Case history. Acoustic diagnostics of an automotive HVAC system

The heating, ventilation, and air conditioning (HVAC) system of an automobile can be a source of acoustic annoyance, particularly when it is operated under maximum airflow conditions. In order to suggest possible design changes for the purpose of noise control, a systematic characterization of the acoustic sources and mechanisms must be conducted initially. This paper addresses such a characterization for a typical automotive system. The approach is based on acoustic intensity measurements of the stand-alone HVAC system operating under maximum airflow conditions in an anechoic chamber. The experimental data indicate that the centrifugal blower is the dominant source of noise, particularly at low frequencies, while separation zones, the flow over sharp edges within the HVAC system, louver flows, and flow-induced casing vibration are secondary sources of noise that become increasingly important as the frequency approaches 3 kHz. Qualification of these identified sources of sound is aided by detailed flow visualizations conducted underwater on an optically clear HVAC system utilizing similitude.