2 Combining Nontraditional Response Variables with Acceleration Data for Experimental Modal Analysis 13 Fig . 2. 5 MAC between 2 accelerometer and 1 strain versus 3 accelerometer 2.5 Accelerometers and One Strain Gauge Modal vectors for data with two acceleration and one strain reference were compared with three acceleration modal vectors. The MAC comes out completely diagonal indicating that the modes are similar. MAC is shown in Fig. 2.5. 2.6 Accelerometer and Two Strain Gauges Modal vectors for data with one acceleration and two strain references were compared with three acceleration modal vectors. The MAC comes out completely diagonal indicating that the modes are similar. MAC is shown in Figs. 2.6 and 2.7 for the two different acceleration references. Another comparison that was done was to compare the density of pole estimates. There were differences in the density of the scatter plots, but it was different for different modes. There was not one set of data that gave better clusters of damping ratio values for all modes. This suggested that there was not any improvement in damping results from replacing accelerometers with strain gauges at some measurement locations. 2.7 Accelerometer and Microphones To combine accelerometer data with microphone data, it was important to consider the fact that there was a large difference in their magnitudes. The microphone data was observed to be around 6 orders of magnitude larger. An equation was set up such that modified pressure FRF data was expressed as . Hpm = R3∗ Hp+ R2/ω 2 + R1 where Hpm is the modified pressure data, Hp is measured pressure FRF, and R1, R2 and R3 are the terms determined in the least square sense by comparing Hpm to the displacement FRF on the same location. The average of all R3 values for all
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