoustic functional devices, the sound wave focusing and the like.The phenomenon that the tension and contraction force the muscle needed can not be recovered after a long time exercise, the muscle can not do continuing repeated contraction movement was named the Muscle Fatigue. Muscle fatigue degree can be used as an important evaluation parameter for the muscle injury, the sports training, and the disease. Muscles are mainly composed of the muscle fibers, and they were connected by the connective tissue. Normally, the fibers that support the motor function of the organism (the movement of the body and the wiggling of the digestive tract) are amount of contractile cylinders that are well-regulated arranged and parallel with the longitudinal axis of the cell under the microscope. Therefore the muscle tissue can be regarded as a kind of sound transmission medium which is arranged by muscle fibers and connective tissues in order. Base on the transmission law of the wave energy in the two dimensional phononic crystal, this paper intends to discuss a new method for Muscle Fatigue Evaluation by studying the acoustic wave transmission effect in muscle tissues.The content of this thesis is discussing the transmission law of the wave energy in the band gap and the pass band of the crystal based on the Finite Difference Time Domain (FDTD) and studying the sound wave transmission effect in the process of muscle fatigue.The primary work of this paper is given as follow:(1) Investigating the transfer characteristics of the acoustic energy through the crystal by FDTD. According to the relation between energy and vibration, the vibration parameters is used to express the energy transfer characteristics of the acoustic propagation. By changing the number of the periods, we analysis the energy loss of the wave with a band gap frequency which propagates through the periodical media; we make a comparison between the propagation of acoustic wave with a band-gap frequency and a pass-band frequency by using the energy transfer characteristics of the acoustic wave with a single frequency, we found that the band-gap acoustic energy is damping quickly during a period and the attenuation changes slower after a long time, as a result, the whole vibration time is longer than the pass-band acoustic wave. The passing of the band-gap acoustic vibration through the media takes a longer time.(2) Based on the discussion on the transmission law of the acoustic energy in the two dimensional crystal, we have analysed the acoustic effect during the fatigue of the skeletal muscle. According the Skeletal Muscle Fatigue Theory, the fibers become harder and the filling rate of the fibers become smaller. In consideration of the change of the muscle’s parameter, we modeled the muscle by two dimensional crystal and studied the acoustic effect during the fatigue of the skeletal muscle base on FDTD.During the process of muscle fatigue, the central frequency of elastic wave passed through the muscle shift to the low frequency gradually.We have conducted specific experiments in this paper to test the theoretical analyses and stimulations. The acoustic signal is transmitted and received by ultrasonic probes and the information of the sound which passed through the gradually exhausted biceps brachii muscle is recorded. The FDTD is used to get the parameter of the acoustic wave transmission and the FFT of the received signal is computed to get the frequency spectrum curve. During the experiments, the inside information of muscles is recording real-time by B ultrasonic to observe and analyze the changes of muscles internal organization during exhausting. The investigating of acoustics effect during the muscles exhausting provide a theoretical basis for the harmless judgments of the muscle damage degree in daily life.