Estimations of non-linearities in structural vibrations of string musical instruments

Under the excitation of strings, the wooden structure of string instruments is generally assumed to undergo linear vibrations. As an alternative to the direct measurement of the distortion rate at several vibration levels and frequencies, we characterise weak non-linearities by a signal-model approach based on cascade of Hammerstein models. In this approach, in a chain of two non-linear systems, two measurements are sufficient to estimate the non-linear contribution of the second (sub-)system which cannot be directly linearly driven, as a function of the exciting frequency. The experiment consists in exciting the instrument acoustically. The linear and non-linear contributions to the response of (a) the loudspeaker coupled to the room, (b) the instrument can be separated. Some methodological issues will be discussed. Findings pertaining to several instruments - one piano, two guitars, one violin - will be presented.Comment: 11th Congr\`es Fran\c{c}ais d'Acoustique, Nantes : France (2012

A method to measure elastic and dissipative material properties of sandwich structures and its numerical validation

Titre du résumé en français joint : Identification par analyse modale haute résolution des propriétés d'élasticité et d'amortissement de matériaux de type "sandwich"National audienceA method to measure elastic and dissipative properties of the constituents of a sandwich structure is proposed and validated. The method relies on the comparison between (a) the modal frequen- cies and dampings of a thick plate as predicted by an extended Rayleigh-Ritz procedure and (b) the their values as given by experimentation or numerical simulation. On real plates, a one-point measurement of free vibrations is sufficient, provided that a high-resolution modal analysis [1] is used [2]. For valida- tion purposes, the experimental modal analysis is replaced by a finite-element model analysis (numerical measurement). Minimising the differences between the modal characteristics yields an estimation of the values of the elastic and dissipative material properties. Agreement between estimated and original mechanical parameters is shown to be good for the parameters which are influential in plate vibration.See http://hal.archives-ouvertes.fr/docs/00/59/28/82/ANNEX/r_0PBFPY04.pd

Identification of elastic and damping properties of sandwich structures based on high resolution modal analysis of point measurements

International audienceA method is proposed to identify the mechanical properties of the skin and core materials of sandwich structures having heterogeneous cores. All the elastic coefficients and loss-factors that matter in the dynamics of such a panel in the thick-plate approximation are identified. To this end, experimental natural modes (i.e. eigenmodes of the damped system) are compared to the numerical modes of large sandwich panels (lx,y/h ≃ 80). The chosen generic model for the visco-elastic behaviour of the materials is E(1 + j ). The numerical modes are computed by means of a Rayleigh-Ritz procedure and their dampings are predicted according to the visco-elastic model. The frequencies and dampings of the natural modes of the panel are estimated experimentally by means of a high-resolution modal analysis technique. An optimisation procedure yields the desired coefficients. A sensitivity analysis assess the reliability of the method. Identification is conducted on two very different kind of sandwich panels to illustrate the method

Measurement of relevant elastic and damping material properties in sandwich thick plates

International audienceAn easy-to-implement method to measure relevant elastic and damping properties of the constituents of a sandwich structure, possibly with a heterogeneous core, is proposed. The method makes use of a one-point dynamical measurement on a thick-plate. The hysteretic model for each (possibly orthotropic) constituent is written generically as "E(1+jη)" for all mechanical parameters. The estimation method of the parameters relies on a mixed experimental/numerical procedure. The frequencies and dampings of the natural modes of the plate are obtained from experimental impulse responses by means of a high-resolution modal analysis technique. This allows for considerably more experimental data to be used. Numerical modes (frequencies, dampings, and modal shapes) are computed by means of an extended Rayleigh-Ritz procedure under the "light damping" hypothesis, for given values of the mechanical parameters. Minimising the differences between the modal characteristics yields an estimation of the values of the mechanical parameters describing the hysteretic behaviour. A sensitivity analysis assesses the reliability of the method for each parameter. Validations of the method are proposed by (a) applying it to virtual plates on which a finite-element model replaces the experimental modal analysis, (b) some comparisons with results obtained by static mechanical measurements, and (c) by comparing the results on different plates made of the same sandwich material

Identification of honeycomb sandwich properties by high-resolution modal analysis

International audienceA method is proposed to identify the mechanical properties of the skin and core materials of honeycomb sandwich. All the elastic coefficients and loss-factors that matter in the dynamics of a panel in the thick-plate approximation are identified. To this end, experimental natural modes (i.e. eigenmodes of the damped system) are compared to the numerical modes of a large sandwich panel (lx,y/h ≃ 80). The chosen generic model for the visco-elastic behaviour of the materials is E(1 + jd). The numerical modes are computed by means of a Rayleigh-Ritz procedure and their dampings are predicted according to the visco-elastic model. The frequencies and dampings of the natural modes of the panel are estimated experimentally by means of a high-resolution modal analysis technique. An optimisation procedure yields the desired coefficients. A sensitivity analysis assess the reliability of the method

SMART-I²: A Spatial Multi-users Audio-visual Real Time Interactive Interface

International audienceThe SMART-I2 aims at creating a precise and coherent virtual environment by providing users with both audio and visual accurate localization cues. It is known that for audio rendering, Wave Field Synthesis, and for visual rendering, Tracked Stereoscopy, individually permit high quality spatial immersion within an extended space. The proposed system combines these two rendering approaches through the use of a large Multi-Actuator Panel used as both a loudspeaker array and as a projection screen, considerably reducing audio-visual incoherencies. The system performance has been confirmed by an objective validation of the audio interface and a perceptual evaluation of the audio-visual rendering

The SMART-I²: A new approach for the design of immersive audio-visual environments

International audienceThe SMART-I² aims at creating a precise and coherent virtual environment by providing users with both audio and visual accurate localization cues. Wave field synthesis, for audio rendering, and tracked passive stereoscopy, for visual rendering, individually permit high quality spatial immersion within an extended space. The proposed system combines these two rendering approaches through the use of large multi-actuator panels used as both loudspeaker arrays and as projection screens, providing a more perceptually consistent rendering

Vibroacoustics of the piano soundboard: (Non)linearity and modal properties in the low- and mid-frequency ranges

Research highlights: - Nonlinearity characterization of a structure independently from its exciting device. - Experimental modal identification, including damping, in the mid-frequency range. - Boundary conditions, low-and high-frequency regimes clarified in the piano soundboard. - Good match between experimental observations and FEM results at low frequencies. - FEM of a non-regularly ribbed soundboard reveals high-frequency localization in piano.International audienceThe piano soundboard transforms the string vibration into sound and therefore, its vibrations are of primary importance for the sound characteristics of the instrument. An original vibro-acoustical method is presented to isolate the soundboard nonlinearity from that of the exciting device (here: a loudspeaker) and to measure it. The nonlinear part of the soundboard response to an external excitation is quantitatively estimated for the first time, at ≈ -40 dB below the linear part at the ff nuance. Given this essentially linear response, a modal identification is performed up to 3 kHz by means of a novel high resolution modal analysis technique (Ege et al., High-resolution modal analysis, JSV, 325(4-5), 2009). Modal dampings (which, so far, were unknown for the piano in this frequency range) are determined in the midfrequency domain where FFT-based methods fail to evaluate them with an acceptable precision. They turn out to be close to those imposed by wood. A finite-element modelling of the soundboard is also presented. The low-order modal shapes and the comparison between the corresponding experimental and numerical modal frequencies suggest that the boundary conditions can be considered as blocked, except at very low frequencies. The frequency-dependency of the modal density and the observation of modal shapes reveal two well-separated regimes. Below ≈ 1 kHz, the soundboard vibrates more or less like a homogeneous plate. Above that limit, the structural waves are confined by ribs, as already noticed by several authors, and localised in restricted areas (one or a few inter-rib spaces), presumably due to a slightly irregular spacing of the ribs across the soundboard

SMART-I²: Spatial Multi-users Audio-visual Real Time Interactive Interface, a broadcast application context

International audienceSMART-I2 is a high quality 3D audio-visual interactive rendering system. In SMART-I2, the screen is also used as a multichannel loudspeaker. The spatial audio rendering is based on Wave Field Synthesis, an approach that creates a coherent spatial perception of sound over a large listening area. The azimuth localization accuracy of the system has been verifed by a perceptual experiment. Contrary to conventional systems, SMART-I2 is able to realize a high degree of 3D audio-visual integration with almost no compromise on either the audio or the graphics rendering quality. Such a system can provide benefits to a wide range of applications. Index Terms-- Audio-visual integratio

Audio, visual, and audio-visual egocentric distance perception by moving participants in virtual environments

International audienceA study on audio, visual, and audio-visual egocentric distance perception by moving participants in virtual environments is presented. Audio-visual rendering is provided using tracked passive visual stereoscopy and acoustic wave fi eld synthesis (WFS). Distances are estimated using indirect blind-walking (triangulation) under each rendering condition. Experimental results show that distances perceived in the virtual environment are accurately estimated or overestimated for rendered distances closer than the position of the audio-visual rendering system and underestimated for distances farther. Interestingly, participants perceived each virtual object at a modality-independent distance when using the audio modality, the visual modality, or the combination of both. Results show WFS capable of synthesizing perceptually meaningful sound fields in terms of distance. Dynamic audio-visual cues were used by participants when estimating the distances in the virtual world. Moving may have provided participants with a better visual distance perception of close distances than if they were static. No correlation between the feeling of presence and the visual distance underestimation has been found. To explain the observed perceptual distance compression, it is proposed that, due to con flicting distance cues, the audio-visual rendering system physically anchors the virtual world to the real world. Virtual objects are thus attracted by the physical audio-visual rendering system