# Noise Identification for an Automotive Wheel Bearing
## 3 Noise Identification  
To confirm whether the noise of the assembled wheel bearing was due to the vibration of the dust shield, modal testing was conducted on the dust shield. Figure 6 shows the configuration for the modal testing of the dust shield. As shown in the figure, the assembled wheel bearing is installed in a stationary experimental device without rotation. The red and blue points on the dust shield indicate the attachment position of the accelerometer and excitation position of the impact hammer, respectively. By attaching one accelerometer to the red point and hitting the impact hammer at the blue point, the excitation force of the impact hammer and acceleration response of the accelerometer were measured. The signals were acquired in a similar manner by moving the accelerometer to another red point. The frequency response function, which represents the relationship between the excitation force (input) and acceleration response (output), was obtained from the acquired signals. Figure 5: Waterfall plot of the noise and vibration for various rotating speeds of the assembled wheel bearing: (**a**) noise, (**b**) vibration of the dust shield, and (**c**) vibration of the outer ring. The natural frequencies and mode shapes were analyzed using modal analysis. The frequency response function for all measurement positions is illustrated in Figure 7, where the peak frequencies denote the natural frequencies of the dust shield. To compare the noise/vibration frequency, which are generated when the assembled wheel bearing rotates, with the natural frequency of the dust shield, the peak noise frequencies in Figure 4a, peak vibration frequencies in Figure 4b, and natural frequencies of the dust shield in Figure 7 are summarized in Table 2. As listed in Table 2, the natural frequencies of dust shield and peak vibration frequencies of the dust shield are almost the same. However, a frequency corresponding to the natural frequency of 888 Hz was not observed in the peak vibration frequency of the dust shield. The reason for this is explained later during the analysis of the mode shape. Among the natural frequencies of the dust shield, 888, 940, 998, 1120, 1200, and 1300 Hz were similar to the peak noise frequencies for the assembled wheel bearing. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline Natural frequencies of the dust shield (Hz) & 104 & 124 & 166 & 180 & 384 & 574 & 779 & 888 & 940 & 998 & 1120 & 1200 & 1300 \\ \hline Peak vibration frequencies of the dust shield (Hz) & 107 & 125 & 170 & 180 & 387 & 578 & 783 & & 940 & 1000 & 1125 & 1206 & 1306 \\ \hline Peak noise frequencies of the assembled wheel bearing (Hz) & & & & & & & & 896 & 940 & 1000 & 1125 & 1206 & 1306 \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of the natural frequencies of the dust shield, peak vibration frequencies of the dust shield, and peak noise frequencies of the assembled wheel bearing. Figure 6: Experimental setup for the modal testing of the dust shield. Figure 7: Frequency response functions obtained from the modal testing of dust shield. The mode shapes of the dust shield close to the peak noise frequencies of the assembled wheel bearing are shown in Figure 8. Each point in Figure 8 is an accelerometer position, and the mode shapes before and after deformation are indicated by dotted and solid lines, respectively. In Figure 8, it can be observed that there is a nodal point where the magnitude of the vibration becomes zero on the mode shape corresponding to 888 Hz. This point is the same as the location where the accelerometer is attached to measure the vibration of the dust shield in the previous analysis (Figure 2). Hence, at the peak vibration frequency of the dust shield, a frequency of 888 Hz was not observed because the vibration was measured at the nodal point. If the accelerometer is attached to a different location, which avoids the nodal point when measuring the vibration of the dust shield, a frequency similar to 888 Hz can be observed in the frequency spectrum.