Damping Measures Research Articles

Material identification of real impact sounds: Effects of size variation in steel, glass, wood, and plexiglass plates

Identification of the material of struck objects of variable size was investigated. Previous studies on this issue assumed recognition to be based on acoustical measures of damping. This assumption was tested, comparing the power of a damping measure in explaining identification data with that of several other acoustical descriptors. Listeners' performance was perfect with respect to gross material categories (steel-glass and wood-plexiglass) comprising materials of vastly different mechanical properties. Impaired performance was observed for materials within the same gross category, identification being based on the size of the objects alone. The damping descriptor accounted for the identification of the gross categories. However other descriptors such as signal duration explained the results equally well. Materials within the same gross category were identified mainly on the basis of signal frequency. Overall poor support for the relevance of damping to material perception was found. An analysis of the acoustical support for perfect material identification was carried out. Sufficient acoustical information for perfect performance was found. Thus, procedural biases for the origin of the effects of size could be discarded, pointing toward their cognitive, rather than methodological nature. Identification performance was explained in terms of the regularities of the everyday acoustical environment.

Measurement of high damping: techniques and analysis

Damping capacity (mechanical energy loss (Δ W/ W) during one cycle σ– ε) can be directly measured using a tensile apparatus, for example. However, it is often more convenient to use others techniques where the sample is submitted to oscillating vibrations. Damping can be connected to the decay of free oscillations, to the width of the resonance peak (high frequency) or to the phase lag between stress and strain during subresonant forced oscillations (low frequency). Simple relationships are mainly used between the various measures of damping but these relationships are not always available. In this paper, the use of the various techniques for very high damping and the determination of actual value are discussed.

Active Noise Control In Fuselage Design

To achieve comfortable noise levels inside the passenger cabin, sound damping measures have to be taken to improve the sound insulation properties of the bare airframe. Usually the sound insulation requirements of a passenger cabin are met after the mechanical design of the fuselage structure is already finished, by adding damping materials. To be effective in the low frequency range, a lot of passive damping material has to be used resulting in considerable added weight. In this frequency range active noise control can be a competitive alternative to passive damping materials. To be able to find an optimal layout of an active noise control system a Design & Engineering Engine (DEE), a design support tool, has been developed. The DEE contains a parametric model initiator starting from user-defined requirements. From the parametric model an input file is generated for analysis with the FEM package ABAQUS. The DEE is capable of investigating active noise control systems comprising piezo electric sensors and actuators. Until now only parts of the fuselage section are investigated (panel level). The next step would be to analyse complete fuselage models. This research is mainly focussed on the influence of aspects like panel stiffness, panel curvature, stiffeners and the positioning of the sensors and actuators on the efficiency of the active noise control system. The efficiency of the active noise control system is expressed by the achieved transmission loss. The transmission loss for the different panel configurations is found using the transmission loss predicting algorithms developed at TNO TPD (Berkhoff [l]). These algorithms are based on transfer functions, which are the responses of the sensors to predefined input signals, and are determined with the DEE. Because the DEE can run automatically, many configurations can be investigated and the optimum layout for highest transmission loss can be found.

Reflex and intrinsic changes induced by fatigue of human elbow extensor muscles.

Fatigue-induced changes in intrinsic and reflex properties of human elbow extensor muscles and the underlying mechanisms for fatigue compensation were investigated. The elbow joint was perturbed using small-amplitude and pseudorandom movement patterns while subjects maintained steady levels of mean joint extension torque. Intrinsic and reflex properties were identified simultaneously using a nonlinear delay differential equation model. Intrinsic joint properties were characterized by measures of joint stiffness, viscous damping, and limb inertia and reflex properties characterized by measures of dynamic and static reflex gains. Fatigue was induced using 15 min of intermittent voluntary isometric (submaximal) exercise, and a rest period of 10 min was taken to allow the fatigued muscles to recover from acute fatigue effects. Identical experimental and data analysis procedures were used before and after fatigue. Our findings were that after fatigue, joint stiffness was significantly reduced at higher torque levels, presumably reflecting the reduced force-generating capacity of fatigued muscles. Conversely, joint viscosity was increased after fatigue potentially because of the reduced crossbridge detachment rate and prolonged relaxation associated with intracellular acidosis accompanying fatigue. Static stretch reflex gain decreased significantly at higher torque levels after fatigue, indicating that the isometric fatiguing exercise might be associated with a preferential change in properties of spindle chain fibers and bag(2) fibers. For matched pre- and postfatigue torque levels, dynamic reflexes contributed relatively more torque after fatigue, displaying higher dynamic reflex gains and larger dynamic electromyographic responses elicited by the controlled small-amplitude position perturbations. These changes appear to counteract the fatigue-induced reductions in joint stiffness and static reflex gain. The compensatory responses could be partly due to the effects of increasing the number of active motoneurons innervating the fatiguing muscles. This shift in operating point gave rise to significant compensation for the loss of contractile force. The compensation could also be due to fusimotor adjustment, which could make the dynamic reflex gain much less sensitive to fatigue than intrinsic stiffness. In short, the reduced contribution from intrinsic stiffness to joint torque was compensated by increased contribution from dynamic stretch reflexes after fatigue.

Perception of Material from Contact Sounds

Contact sounds can provide important perceptual cues in virtual environments. We investigated the relation between material perception and variables that govern the synthesis of contact sounds. A shape-invariant, auditory-decay parameter was a powerful determinant of the perceived material of an object. Subjects judged the similarity of synthesized sounds with respect to material (Experiment 1 and 2) or length (Experiment 3). The sounds corresponded to modal frequencies of clamped bars struck at an intermediate point, and they varied in fundamental frequency and frequency-dependent rate of decay. The latter parameter has been proposed as reflecting a shape-invariant material property: damping. Differences between sounds in both decay and frequency affected similarity judgments (magnitude of similarity and judgment duration), with decay playing a substantially larger role. Experiment 2, which varied the initial sound amplitude, showed that decay rate—rather than total energy or sound duration—was the critical factor in determining similarity. Experiment 3 demonstrated that similarity judgments in the first two studies were specific to instructions to judge material. Experiment 4, in which subjects assigned the sounds to one of four material categories, showed an influence of frequency and decay, but confirmed the greater importance of decay. Decay parameters associated with each category were estimated and found to correlate with physical measures of damping. The results support the use of a simplified model of material in virtual auditory environments.

The Use of Imposed Displacements to Determine Impact Forces in a Multiple Blade Shed Incident

Experimental data from a multiple blade shed incident are used to determine the forces exerted by the blades on a containment ring. A transient, dynamic, finite element procedure is used to model the ring during the blade shed. This work focuses on the selection of the proper numerical parameters that lead to a stable and accurate numerical solution while maintaining physical reality. Examination of the degree of implicitness and various measures of damping, as well as incorporation of large displacement algorithms, has lead to a simulation that successfully determines the forces on the ring. Accurate determination of these forces is necessary for optimal design of containment systems.

A general program for linear parameter estimation and uncertainty analysis

A multimethod generalized matrix inversion program GMINV is presented in FORTRAN 77 for linear parameter estimation and error analysis using a variety of least-squares and most-squares criteria. The program handles unconstrained and constrained problems and the resulting system of equations are solved using the singular value decomposition (SVD) technique. For ill-posed linear problems, the use of Marquardt-type damping and Twomey-Tikhonov solution regularization measures insures numerical stability and assures feasibility of the estimates. Sufficient examples of practical applications in geophysics and geology are provided to guide potential users.

5235283 Gradient coil system for a nuclear magnetic resonance tomography apparatus which reduces acoustic noise

The supply of current pulses to gradient coils in a magnetic resonance imaging tomography apparatus results in physical vibration of the coils, causing acoustic noise. For reducing such acoustic noise, a gradient coil system is disclosed wherein the individual gradient coils are secured on a tubular carrier formed by a two concentric shells. The two shells are joined to each other in a manner which is rigid against shearing in the longitudinal direction by connector elements. The self-excited vibrational behavior of the gradient coil system is thereby tuned to lower vibrational amplitudes and to higher vibrational frequencies, so that a more effective damping can be achieved with acoustic damping measures.

Gravitational radiation reaction in quasistatic, axially symmetric systems with possibly strong fields

The leading radiation reaction in quasistatic, axially symmetric systems is calculated by matched asymptotic expansions. Earlier results on inductive transfer of near-zone “energy” are combined via matching with a wave-zone expansion in the Regge-Wheeler gauge. Two independent measures of the damping are computed: (1) the change of the 1/Rcoefficient in a Weyl-Levi-Civita type of multipole expansion in the near zone and (2) the change in Bondi energy at future null infinity. When an averaging process is justified, such as in (nearly) periodic systems, both quantities yield a generalized version of the usual quadrupole formula, provided the radiation is outgoing at future null infinity. It is also shown that terms nonanalytic in the slow-motion parameter ɛ make a time-even contribution to the near-zone monopole coefficient. The principal advantage of this calculation over previous work is that the sources are permitted to have strong gravitational fields. Moreover, no specific model for the sources is assumed.

Surface-wave diffraction by a periodic row of submerged ducts

The diffraction of a gravity wave of length λ that is obliquely incident upon, and the radiation from, a periodic row of vertical circular ducts of radii a and horizontal spacing b with mouths at a distance h below the free surface of a deep ocean are determined through integral-equation and variational formulations. Numerical results for the reflection coefficient, the pressure-amplification factor (the ratio of the complex amplitude of the wave-induced pressure in the depths of the duct to that of the incident wave at the level of the mouths), and the radiation impedance (the real and imaginary parts of which are measures of the radiation damping and the virtual mass or stiffness of the fluid external to the ducts) are presented as functions of a/λ with a/b and a/h as parameters for the special case of normal incidence with λ > b (which implies that the crests of the scattered waves are parallel to the midplane of the ducts). These results, which complement those of Simon (1981) for a single duct, are of practical interest for wave-power absorption.

Cavitation dynamics: II. Free pulsations and models for cavitation bubbles

A mathematical formalism developed for investigating the dynamics of cavitation bubbles has been used to obtain numerical solutions describing the behavior of bubbles of different initial radii that are damped by heat conduction, viscosity, and compressibility. Calculations have been made to determine two measures of damping, the maximum temperatures, the maximum pressures, and the resonance frequencies of bubbles set into pulsations by a pressure pulse. In general, these quantities are controlled by heat conduction and viscosity at small amplitudes and mainly by compressibility at large amplitudes of motion. One measure of damping—the energy dissipation modulus—has a peak at a well-defined maximum radius. This peak serves to define a transition radius; for pulsations with amplitudes greater than this transition radius, the fraction of available energy dissipated in a cycle decreases and hence very large internal energy densities may occur. A second transition occurs at a radius called the critical radius; for pulsations with amplitudes greater than this critical radius, inertial forces control the collapse of a bubble and very large kinetic energy densities may occur in the liquid at the interface. The transition radius and the critical radius, which are functions of the initial size of a cavity, have been used to define a dynamical threshold for cavitation. In a motion where the maximum radius exceeds this proposed threshold, the characteristic phenomena of cavitation may be expected to appear. The dynamical threshold has in turn been used as the basis for models of cavitation bubbles useful in interpreting such phenomena. Subject Classification: 30.70, 30.60.