Paolo Chiariotti

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part I. Development of the technique and preliminary validation

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The first aim of this work is to develop and validate the technique in reverberating environments where the location and the quantification of each source are difficult by traditional techniques. The reverberant field is clearly challenging also for the proposed technique, affecting the achievable accuracy, mainly due to the fact that coherence between radiated and reverberated fields is often unknown and may be relevant. Secondly, the applicability of the method to real cases is demonstrated. A laboratory test case has been developed using a large wooden panel. The measurement is performed both in anechoic environment and under simulated reverberating conditions, for testing the ability of the selective intensity method to remove the reverberation.

Exploiting continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation methods for noise source identification

This paper proposes the use of continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation techniques that aim at characterizing the structure-borne contributions of the noise emission of a mechanical system. The time domain correlation technique presented in this paper is based on the use of FIR (finite impulse response) filters obtained from the vibro-acoustic transfer matrix when vibration data are collected by laser Doppler vibrometry (LDV) exploited in continuous scan mode (CSLDV). The advantages, especially in terms of source decorrelation capabilities, related to the use of CSLDV for such purpose, with respect to standard discrete scan (SLDV), are discussed throughout the paper. To validate this approach, vibro-acoustic measurements were performed on a planetary gear motor for home appliances. The analysis of results is also supported by a simulation.

The application of advanced beamforming techniques for the noise characterization of installed counter rotating open rotors

This paper reports on the results of a series of aeroacoustic measurements of a counter rotating open rotor (CROR) installed on a 1/7 scale model of an advanced regional aircraft design. The tests were conducted in a large low speed wind tunnel for a variety of aircraft geometries, angles of attack and flow speeds. Data were acquired on three far field beamforming arrays. The study attempts to characterize the installed CROR noise source through the application of several beamforming techniques applied to each individual array and to a global array of the three arrays in combination. Although some results have been achieved in this preliminary study, there are also drawbacks and limitations of the beamforming processes for the case of model with CRORs, therefore a deeper investigation will be necessary. This work has been conducted as part of the European Clean Sky funded WENEMOR project which will be completed in August 2013, the testing phase of which was completed in May 2013.

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part II. Application to an aircraft cabin

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The work shown in this paper is the application of part of the results of the European project CREDO (Cabin Noise Reduction by Experimental and Numerical Design Optimization) carried out within the framework of the EU. Therefore the aim of this paper is to illustrate a real application of the method to the interior acoustic characterization of an Alenia Aeronautica ATR42 ground test facility, Alenia Aeronautica being a partner of the CREDO project.

Exploiting Continuous Scanning Laser Doppler Vibrometry and Wavelet Processing for Damage Detection

The present paper proposes a novel damage detection approach based on the exploitation of the simultaneous time and spatial sampling provided by CSLDV and the feature extraction capabilities of wavelet-domain processing. Superficial defects are analysed in the paper. The damage detection procedure is presented and its performances studied in a simulated application on a plate with different crack scenarios (varying crack depth ratio). Both line and area scans are analysed, considering also the influence of measurement noise. The method shows promising results, since cracks are identified in all severity conditions. An example on a sub-surface defect on a carbon-fiber panel is also presented.

Uncorrelated Noise Sources Separation Using Inverse Beamforming

The separation of a measured sound field in uncorrelated sources distributions can be very useful when dealing with sound source localization problems. The use of the Principal Component Analysis (PCA) principle, combined with a Generalized Inverse Beamforming (GIBF) technique, offers the possibility to resolve complex and partially correlated sound sources distributions.

Recovery of Mode Shapes from Continuous Scanning Laser Doppler Vibration Data: A Mode Matching Frequency Domain Approach

The paper illustrates a method for processing, in a blind way, data obtained by Continuous Scanning Laser Doppler Vibrometry (CSLDV). CSLDV makes it possible to measure the structure vibration joining together the spatial and time information. The vibration datum obtained from the laser, which continuously scans (over time and space) the structure under test, is in fact modulated by the Operational Deflection Shape (ODS) excited during the experiment. The idea that we propose in this paper is based on the fact that, if the mode shapes of the structure under test are known a priori, e.g. from a numerical model or from an analytical formulation, it is possible to settle a procedure that searches for similarities between those known mode shapes (the candidate mode shapes) and ODSs that actually modulate the signal. This procedure can be considered a pattern matching technique that makes it possible to identify the resonance frequency related to each ODS and the mode shapes that better match with ODSs excited. A detailed description of the algorithm is given in this paper.

Wavelet processing of continuous scanning laser doppler vibrometry data in non-destructive testing

The present paper proposes a novel non-destructive testing procedure based on the exploitation of the simultaneous time and spatial sampling provided by Continuous Scanning Laser Doppler Vibrometry (CSLDV) and the feature extraction capabilities of wavelet-based processing. Two criteria for selecting in an objective way the mother-wavelet to be used in the decomposition procedure, the Relative Wavelet Energy and Energy to Shannon Entropy Ratio, are compared in terms of capability of best locating the damage. The paper demonstrates the applicability of the procedure for the identification of superficial and in-depth defects in simulated and real test cases when an area scan is performed over the test sample. The method shows promising results, since defects are identified in different severity conditions.

Mode Filtering of Continuous Scanning Laser Doppler Vibration Data

The paper illustrates the idea of using theoretical knowledge of specific structure’s mode shapes as a way of filtering time domain data obtained by Continuous Scanning Laser Doppler Vibrometry (CSLDV). The CSLDV output measures the structure vibration joining together the spatial and time information. It is proposed here to exploit the a priori knowledge of the candidate mode shape spatial distributions to extract from the vibration data the resonance frequency information with high accuracy. That technique is based on the concept that modal analysis ends up with a final abstraction and labelling of the mode shapes on the basis of their nodal lines position, i.e. first, second bending and/or torsional, etc. The expected mode is compared with the experimental data in order to evidence the information on the frequency at which that mode occurs.

Diagnostic procedure on brake pad assembly based on Young's modulus estimation

Quality control of brake pads is an important issue, since the pad is a key component of the braking system. Typical damage of a brake pad assembly is the pad?backing plate detachment that affects and modifies the mechanical properties of the whole system. The most sensitive parameter to the damage is the effective Youngs modulus, since the damage induces a decrease of the pad assembly stiffness and therefore of its effective Youngs modulus: indeed its variation could be used for diagnostic purposes. The effective Youngs modulus can be estimated from the first bending resonance frequency identified from the frequency response function measured on the pad assembly. Two kinds of excitation methods, i.e. conventional impulse excitation and magnetic actuation, will be presented and two different measurement sensors, e.g. laser Doppler vibrometer and microphone, analyzed. The robustness of the effective Youngs modulus as a diagnostic feature will be demonstrated in comparison to the first bending resonance frequency, which is more sensitive to geometrical dimensions. Variability in the sample dimension, in fact, will induce a variation of the resonance frequency which could be mistaken for damage. The diagnostic approach has been applied to a set of undamaged and damaged pad assemblies showing good performance in terms of damage identification. The environmental temperature can be an important interfering input for the diagnostic procedure, since it influences the effective Youngs modulus of the assembly. For that reason, a test at different temperatures in the range between 15??C and 30??C has been performed, evidencing that damage identification technique is efficient at any temperature. The robustness of the Youngs modulus as a diagnostic feature with respect to damping is also presented.