Effects of cavity resonances on sound transmission into a thin cylindrical shell

In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model is presented for the effects of internal cavity resonances on sound transmission into a thin cylindrical shell. The “noise reduction” of the cylinder is defined and computed, with and without including the effects of internal cavity resonances. As would be expected, the noise reduction in the absence of cavity resonances follows the same qualitative pattern as does transmission loss. Numerical results show that cavity resonances lead to wide fluctuations and a general decrease of noise reduction, especially at cavity resonances. Modest internal absorption is shown to greatly reduce the effect of cavity resonances. The effects of external airflow, internal cabin pressurization, and different acoustical properties inside and outside the cylinder are also included and briefly examined.     

粘弹性夹层板结构声传入的理论分析

应用有阻尼的简正模态概念,推导出夹层板与腔耦合条件下的声波基本方程,并通过数值计算研究了粘弹性层的特性参数对板结构振动及其辐射噪声的影响。结果表明,利用粘弹性层的剪切效应,可以有效地抑制结构的振动和由此所建立的闭空间声压。     

Acoustic streaming with resonance gas oscillations in a cylindrical tube

Acoustic streaming accompanying acoustic resonance oscillations of gas in a tube is considered. The effect of both the Prandtl number and the wall loss on the velocity of acoustic streaming in a viscous heat-conducting medium is investigated. Expressions for the longitudinal and transverse components of the flow velocity are obtained.     

水下充水圆柱声腔内声场自适应有源控制实验研究

本文研究了有限长充水圆柱置于水中，外声场透射形成的腔内声场自适应有源控制实验研究，结果显示，由于圆柱结构与水介质的耦合，有源控制中的声控制方法能够较好的抵消声腔主导模态和强耦合的结构主导模态，因而能够抵消在圆柱腔内较宽频带范围的声场，实验还研究了消声频带，误差传感器布放位置及肖声区域等问题。     

基于近场声全息的圆柱壳体内部声场重构效果的影响因素

在Neumann边界条件下基于近场声全息原理(NAH)重构了两端封闭的有限长圆柱壳体内部声场,计算出的结果与基于模态叠加法构建的内部声场进行比较,证明了重构方法的正确性,然后分别讨论了激励频率和全息面选择对NAH重构声场精度的影响以及重构面上不同位置的重构效果。     

Mechanisms of active control of sound transmission through a linked double-wall system into an acoustic cavity

The mechanism of active control on sound transmission through a mechanically linked double-wall structure into an acoustic cavity is investigated in this paper. Two control methods, i.e., structural control and acoustic control under two linkage cases (soft and hard) are investigated to analyze the effect of the links on the selection of control strategies and the corresponding control mechanisms. Simulations are performed to examine the dominant control mechanism (modal suppression or modal rearrangement) in different frequency ranges for each control case. The alteration in the structural-acoustic coupling is also analyzed so as to explain the mechanisms of sound attenuation. In addition, the dominance of the acoustic mode (0, 0, 0) in the energy transmission process as well as its use in designing a more effective sensor/actuator arrangement is discussed.     

Acoustic resonance scattering from a multilayered cylindrical shell with imperfect bonding

The method of wave function expansion is adopted to study the three dimensional scattering of a time-harmonic plane progressive sound field obliquely incident upon a multi-layered hollow cylinder with interlaminar bonding imperfection. For the generality of solution, each layer is assumed to be cylindrically orthotropic. An approximate laminate model in the context of the modal state equations with variable coefficients along with the classical T-matrix solution technique is set up for each layer to solve for the unknown modal scattering and transmission coefficients. A linear spring model is used to describe the interlaminar adhesive bonding whose effects are incorporated into the global transfer matrix by introduction of proper interfacial transfer matrices. Following the classic acoustic resonance scattering theory (RST), the scattered field and response to surface waves are determined by constructing the partial waves and obtaining the non-resonance (backgrounds) and resonance components. The solution is first used to investigate the effect of interlayer imperfection of an air-filled and water submerged bilaminate aluminium cylindrical shell on the resonances associated with various modes of wave propagation (i.e., symmetric/asymmetric Lamb waves, fluid-borne A-type waves, Rayleigh and Whispering Gallery waves) appearing in the backscattered spectrum, according to their polarization and state of stress. An illustrative numerical example is also given for a multi-layered (five-layered) cylindrical shell for which the stiffness of the adhesive interlayers is artificially varied. The sensitivity of resonance frequencies associated with higher mode numbers to the stiffness coefficients is demonstrated to be a good measure of the bonding strength. Limiting cases are considered and fair agreements with solutions available in the literature are established.     

Study on sound transmission characteristics of a cylindrical shell using analytical and experimental models

A circular cylindrical cavity enclosed by a thin elastic shell is found in many practical devices such as expansion volume mufflers, hermetic compressors and aircraft cabins. Analytical and experimental studies are conducted in this work to understand the characteristics of sound transmission through the cylindrical wall of such a system. Using an infinitely long circular cylindrical shell subjected to a plane incident wave, an exact solution is obtained by solving the classical shell vibration equations and the acoustic wave equations simultaneously. Transmission losses obtained from the solution are compared to the transmission losses that are measured for a cylindrical shell of finite length and the same cross-sectional dimensions. The comparison suggests that the theoretical model can be used as an effective design tool despite considerable simplifications involved.     

Modeling sound transmission through the pulmonary system and chest with application to diagnosis of a collapsed lung

A theoretical and experimental study was undertaken to examine the feasibility of using audible-frequency vibro-acoustic waves for diagnosis of pneumothorax, a collapsed lung. The hypothesis was that the acoustic response of the chest to external excitation would change with this condition. In experimental canine studies, external acoustic energy was introduced into the trachea via an endotracheal tube. For the control (nonpneumothorax) state, it is hypothesized that sound waves primarily travel through the airways, couple to the lung parenchyma, and then are transmitted directly to the chest wall. In contradistinction, when a pneumothorax is present the intervening air presents an added barrier to efficient acoustic energy transfer. Theoretical models of sound transmission through the pulmonary system and chest region to the chest wall surface are developed to more clearly understand the mechanisms of intensity loss when a pneumothorax is present, relative to a baseline case. These models predict significant decreases in acoustic transmission strength when a pneumothorax is present, in qualitative agreement with experimental measurements. Development of the models, their extension via finite element analysis, and comparisons with experimental canine studies are reviewed.     

Acoustic transmission performance of double-wall active sound packages in a tube: Numerical/experimental validations

A method based on the combination of the (u, p) formulation and the finite element approach is proposed for calculating the transmission loss of double-wall active sound packages with porous-material cores in a rectangular tube. The (u, p) formulation based on the displacement in the solid phase and the pressure in the fluid phase is rewritten to investigate sound propagation in porous materials, and the coupled boundary conditions and related parameters involved are given. The transmission loss of the double-wall active sound packages excited by a plane wave is calculated via the COMSOL environment with passive and active control. Moreover, a two source-location method is developed to measure the transmission loss of these double-wall active sound packages in a rectangular tube. Some results from numerical examples are shown to be in good agreement with the measured data. It is concluded that with active control, the transmission loss of double-wall active sound packages can be improved by more than 10 dB around resonance frequencies.     

圆柱腔低频声场的球谐函数分解及声场再现

针对圆柱腔低频声场的球谐函数分解,提出了基于移动式球形传声器阵列(简称球阵)测量的空间域直接求解法。利用空间域测量数据,在压缩感知(Compressive Sensing,CS)理论框架下通过一次线性方程组求解,获得整体坐标系下的声场展开系数。首先讨论了感知矩阵的列相关性,比较不同球阵形式及球面采样点数目,不同球谐函数截断阶数以及空间测量位置等的影响。随后在飞机舱室模型内利用文中提出的方法求解声场展开系数,再现腔内水平面的声压分布,并与传统球坐标变换方法做比较。实验结果表明,利用空间域直接求解法,通过球面随机挑选10个采样点的刚性球阵在声腔3个位置进行声场测量,不仅能够有效求解声场展开系数,而且声场再现精度更高,同时计算效率也显著提高。     

基于声强测量的圆柱内部全息柱面复声压相位重构

本文把基于声强测量的全息相位重构原理应用到圆柱内部声场中,阐述了圆柱面相位重构原理,推导了相应的有限空间离散算法并计算了圆柱声腔内主要声模态。结果表明,重构相位和理论值有较好的吻合。最后,分析了重构频率、全息孔径及全息柱面与壳体的间距等重构参数对相位重构精度的影响。     

On sound transmission into a thin cylindrical shell under “flight conditions”

In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model for sound transmission into a thin cylindrical shell is used to study sound transmission under “flight conditions”: i.e., under conditions of external air flow past a pressurized cylinder at flight altitude. Numerical results for different incidence angles are presented for a typical narrow-bodied jet in cruising flight at 10 660 m (35 000 ft) with interior pressure at 2440 m (8000 ft). A comparison is made between no-flow sound transmission at standard conditions on the ground to sound transmission under flight conditions. It is shown that at M = 0, the cylinder transmission loss has dips at f_R (cylinder ring frequency) and f_c (critical frequency for a flat panel of same material and thickness as shell). Below f_R cylinder resonances affect TL. Between f_R and f_c, cylinder TL follows a masslaw behavior. Flow provides a modest increase in TL in the mass-law region, and strongly interacts with the cylinder resonances below f_R. For normally-incident waves, TL is unaffected by flow.     

Acoustic roles of the laryngeal cavity in vocal tract resonance

The acoustic effects of the laryngeal cavity on the vocal tract resonance were investigated by using vocal tract area functions for the five Japanese vowels obtained from an adult male speaker. Transfer functions were examined with the laryngeal cavity eliminated from the whole vocal tract, volume velocity distribution patterns were calculated, and susceptance matching analysis was performed between the laryngeal cavity and the vocal tract excluding the laryngeal cavity (vocal tract proper). It was revealed that the laryngeal cavity generates one of the formants of the vocal tract, which is the fourth in the present study. At this formant, the resonance of the laryngeal cavity (the 1/4 wavelength resonance) induces the open-tube resonance of the vocal tract proper (the 3/2 wavelength resonance). At the other formants, on the other hand, the vocal tract proper acts as a closed tube, because the laryngeal cavity has only a small contribution to generating these formants and the effective closed end of the whole vocal tract is the junction between the laryngeal cavity and the vocal tract proper.     

The application of mode coupling theory to the transmission of sound in the sidebranch of a rectangular duct system

Mode coupling is applied to rectangular ducts of the size encountered in modern ventilation systems. Theory is developed to describe the transmission of sound along a straight duct, and along a sidebranch. Acoustic impedance boundary conditions at the duct walls are included. Experimental and predicted results are presented for the frequency spectra, cross-sectional variation and longitudinal variation in the entry and sidebranch ducts caused by a pure tone source. Good agreement is shown between the two sets of results.     

Experimental and numerical analysis of floating floor resonance and its effect on impact sound transmission

This paper is concerned with the analysis of the dependence of low frequency impact sound transmission through floating floor systems on in situ matched resonances. The evidence for the floating floor matched resonances has been found previously considering laboratory and numerical tests for one concrete slab and floating floor with varying resilient layers. In the present paper, considering laboratory and field tests for concrete slabs and floating floors with different plan configurations, this evidence is strengthened as differences between laboratory and field measurements of the impact sound level were negligible for the bare concrete slab but not with the floating floor installed. These results were also confirmed numerically. The analysis indicates that the dependence of low frequency impact sound transmission through floating floor systems on in situ matched resonances should be considered in addition to the conventional single degree of freedom models in order to improve accuracy.     

Electron cyclotron resonance heating in a short cylindrical plasma system

Electron cyclotron resonance (ECR) plasma is produced and studied in a small cylindrical system. Microwave power is delivered by a CW magnetron at 2.45 GHz in TE¹⁰ mode and launched radially to have extraordinary (X) wave in plasma. The axial magnetic field required for ECR in the system is such that the first two ECR surfaces (B = 875.0 G andB = 437.5 G) reside in the system. ECR plasma is produced with hydrogen with typical plasma density n^e as 3.2 × 10¹⁰ cm^-3 and plasma temperature T^e between 9 and 15 eV. Various cut-off and resonance positions are identified in the plasma system. ECR heating (ECRH) of the plasma is observed experimentally. This heating is because of the mode conversion of X-wave to electron Bernstein wave (EBW) at the upper hybrid resonance (UHR) layer. The power mode conversion efficiency is estimated to be 0.85 for this system. The experimental results are presented in this paper.     

考虑任意阻抗壁面条件管腔结构声场特性分析

针对任意阻抗壁面条件一维管腔声学系统建模,对系统动力学特性进行预报。为了满足阻抗边界条件对声压一阶导数连续性要求,管腔声压函数通过在标准傅里叶级数端点位置引入边界光滑辅助多项式进行构建。结合壁面阻抗声学边界和管腔声学Helmholtz控制微分方程得到强形式标准特征值问题,获得相应的声学模态信息。在数值仿真中,通过算例给出各种边界条件下管腔声学模态频率、声压振型、声压和质点振速频率响应曲线,与现有文献中相关结果进行对比,充分验证了本文求解方法的正确性和有效性,证明该方法可对任意阻抗壁面条件管腔系统声学特性进行准确预报。     

Acoustic characteristics analysis of slender cavity system with arbitrary impedance end conditions

A modeling method for the dynamic characteristics analysis of a slender acoustical cavity with impedance end conditions is established. In order to satisfy the continuity requirement at impedance ends for the first order differential of sound pressure, field function is constructed as the standard Fourier series supplemented by boundary smoothed auxiliary polynomials. System characteristic equation is derived by solving the governing differential equation and impedance acoustic boundary of slend...