On the basis of analyzing the longitudinal forced vibration of the gas column in the resonator of a half-wavelength standing wave thermoacoustic refrigerator, the idea of replacing the standing wave vibration with the first-order piston mode vibration of the gas column in the thermoacoustic refrigerator was proposed, so as to increase the vibration amplitude of the gas in the resonator, realize the large alternating flow of gas, and improve the working efficiency of the thermoacoustic refrigerator. At the same time, its rationality and feasibility are discussed from the aspects of modal vibration, pressure distribution of gas column piston, and temperature variation of gas microcluster vibration in equilibrium position. Combined with the existing research results, the superiority of the application of the first-order piston mode vibration of the gas column in the thermoacoustic refrigerator is further explained. The results provide a new research idea and direction for further reducing the cold end temperature of thermoacoustic refrigerator.
The propagation characteristics of ultrasonic waves on the periodic rough interface between fluid and porous medium are studied,and the effect of porosity on reflection and transmission in the porous medium with open pores and sealed pores is analyzed.Based on Biot theory of porous medium and the grating equation theory,the linear equation including reflection coefficient and transmission coefficient of each order is obtained..The relationships of porosity with reflection and transmission coefficients of the periodic rough interface are calculated by Fourier transform.The results show that the influence of frequency on the reflection and transmission coefficients is irregular due to the periodic characteristics of the interface.However,the variation of porosity has significant effect on reflection and transmission coefficients.Moreover,the coefficient changes are different in open pores and sealed pores because of the discrepancy in the porous medium states.
Abstract Photoacoustic (PA) technology shows great potential in bone microstructure assessment. Due to the solid-liquid two-phase characteristics of cancellous bone, the PA signal received by transducers is aliasing of distributed PA source characteristics and acoustic propagation characteristics of cancellous bone. The contribution of various multi-scale distributed PA sources to the PA signal complexity leads to difficulty in identifying the sound propagation characteristics, making it difficult for quantitative assessment of bone microstructure. To eliminate the spectrum contribution of the PA source to the PA signal, we proposed an eccentric excitation-differential detection system and the acoustic propagation characteristics of cancellous bone can be obtained by calculating the PA differential attenuation spectrum (PA-DAS). The PA-DAS quantization parameter-slope is further extracted to quantify the attenuation of different frequency components. Simulation results show that slope strongly correlates to the porosity of cancellous bone, demonstrating the eccentric excitation-differential detection system combined with the PA-DAS method can realize the quantitative bone assessment.
基于热声效应,对基底厚度小于热渗透深度的石墨烯薄膜发声器进行理论和仿真研究.首先,利用石墨烯薄膜发声器的热功率平衡方程与气体中的热弹性耦合线性方程组推导出了石墨烯薄膜发声器的近/远场声压表达式.将理论计算结果与实验测试值进行对比,二者吻合良好,验证了理论模型的正确性.然后,利用仿真软件对石墨烯薄膜的近远场声压值进行了仿真计算,并将仿真值与理论值进行对比,二者有良好的一致性,验证了仿真计算方法的有效性.研究表明,石墨烯薄膜发声器的远声场为球面波,近声场近似为平面波.在远场低频段,声压级随输出声频率的增加而缓慢增大;在近场高频段,声压级几乎不受输出声频率的影响.研究结果为基底厚度小于热渗透深度情形下的热致发声器提供了理论计算和分析方法,对石墨烯薄膜声源器件的实验研究具有指导意义.
通过稀疏重构得到传感器阵列输出数据的稀疏表示模型,研究了单快拍采样情形下的信号到达角(Direction of Arrival,DOA)估计问题.提出了一种基于最小均方误差(Minimum Mean-Square Error,MMSE)准则迭代实现的单快拍到达角估计算法(Iterative Implementation of MMSE,II-MMSE).该算法将原有的稀疏表示模型中稀疏信号矢量的求解问题,转化为迭代求解稀疏功率对角阵,进而估计多目标信号的DOA.给出了算法的完整实现流程,从理论上分析了II-MMSE算法的迭代收敛性和对阵列模型误差的鲁棒性.仿真结果表明,II-MMSE算法在低信噪比、相干背景、小样本、阵列未校准等条件下都具有良好的测向精度和多目标分辨能力.
