Plate mode identification using modal analysis based on microphone array measurements

Regular paper

University of Technology Eindhoven

Tuesday 2 june, 2015, 10:40 - 11:00

0.8 Rome (118)

Abstract:

In vibration control of for example accurate positioning systems, vibrations are of influence and therefore a modal model of the dynamics is needed to predict these vibrations. The goal of this study is to investigate the possibilities of identifying the modal properties of plate-like structures by applying modal analysis on acoustic pressure measurements using a microphone array. Because the method is non-contact, the system dynamics are not influenced during a measurement.
The modal properties are determined by using the rational fraction polynomial method(RFP), which is an indirect frequency domain modal analysis method. Two measurement techniques are compared; (1) measurements with accelerometers and (2) with a microphone array. Using planar near-field acoustic holography (PNAH), the displacement field of the vibrating source is calculated from the pressure data captured with the microphone array. The mobility frequency response functions are determined from the displacement field. The measurements are performed on two plates which differ in thickness. All plate dimensions
are smaller than the coincidence wavelength of the first mode, which makes it important to measure in the near-field. The plates have completely free boundary conditions. The dimensions of the microphone array are larger than the plate dimensions. The frequency domain of interest is 0-1500Hz.
The mode shapes are clearly visible in the from the acoustic pressure reconstructed displacement field. The frequency response functions based on both methods are similar. As expected, modal analysis based on acceleration measurements give good results, all modal properties are found very well. The eigenfrequencies resulting from the modal analysis based on PNAH are comparable to the eigenfrequencies found using the accelerometers. However, the obtained mode shapes are not accurate. We think that one of the possible reasons for
this is the limited quality of the driving point frequency response function.