# 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 an Actual Vehicle  
Noise and vibration measurements were carried out using an actual vehicle--a compact passenger car equipped with a 1.6 L, four-cylinder turbocharged engine. The coolant used was a 50/50 mixture of ethylene glycol and water. As described in the Introduction, cavitation noise generated by the heater core tends to occur when the engine operates at high speed under cold conditions. To replicate such conditions, the test was conducted in a cooling chamber maintained at \(-10\)\({}^{\circ}\)C, ensuring that the coolant temperature was initially 1 \({}^{\circ}\)C. The engine was then started and gradually accelerated to 5000 RPM over a five-second period. Once this speed was reached, it was held constant while noise and vibration data were recorded. The engine speed was specifically maintained at 5000 RPM because cavitation noise does not occur at lower speeds, such as 4000 RPM. For experimental accessibility, a section of the bulkhead and heater core pipe cover was removed, as illustrated in Figure 8. A microphone was positioned 1 cm from its side surface to capture noise, while an accelerometer was mounted on the heater core pipe to measure vibrations. The acquired signals were analyzed using a spectrum analyzer to extract dynamic response and frequency spectra. Figure 7: Frequency spectra for the noise and vibration signals generated from the heater core in the experimental setup: (**a**) noise and (**b**) vibration. To ensure the reliability of the measured noise and vibration signals and minimize mutual interference between different sources, several precautions were taken during data collection and preprocessing. Prior to the main experiments, a preliminary frequency analysis was performed to identify the characteristic frequency ranges of various sources such as the engine, coolant pump, and fan. Based on these findings, the analysis of cavitation effects focused on signals above 2 kHz, a range in which the influence of cavitation is more pronounced and interference from other sources is relatively limited. To further minimize mechanical cross-talk, microphones (PCB 378B02, PCB Piezotornics, Depew, NY, USA) and accelerometers (B&K 4383, Bruel & Kjaer, Naerum, Denmark) were mounted on structurally isolated locations, preventing direct mechanical coupling between the acoustic and vibration sensing elements. The microphones used had a frequency response of 3.15 Hz to 20 kHz with an accuracy of \(\pm\)1 dB, while the accelerometers operated within a range of 1 Hz to 16 kHz with an accuracy of \(\pm\)5%. All signals were simultaneously recorded using a B&K 3050-B-060 spectrum analyzer (Bruel & Kjaer, Naerum, Denmark), which supports up to six input channels with a frequency range extending from DC to 51.2 kHz. The analyzer accepts both CCLD (IEPE) and voltage input types and supports high-fidelity data transfer and synchronization via LAN with Power over Ethernet (PoE). The front panel (UA-2100-060, NF Corporation, Yokohama, Japan) provided six BNC input connectors for convenient sensor connection, and the system was operated in conjunction with BK Connect(Bruel & Kjaer, Naerum, Denmark) and PULSE(tm) LabShop software 25.1 (Bruel & Kjaer, Naerum, Denmark) for data acquisition and post-processing. These configurations and methodologies ensured high-precision measurement of cavitation-induced signals while minimizing interference from unrelated noise and vibration sources. Experimental results indicated that an abnormal noise, resembling gravel or sand passing through a pipe, persisted for approximately 30 s after reaching 5000 RPM before dissipating. Figure 9 compares noise signals under cavitation-free (Figure 9a) and cavitation-present (Figure 9b) conditions, revealing higher sound pressure levels in the presence of cavitation. Similarly, vibration signals, as shown in Figure 10, exhibit increased magnitudes with cavitation, characterized by impulse-type signals absent in non-cavitating conditions.