# Reduction of the Cavitation Noise in an Automotive Heater Core
## 2 Measurement and Analysis of Noise and Vibration from the Heater Core
### Measurement and Analysis in the Experimental Setup  
To investigate noise and vibration characteristics, coolant was supplied to the heater core using controlled equipment capable of regulating temperature and flow rate, as depicted in Figure 3. The experimental setup shown in Figure 4 was employed to measure the noise and vibration generated by the heater core. Noise measurements were conducted using microphones (PCB 378B02, PCB Piezotronics, Depew, NY, USA), while accelerometers (B&K 4383, Briel & Kjaer, Narum, Denmark) captured vibration data. The acquired signals were processed using a spectrum analyzer (B&K 3050-B-060, Bruel & Kjaer, Narum, Denmark) to analyze responses in both time and frequency domains. To examine cavitation-induced bubble formation, transparent tubes were installed in the inlet and outlet pipes for visual observation. Figure 3: Equipment for controlling the temperature and flow rate of the coolant. Figure 2: Heater core of an automobile (the red arrows indicate the flow direction of the coolant). Cavitation bubble formation was investigated under varying coolant conditions, with temperatures set at 75 \({}^{\circ}\)C, 80 \({}^{\circ}\)C, and 85 \({}^{\circ}\)C and flow rates of 20, 25, 30, and 35 L/min. Table 1 summarizes the observations, where \"O\" denotes cavitation occurrence and \"X\" indicates its absence. The results demonstrate that cavitation is more likely to occur at lower coolant temperatures and higher flow velocities. Specifically, at 75 \({}^{\circ}\)C, cavitation occurred at 30 and 35 L/min but was absent at lower flow rates. At 80 \({}^{\circ}\)C, cavitation was observed only at 35 L/min, while at 85 \({}^{\circ}\)C, cavitation did not occur at any tested flow rate. Since the cross-sectional area of the inlet and outlet pipes remains constant, an increase in flow rate leads to a corresponding increase in flow velocity, promoting cavitation formation under specific conditions. Noise and vibration measurements were conducted under two operating conditions: (1) without cavitation (85 \({}^{\circ}\)C, 20 L/min) and (2) with cavitation (75 \({}^{\circ}\)C, 35 L/min). Figure 5 presents noise signals in the time domain, expressed in A-weighted sound pressure levels. The comparison between Figure 5a,b indicates that noise levels are higher in the presence of cavitation. However, it should be noted that the difference in flow rate between the two cases may also influence the acoustic characteristics, and therefore, the specific contribution of cavitation to the noise cannot be completely isolated. This limitation is acknowledged in interpreting the results. In contrast, vibration signals recorded using an accelerometer mounted on the upper tank of the heater core exhibit a more distinct difference between conditions. As shown in Figure 6, the vibration amplitudes are significantly higher in the cavitation-present condition (Figure 6b) due to cavitation-induced impacts on the inner wall of the upper tank. Moreover, high-peak impulses observed in the time-domain vibration signals suggest the presence of strong, transient forces, which are consistent with the collapse of cavitation bubbles rather than a mere increase in flow rate. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multicolumn{4}{c}{**Flow Rate (L/min)**} \\ \cline{2-5}  & **20** & **25** & **30** & **35** \\ \hline
75 & X & X & O & O \\
80 & X & X & X & O \\
85 & X & X & X & X \\ \hline \hline \end{tabular} \({}^{\star}\)O\({}^{\star}\) indicates that cavitation occurred; \({}^{\star}\)X\({}^{\star}\) indicates that cavitation did not occur.  
\end{table} 
Table 1: Cavitation occurrence, depending on the flow rate and temperature of the coolant. Figure 4: Experimental setup for measuring the noise and vibration generated in the heater core. To further analyze the effects of cavitation, time-domain signals (Figures 5 and 6) were transformed into frequency-domain spectra (Figure 7).