Mechanical impedance gauge based on measurement of strains on a vibrating rod

An impedance gauge based on measurement of strains at two different cross-sections of a vibrating rod is analyzed and tested. The gauge rod, which may have variable characteristic impedance, is in contact with the object at one end and is driven by a harmonic vibrator at the other end. For conical and cylindrical rods explicit relations between point impedance and measured strains are derived. For a cylindrical gauge rod of steel with length 800 mm, diameter 10 mm, and distance between strain gauges 250 mm a fair agreement was generally obtained between experimental and theoretical point impedances of cylindrical test objects in the frequency range 50 Hz to 1·7 kHz. Significant improvements in accuracy over that of the tested prototype are expected to be feasible.     

Experimental determination of mechanical impedance through strain measurement on a conical rod

An impedance gauge based on measurement of strains at two different cross-sections of a conical gauge rod is tested. The gauge rod is in contact with the object at its narrow end while its wide end is driven by a harmonic vibrator. The length of the gauge rod is 260 mm and its end diameters are 9·8 mm and 20·1 mm. For cylindrical test objects a fair agreement is generally obtained between experimental and theoretical point impedances in the frequency range 50 Hz to 5 kHz. Further improvements in accuracy are expected to be feasible.     

Acoustic load on the ear caused by headphones

The standardized method for measurement of complex impedances according to ISO 10534 Part 2 is applied to the acoustic impedance of the ear with an "open-pinna" condition and with different types of headphones. The method is based on measurement of the transfer function of two microphone locations in an impedance tube and subsequent signal processing of the complex signal spectra. The termination of the tube is interpreted as ear canal entrance, while the measurement direction is, apparently, from "inside" the head towards outside. A tube which was specifically designed for this purpose works well, even though extremely small impedances must be measured. The impedances of the free pinna are similar to the "soft" end condition in the open tube, approximately following the radiation impedance of a piston into free space. The headphone impedances can be separated according to the type of headphone. In addition, the absolute impedances as the differences to the open ear compared with a number of headphones are interesting and may be starting point for further investigations. One possibility is, of course, quality control of headphones. The results are also expected to be useful for psychoacoustic research, for better understanding of sound perception, and for use in development of audio equipment.     

The acoustical input impedance of excised human lungs--measurements and model matching

The input impedance at primary bronchi of excised human lungs was measured in the frequency range from 2-5000 Hz. For the measurements, a self-developed acoustic impedance head and a narrow-band measuring system with sinusoidal excitation were used. The lungs were inflated and deflated by using an arrangement called respiratory state controller. The impedances were thus measured at different states of lung inflation. An already existing mathematical model was developed further to cover not only fairly inflated lungs, but also deflated ones. The parameter sensitivity of this model is investigated. The acoustomechanical parameters of the model were fitted to match the impedances measured. It turns out that some of these parameters are hardly calculable. The values given in this paper were chosen to agree with the measurements and to be physically reasonable. Although the measurements were performed at primary bronchi, the model is able to predict also impedances at the top end of the trachea (at different respiratory states). This impedance is useful for speech signal processing applications. The model prediction of the trachea impedances agrees well with previous results of other authors.     

Determination of acoustic impedances of multi matching layers for narrowband ultrasonic airborne transducers at frequencies <2.5 MHz - Application of a genetic algorithm

The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. One major problem of ultrasonic transducers, radiating acoustic energy into air, is to find the proper acoustic impedances of one or more matching layers. This work aims at developing an original solution to the acoustic impedance mismatch between transducer and air. If the acoustic impedance defences between transducer and air be more, then finding best matching layer(s) is harder. Therefore we consider PZT (lead zirconate titanate piezo electric) transducer and air that has huge acoustic impedance deference. The vibration source energy (PZT), which is used to generate the incident wave, consumes a part of the mechanical energy and converts it to an electrical one in theoretical calculation. After calculating matching layers, we consider the energy source as layer to design a transducer. However, this part of the mechanical energy will be neglected during the mathematical work. This approximation is correct only if the transducer is open-circuit. Since the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realized and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness and different attenuation enabling a compensation of non-ideal real values by changing their thickness was computer analyzed (base on genetic algorithm). Firstly, three theoretical solutions were investigated. Namely, Chebyshev, Desilets and Souquet theories. However, the obtained acoustic impedances do not necessarily correspond to a nowadays available material. Consequently, the values of the acoustic impedances are switched to the nearest values in a large material database. The switched values of the acoustic impedances do not generally give efficient transmission coefficients. Therefore, we proposed, in a second step, the use of a genetic algorithm (GA) to select the best acoustic impedances for matching layers from the material database for a narrow band ultrasonic transducer that work at frequency below the 2.5 MHz by considering attenuation. However this bank is rich, the results get better. So the accuracy of the propose method increase by using a lot of materials with exact data for acoustic impedance and their attenuation, especially in high frequency. This yields highly more efficient transmission coefficient. In fact by using increasing number of layer we can increase our chance to find the best sets of materials with valuable both in acoustic impedance and low attenuation. Precisely, the transmission coefficient is almost equal to unity for the all studied cases. Finally the effect of thickness on transmission coefficient is investigated for different layers. The results showed that the transmission coefficient for air media is a function of thickness and sensitive to it even for small variation in thickness. In fact, the sensitivity increases when the differences of acoustic impedances to be high (difference between PZT and air).     

Travelling Modes in Wave-Heated Plasma Sources

This paper describes a theoretical and experimental study of surface- and helicon-wave-heated plasma sources in which standing waves are set up in the cavity between the closed end plate to a plasma vessel and a wave launcher while travelling waves propagate from the opposite side of the launcher into a region which is long compared with the attenuation distance of the waves. We model the situation as a lossy transmission line of finite length coupled at the launcher to a lossy transmission line of infinite extent. RF power applied to the launcher divides in the ratio of the input impedances of the two transmission lines. For a conducting end plate, the power delivered to the travelling waves is a maximum when the cavity length is an odd number of 1/4 wavelengths long for which its input impedance is a maximum. Similarly, for an insulated end plate, the power delivered to the travelling waves is a maximum for a cavity with a length equal to an integer number of half wavelengths for which its input impedance is again a maximum.      

Measurements of mutual radiation impedance between baffled cylindrical shell transducers

The mutual radiation impedances between two baffled cylindrical shell transducers, made of PZT-4 piezoelectric rings with one half of the cylindrical surface covered by a compliant baffle, were determined in experiments as a function of separation distance using an experimental technique recently described [B. Aronov, J. Acoust. Soc. Am. 119, 3822-3830 (2006)]. Two baffled cylindrical shell transducers were connected electrically in parallel and the input electrical admittance was measured at different separations when the two transducers were operated in phase and then 180 degrees out-of-phase in both the coaxial and horizontal (side-by-side) alignments. The mutual radiation impedances were then computed using an electrical equivalent circuit to determine the significance of interaction for cylindrical elements in these array geometries.     

Vibration of continuous systems with compliant boundaries

A theoretical study of two types of continuous systems with a general form of compliant boundary conditions is presented. The systems considered are elastic beams and circular plates with elastic damped edge constraints. Beam studies are restricted to those with identical boundary conditions at each end. The method of solution consists of formulating the edge condition of the system in terms of the impedance of the compliant boundary material and of using classical solution techniques to solve the equations of motion. The result of matching the boundary conditions of the system with constraining conditions is the system frequency equation in terms of the constraint impedances.A discussion is presented giving the influence of the compliant material on the vibration of the structure. The models give numerically the effect of elasticity and damping of the supports on the resonant frequencies of the systems. Parameters are obtained which indicate when one may assume simply supported or clamped boundaries for the actual case of elastic damped constraints without introducing large errors in the natural frequencies.     

An electro-acoustics impedance error criterion and its application to active noise control

Characteristics of radiation impedance and its inducing variation of electrical impedance for a controllable source have been investigated. An impedance-based error criterion has been proposed and its application to active noise control is demonstrated through a coil driven loudspeaker. A general formula of radiation impedance is derived for two control strategies, according to the criterion of total acoustic power output. The radiation impedances of some commonly used sound sources are calculated. We discuss in detail the relation between variation of the input electrical impedance and radiation impedance for the two control strategies. An AC-bridge circuit is designed to measure the weak variation of electrical impedance resulted from radiation impedance. The input electrical impedance of a loudspeaker was measured and the experimental result is consistent with that of theoretical analysis. An impedance-based error criterion is proposed since the AC-bridge relative output is unique for a certain control strategy. The implementation of this criterion applied to an active control system is analyzed by simulations. An analogue control system is set up and experiments are carried out in a semi-anechoic chamber to verify the new control approach.     

The detection of thin embedded layers using normal incidence ultrasound

A theoretical investigation of the use of normal incidence ultrasonic reflection measurements for the detection and characterization of thin layers embedded between two much thicker media has been carried out. It has been shown that the form of the relationship between the normal incidence longitudinal reflection coefficient and frequency is defined by the reflection coefficients at zero frequency and at half the resonance frequency of the layer. The reflection coefficient at zero frequency is solely a function of the impedances of the media on either side of the layer, while that at half the resonance frequency of the layer is a function of the impedances of all three media. In general, the sensitivity of the reflection coefficient to the presence of the layer increases as the product of frequency and layer thickness increases, the maximum sensitivity being at half the resonance frequency of the layer. Unfortunately, with thin layers, it is generally not practical to test at this frequency. However, the reflection coefficient curve can, in principle, be reconstructed from data measured at lower frequencies and the sensitivity of the reflection coefficient at lower frequencies to the characteristics of the layer can be predicted from the sensitivity at half the resonance frequency. The sensitivity is also critically dependent on the relative impedances of the three media and is generally greatest when the half spaces on either side of the layer have the same impedance. With favourable impedances, it is possible to detect layers whose thickness is a small fraction of the wavelength of the ultrasonic waves employed. However, with other combinations of impedances, the detection of much thicker layers is not possible.     

Forced torsional vibrations of a cylindrical rod connected to an elastic half-space

An analysis is presented of the forced torsional vibrations of a cylindrical rod connected to an elastic half-space under the condition that the circumferential displacement at the free end of the rod, where the disturbing moment is applied, varies proportionally with the distance from the rod axis. Both the Pochhammer-Chree and elementary theory are utilized. The response curve of the rotational amplitude at the free end of the rod, obtained from the Pochhammer-Chree theory, and that obtained from the elementary theory almost coincide with each other except in the case where the rod and the half-space are of the same material. The amplitude attenuation is largest when the rod and the half-space are of the same material. The maximum values of half-space displacement distribution at the interface lie within the rod edge.     

Directional properties of an array of sound receivers positioned in an impedance screen recess

Expressions for calculating the directional characteristics of an array of sound receivers positioned in a waveguide with impedance walls are obtained from the solution to the problem on the diffraction of a plane sound wave by the waveguide open end with impedance flanges. The waveguide can be of a finite length, and, in this case, it can be considered as an open cavity in an impedance screen. The solution of the integral equation for the sound pressure distribution over the opening area is reduced to the solution of an infinite system of algebraic equations for the coefficients of the field expansion in normal waveguide waves. Examples of calculated directional characteristics are presented for arrays with receivers positioned at different distances from the opening and for different values of the impedances of the waveguide walls and flanges.     

雷电在水平导体中产生感应电压的观测及数值模拟研究

对自然雷电的9次回击过程在水平导体上产生的感应电压特征进行了分析,并利用数值模拟分析了各种参量对感应电压的影响.自然雷电9次回击在导体上产生的感应电压的变化范围为4.6—18.6 kV,平均值为11.2 kV.感应电压的半峰值宽度和下降时间的几何平均值分别为0.87和2.9 μs.数值模拟结果表明,回击在导体两端产生的感应电压随回击速度的增加而增大,随导体高度的增加而增大.当导体两端的接地电阻匹配时,感应电压随电阻的增加而增大,但并不满足线性关系.当导体两端的接地电阻不匹配时,高电阻端的感应电压远大于低电 关键词： 感应电压 自然雷电 数值模拟     

Improved precision in measurements of acoustic impedance spectra using resonance-free calibration loads and controlled error distribution

Resonances and/or singularities during measurement and calibration often limit the precision of acoustic impedance spectra. This paper reviews and compares several established techniques, and describes a technique that incorporates three features that considerably improve precision. The first feature is to minimize problems due to resonances by calibrating the instrument using up to three different acoustic reference impedances that do not themselves exhibit resonances. The second involves using multiple pressure transducers to reduce the effects of measurement singularities. The third involves iteratively tailoring the spectrum of the stimulus signal to control the distribution of errors across the particular measured impedance spectrum. Examples are given of the performance of the technique on simple cylindrical waveguides.     

Influence of the thickness of multilayer matching systems on the transfer function of ultrasonic airborne transducer

The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. This problem is of particular importance in the case of ultrasonic transducers working at a frequency above 1 MHz. Because the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realised and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness enabling a compensation of non-ideal real values by changing their thickness was computer analysed. The result of this analysis is the conclusion that from the technological point of view a layer with defined thickness is easier and faster to produce than elaboration of a new material with required acoustic parameter.     

Theoretical and experimental investigations on microstrip active array antennas and power combiners

In this paper, microstrip active array antennas and power combiners are investigated. The mutual impedance of a rectangular microstrip patch array is analyzed based on the Richmond's reaction integral equation and Galerkin technique in spectral-domain. The computer-aided analysis and design of the active array are then carried out by coupling the mutual impedances to the large-signal characteristics of Gunn diodes with a harmonic balance technique. Based on the theoretical study, microstrip active patch array antennas are investigated experimentally.     

An analysis on the Frequency-dependent grounding impedance based on the ground current dissipation of counterpoises in the two-layered soils

Because of the increased usage of power conversion equipment and their susceptibility to lightning strikes, high frequency fault currents flowing through grounding systems are increased. As grounding systems change from individual groundings to common or integrated groundings, the improved performance of grounding systems against high frequency fault currents is urgently required. When regarding the common or integrated grounding systems, most of them are required to satisfy the ground resistance defined by standards or regulations. The grounding system performance against high frequency fault currents is defined by the grounding impedance. A number of studies have reported that grounding system performance is reduced due to the increase of the grounding impedance with increasing of the frequency of fault currents. In this paper, in order to investigate the characteristics of the frequency-dependent grounding impedances of counterpoises installed in the two-layered soils, the ground current dissipation rates in each 10 m section of the counterpoises were measured and analyzed as functions of the length of grounding electrodes and the current injection point. As a result, most of the ground currents above the frequency of 100 kHz are dissipated into the earth near the current injection point and the high frequency grounding impedance of long counterpoises is converged into that of short counterpoises. The high frequency grounding impedance measured at the low soil-resistivity end of the counterpoise buried in the two-layered soils is much lower than that measured at the high soil-resistivity end.     

Double basses on the stage floor: tuning fork-tabletop effect, or not?

The question whether or not double basses can benefit from a compliant and radiating stage floor in the low end of their tonal register, similar to the well-known tuning fork-tabletop effect, was examined through field experiments in five concert halls. The topic comprises several aspects: (1) How well the mechanical impedances of double basses and the stage floor match, (2) amount of vibration velocity transmitted to the floor through the end pin of the bass, and (3) radiation efficiency of point-excited bending waves in the stage floor far below the coincidence frequency. Each aspect represents a prerequisite for the tuning fork-tabletop effect to take place. The input impedance at the end pin was measured for three representative double basses. The stage floors of five orchestra halls were measured with respect input impedance and damping, while sound radiation to the audience area was measured for two of them. In Lindeman Hall, Oslo, all conditions for the tuning fork-tabletop effect to take place were clearly met. The contribution from the stage-floor radiation to the sound pressure level in the audience area was found to be about 5 dB between 40 and 60 Hz, and even higher between 30 and 40 Hz.     

球面端面的锥形梯度折射率透镜的近轴成像特性

本文根据光线在球面上的折射公式及光线微分方程,研究了光线经两端面为球面的锥形梯度折射率透镜的传播和变换,基于光线传递ABCD矩阵,提出了球面端面的锥形梯度折射率透镜的一种等效光学系统.文中给出了该透镜的主平面、焦平面和焦距计算公式,以及近轴成像高斯公式.当锥度为零时即得到球面端面的柱形或径向梯度折射率透镜的相应结果.     

Broadband impedance calculations and single bunch instabilities estimations of of the HLS-Ⅱ storage ring

The upgrade project of the Hefei Light Source storage ring is under way.In this paper,the broadband impedances of resistive wall and coated ceramic vacuum chamber are calculated using the analytic formula,and the wake fields and impedances of other designed vacuum chambers are simulated by CST code,and then a broadband impedance model is obtained.Using the theoretical formula,longitudinal and transverse single bunch instabilities axe discussed.With the carefully-designed vacuum chamber,we find that the thresholds of the beam instabilities are higher than the beam current goal.