声功率测定中的低频率差异问题

在混响室中测定声功率,方法简单,但在低频率所得结果比自由场值要低。本文证明,在混响室中测定功率不但与声源的自由空间功率有关,并且与声源和接收器的位置以及测量方法、平均方法有关。文中导出了反映这些关系的准确公式及统计公式。所得结果解释了低频差异问题,与前人报道的实验结果相符。根据所得理论,提出容易实现的声功率测定技术,以及适用的正确公式。     

低频声音在水雾中衰减的测量

一.引言 低频声音在霧气中的衰减的测量可以在充霧的混响室中行之,但是这种测定方法只限制在一些分立的低频率——即相当于混响室的低简正振动方式。为了使这种测量能够在较广的连续频程中进行,作者採取了阻抗管的驻波分析方     

EXISTENCE OF NATURAL FREQUENCIES OF SYSTEMS WITH ARTIFICIAL RESTRAINTS AND THEIR CONVERGENCE IN ASYMPTOTIC MODELLING

A major limitation of the Rayleigh-Ritz method for determining the natural frequencies of a system is the need to choose admissible functions that do not violate the geometric constraints of that system (Courant 1943 Bulletin of the American Mathematical Society49, 1-23). Several researchers have attempted to overcome this problem by asymptotically modelling the rigid constraints with artificial (imaginary) restraints of very large stiffness (Courant 1943Bulletin of the American Mathematical Society49 , 1-23; Warburton and Edney 1984 Journal of Sound and Vibration95, 537-552; Gorman 1989 Journal of Applied Mechanics56, 893-899; Kim et al. 1990 Journal of Sound and Vibration143, 379-394; Yuan and Dickinson 1992 Journal of Sound and Vibration153, 203-216; Yuan and Dickinson 1992 Journal of Sound and Vibration159, 39-55; Cheng and Nicolas 1992 Journal of Sound and Vibration155, 231-247; Yuan and Dickinson 1994Computers and Structures53 , 327-334; Lee and Ng 1994 Applied Acoustics42, 151-163; Amabili and Garziera 1999 Journal of Sound and Vibration224, 519-539; Amabili and Garziera 2000 Journal of Fluids and Structures14, 669-690). While the numerical results thus obtained for the systems considered in the literature were in close agreement with exact values for the natural frequencies corresponding to the first few modes, sample calculations show that the error introduced by the asymptotic modelling increases with mode number and therefore to obtain accurate results for higher modes the magnitude of stiffness should also be increased. In any event, the error due to the asymptotic modelling would remain uncertain, except when the correct frequency values are known. However, the use of artificial restraints with negative stiffness, a new concept which was introduced in a recent publication (Ilanko and Dickinson 1999 Journal of Sound and Vibration219, 370-378) paves the way for estimating the error due to asymptotic modelling. This is possible since in this work, the Rayleigh-Ritz frequencies of the constrained system were found to be bracketed by the frequencies of the asymptotic models with positive and negative restraints. However, the use of artificial restraints with negative stiffness has raised some important questions: would a system with a large negative restraint become unstable, and if so what is the guarantee that the frequencies of the asymptotic model would converge to that of the constrained system? This paper is the result of the author's attempt to answer these questions and gives a proof of existence of natural frequencies for systems with artificial restraints (springs) having positive or negative stiffness coefficients, and their convergence towards constrained systems. Based on Rayleigh's theorem of separation, it has been shown that a vibratory system obtained by the addition of h restraints to an n -degree-of-freedom (d.o.f.) system, where h<n, will have at least (n÷h) natural frequencies and modes and that as the magnitude of the stiffness of the added restraints becomes very large, these (n÷h) natural frequencies will converge to the (n÷h) natural frequencies of a constrained system in which the displacements restrained by the springs are effectively constrained.     

EXPERIMENTAL INVESTIGATION ON THE HEAT TRANSFER OF TURBULENT FLOW IN A PIPE OSCILLATING AT LOW FREQUENCIES

An experimental study was performed focusing on heat transfer and friction coefficient associated with turbulent oscillating tube flow. For this goal an oscillating mechanism was designed. Experiments were conducted for the low oscillating frequency in the range of 0.008–1.988 Hz and dimensionless amplitude was chosen as X₀ = 0.3, 0.6, and 0.9. Reynolds number was changed from 0.5 × 10⁴ to 2.5 × 10⁴. The bulk temperature of the fluid at the exit of the oscillating section was fond to be increasing with oscillating frequency and amplitude. For the oscillating cases, heat transfer enhancement is obtained 52% for f = 1.988 s^?1, 40% for f = 1.320 s^?1, and 28% for f = 0.008 s^?1, in comparison with the smooth pipe at the highest Reynolds number. The results also showed that Nusselt number and friction coefficient also increased with increasing frequency and amplitude.     

轴向极化压电陶瓷圆片的自由振动低阶振动方式分析

本文利用解微分方程二点边值问题的伴随法^[6],对应力自由边界条件下的轴向极化压电陶瓷圆片振子的压电运动方程进行了数值积分。求出了厚度与直径比小于0.45的振子在厚度方向运动影响下的低阶径向模的谐振频率及质点位移分布图象。给出了基频的厚度影响修正曲线及与泊松比有关的泛音比曲线。
本文的数值计算全面考虑了压电性的作用,未对振子尺度作任何限制.实验结果与计算结果很好相符。