Effect of handset proximity on hearing aid feedback

The increased sensitivity of hearing aids to feedback as a telephone handset is brought near has been studied experimentally and numerically. For the measurements, three different hearing aids were modified so that the open-loop transfer function could be measured. They were mounted in the pinna of a mannikin and the change in open-loop transfer function determined as a function of handset proximity. Increases of over 20 dB were observed, most of this change occurring within the first 10 mm of separation between pinna and handset. Numerical calculations performed using a boundary element technique were in good agreement with the measurements.     

Room reverberation effects in hearing aid feedback cancellation

Room reverberation can affect feedback cancellation in hearing aids, with the strength of the effects depending on the acoustical conditions. These effects were studied using a behind the ear (BTE) hearing aid mounted on a dummy head and coupled to the ear canal via an open fitting. The hearing aid impulse response was measured for the dummy head placed at eight closely spaced locations in a typical office. The feedback cancellation in the hearing aid used a set of filter coefficients that were initialized for one location within the room, and then allowed to adapt to the feedback path measured at the same or to a different location. The maximum stable gain for the hearing aid was then estimated without feedback cancellation, for the initial set of feedback cancellation filter coefficients prior to adaptation, and for the feedback cancellation filter after adaptation. A low-order ARMA model combining a fixed set of poles with an adaptive FIR filter is shown to be effective in representing the feedback path exclusive of reverberation. Increasing the adaptive filter length has only a small benefit in improving the feedback cancellation performance due to the inability of the system to model the room reverberation. The mismatch between the modeled and actual feedback paths limits the headroom increase that can be achieved when using feedback cancellation, and varies with the location within the room.     

Adaptive feedback cancellation in hearing aids with clipping in the feedback path

Adaptive linear filtering algorithms are commonly used to cancel feedback in hearing aids. The use of these algorithms is based on the assumption that the feedback path is linear, so nonlinearities in the feedback path may affect performance. This study investigated the effect on feedback canceller performance of clipping of the feedback signal arriving at the microphone, as well as the benefit of applying identical clipping to the cancellation signal so that the cancellation path modeled the nonlinearity of the feedback path. Feedback signal clipping limited the amount of added stable gain that the feedback canceller could provide, and caused misadjustment in response to high-level inputs, by biasing adaptive filter coefficients toward lower magnitudes. Cancellation signal clipping mitigated these negative effects, permitting higher amounts of added stable gain and less misadjustment in response to high-level inputs, but the benefit was reduced in the presence of the highest-level inputs.     

Pressure dependence of space charge deposition in piezoelectric polymer foams: simulations and experimental verification

The piezoelectric activity of PQ-50 cellular polypropylene (PP) foam (an example of a so-called ferroelectret) is measured after repeated charging in a nitrogen atmosphere at a range of pressures between 61 and 381?kPa. The results are compared against simulations using a multilayer electromechanical model based on Townsend??s model of Paschen breakdown and a realistic distribution of void heights determined from scanning electron micrographs. The modeled piezoelectric coefficients versus pressure are in good agreement with experimental data when adjusted Paschen coefficients are used, indicating that the Paschen curve for electric breakdown in gases needs to be modified for dielectric barrier discharges in microcavities. The highest d ₃₃ coefficients were achieved for pressures above 251?kPa. For previously uncharged PP foam, the model predicts an optimal charging pressure of 186 kPa.     

Liu混沌系统的线性反馈同步控制及电路实验的研究

研究了新型混沌系统——Liu混沌系统的同步控制问题，基于Liu混沌系统的混沌特性，采用线性反馈控制方法，给出了实现Liu混沌系统同步的控制参数取值范围，数值仿真和电路实验证实了该方法的有效性. 关键词： Liu混沌系统 线性反馈控制 混沌同步 电路     

The constant-volume-velocity nature of hearing aids: conclusions based on computer simulations

In the literature there are several references which imply that various parts of a hearing aid are sources of constant volume velocity. Reported herein are the findings of an investigation of the validity of such statements. A computer scheme, referenced elsewhere, for modeling in situ hearing aids was utilized to test the constant-volume-velocity hypothesis. In particular, capabilities of the receiver, ear hook, and earmold tip to deliver constant volume velocity were investigated via a computer. To facilitate such an investigation, a universal receiver/earmold model was created. This model was broken down into "source" and "load"at three locations: the receive output, output of the ear hook, and medial tip of the earmold. At each location comparisons were made between computed values of source and load impedance. The constant-volume-velocity hypothesis was assumed to be valid for those cases where source impedance was much, much greater than load impedance. Plots of such impedances show that, for the cases investigated, this rarely occurred, except over certain frequency bands. With the exception of in-the-ear hearing aids, these results appear to contradict inferences made in the literature about the constant-volume-velocity nature of hearing aids.     

Performance of an adaptive beamforming noise reduction scheme for hearing aid applications. II. Experimental verification of the predictions

A method to predict the amount of noise reduction which can be achieved using a two-microphone adaptive beamforming noise reduction system for hearing aids [J. Acoust. Soc. Am. 109, 1123 (2001)] is verified experimentally. 34 experiments are performed in real environments and 58 in simulated environments and the results are compared to the predictions. In all experiments, one noise source and one target signal source are present. Starting from a setting in a moderately reverberant room (reverberation time 0.42 s, volume 34 m3, distance between listener and either sound source 1 m, length of the adaptive filter 25 ms), eight different parameters of the acoustical environment and three different design parameters of the adaptive beamformer were systematically varied. For those experiments, in which the direct-to-reverberant ratios of the noise signal is +3 dB or less, the difference between the predicted and the measured improvement in signal-to-noise ratio (SNR) is -0.21+/-0.59 dB for real environments and -0.25+/-0.51 dB for simulated environments (average +/- standard deviation). At higher direct-to-reverberant ratios, SNR improvement is systematically underestimated by up to 5.34 dB. The parameters with the greatest influence on the performance of the adaptive beamformer have been found to be the direct-to-reverberant ratio of the noise source, the reverberation time of the acoustic environment, and the length of the adaptive filter.     

Intelligibility ratings of continuous discourse: application to hearing aid selection

Twelve normal-hearing subjects rated the intelligibility of 35-s, hearing-aid-processed continuous discourse (CD) passages. Three talkers (two male, one female), four hearing aids, and two signal-to-babble (S/B) ratios were used in a completely crossed design. Research questions concerned: (1) ability of listeners to rate intelligibility, (2) sensitivity of hearing aid rankings were based on intelligibility ratings for three CD passages per instrument, and (3) dependence of hearing aid rankings on (a) S/B ratio, and (b) talker characteristics. Results were: (1) listeners were able to rate intelligibility, (2) rankings based on intelligibility ratings of three CD passages per hearing aid were capable of identifying two superior instruments within a group of four hearing aids that were similar in frequency/gain function, (3) listening in a more difficult S/B ratio substantially decreased the sensitivity of the hearing aid rankings for the female talker but had only minor effects on the rankings for the male talkers, and (4) hearing aid intelligibility rankings were found to be different for different talkers. Applications to hearing aid selection are discussed.     

Noise-induced hearing damage caused by metabolic exhaustion: a mathematical model

A mathematical model for noise-induced hearing loss is based on the assumption that hair cells are damaged, temporarily or permanently, by metabolic exhaustion, and that the number of damaged hair cells and the hearing loss are monotonically increasing functions of an energy deficiency. The purpose of the model is to focus on the influence of sound intensity, exposure duration, and temporal pattern of the sound exposure on the noise-induced hearing loss from long-duration exposures. The model is restricted to the range of sound levels where metabolic exhaustion probably is the main reason for the hair cell damage. Only exposures with similar frequency spectra and producing moderate hearing losses are considered; frequency dependence is not discussed.     

A generalized multiparticle Mie-solution: further experimental verification

We further test our electromagnetic multisphere-scattering solution developed earlier by comparing theoretical predictions from the theory with a set of laboratory measurements of microwave analog to light scattering by aggregated spheres. This solution is an extension of Mie theory to the multisphere case, generally applicable to an arbitrary aggregate of spherical and/or nonspherical particles. It is demonstrated once again that the theory is in a uniform agreement with experiment, convincingly confirming the veracity of the multiparticle-scattering formulation. The computer code for the calculation of the scattering by an aggregate of spheres in a fixed orientation and the experimental data havebeen made publically available.     

Molecular structure calculations via path integral simulations: Estimating finite-discretization errors

Path integral simulations are now recognized as a useful tool to determine theoretically the structure of complex molecules at finite temperatures including quantum effects. In addition to statistical errors due to incomplete sampling, also systematic errors are inherent in this procedure because of the finite discretization of the path integral. Here, useful “back of the envelope” estimates to assess the systematic errors of bond-length distribution functions are introduced. These analytical estimates are tested for two small molecules, HD⁺ and H₃ ⁺, where quasi-exact benchmark data are available. The accuracy of the formulae is shown to be sufficient in order to allow for a reliable assessment of the quality of the discretization in a given simulation. The estimates will also be applicable in condensed phase path integral simulations, and the basic idea can be generalized to other observables than those presented. Received 13 September 1999 and Received in final form 18 November 1999     

马吕斯定律用于退偏振片时的修正及实验验证

在考虑二向色性偏振片存在退偏振的情况下对马吕斯定律进行了推广,建立了自然光通过两个非理想偏振片时透射光强度的物理模型,并进行了实验验证.实验结果表明,该物理模型与实验数据符合较好,大大改善了"验证马吕斯定律"的普通物理实验中数据与理论的符合程度,实验误差明显减小,相对误差在5%左右.本模型实验原理简单,物理图像清晰,是与实验符合很好的简化模型,有助于降低实验室的维护成本,开阔学生的思维,培养学生的创新素质和创新能力.     

空调器数字化仿真软件开发与实验验证

针对制冷和制热两种情况,建立了以稳态仿真为基础的空调数字化仿真系统。首先运用新的制冷剂显式快速计算模型,对制冷剂物性计算程序进行完善。然后采用集中参数法建立了压缩机模型,对压缩机的性能参数进行二元十项的拟合回归。对毛细管建立了均相流分布参数模型,并且针对该计算模型提出了新的无量纲关联式。在部件模型基础上,建立了家用空调系统仿真计算模型,并且对以往的迭代算法进行了改进,提高了系统稳态仿真算法的稳定性。在系统数值模型的基础上,完成了完全面向对象方式的空调器数字化仿真软件。该仿真结果与实验数据吻合较好,软件可有效预测空调器的性能参数。     

The experimental verification of the absolute space-time theory—I

The fundamental postulate of our theory is the constancy of light velocity only with respect to absolute space. This postulate was proved right by our recently performed ‘coupled-mirrors’ experiment (Marinov, 1974). In the present paper it is shown that the so-called (by us) Newtonian and Einsteinian time synchronisations lead respectively to the Galilean and Lorentz transformations. Both types of synchronisation can be practically realised, hence both corresponding transformations describe the physical reality at low as well as at high velocities of the material points. The conception that the Einstein time dilation is an absolute phenomenon and the Lorentz length contraction a fiction is defended.     

Double-quantum filtration under rotational-resonance conditions: numerical simulations and experimental results

The dependence of the performance of a recently introduced pulse sequence to achieve double-quantum excitation under the n = 1 rotational-resonance condition (T. Karlsson, M. Edén, H. Luthman, and M. H. Levitt, 2000, J. Magn. Reson. 145, 95–107) on different spin-system properties is investigated by means of numerical simulations and ¹³C MAS NMR experiments. For spin systems where chemical shielding anisotropies amount to only an insignificant fraction of the isotropic chemical shielding difference, high efficiencies are found for large and small dipolar coupling interactions. In the presence of significant chemical shielding anisotropies the overall efficiencies decrease and become strongly dependent on the duration of the excitation period. It is demonstrated that those spin-system parameters which are sensitively encoded in the lineshapes of a conventional n = 1 rotational-resonance spectrum are similarly sensitively encoded in the corresponding rotational-resonance double-quantum-filtered lineshapes and may be quantitatively recovered by iterative lineshape-fitting approaches. In certain favorable circumstances, the in-built selectivity of the rotational-resonance double-quantum-filtration approach permits successful application of the experiment on spin systems with more than two spins.     

Ultrasonically assisted turning of aviation materials: simulations and experimental study

Ultrasonically assisted turning of modern aviation materials is conducted with ultrasonic vibration (frequency f approximately 20 kHz, amplitude a approximately 15 microm) superimposed on the cutting tool movement. An autoresonant control system is used to maintain the stable nonlinear resonant mode of vibration throughout the cutting process. Experimental comparison of roughness and roundness for workpieces machined conventionally and with the superimposed ultrasonic vibration, results of high-speed filming of the turning process and nanoindentation analyses of the microstructure of the machined material are presented. The suggested finite-element model provides numerical comparison between conventional and ultrasonic turning of Inconel 718 in terms of stress/strain state, cutting forces and contact conditions at the workpiece/tool interface.     

利用反馈光路的弹光调制器定标及稳定控制

为了实现弹光调制器（photoelastic modulator,PEM）精确定标和长时间稳定工作,提出了一种利用反馈光路的相位延迟幅值定标及控制方案,理论计算并仿真分析了PEM通光孔径上相位延迟幅值的空间分布。在偏离PEM中心的位置设计了集成化的定标反馈光路;结合倍频项比值的相位延迟幅值定标方法,采用数字锁相技术同时提取反馈光路调制光强信号中倍频项,求解出PEM相位延迟幅值。按照上述方案加工制作了PEM实物,并进行了定标及稳定控制实验。实验结果表明,PEM中心相位延迟幅值的定标值与实测值相对偏差不超过0.22%;利用反馈光路控制约100 min,PEM相位延迟幅值标准偏差为0.001 8 rad,最大偏差低于0.42%,实现了PEM相位延迟幅值精确、实时定标,同时实现了PEM长时间稳定控制。