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Audible tones in vehicle interiors are undesirable because of their impact on customer satisfaction and quality metrics. Most of the loudest tonal noise sources located in the engine compartment are isolated from the vehicle interior by the dash-wall. A majority of the automotive blower/s are located in the vehicle interior in the close proximity of the driver and passengers. Hence, the blower induced tones (if present) become audible and airborne, they readily propagate to the vehicle occupants. The severity of these audible tones is going to be most annoying in future especially in vehicles equipped with hybrid, electric and start/stop at idle technologies, for improved fuel economy mandates in future. Due to increased demands for quieter vehicle interiors with higher airflow for achieving quick thermal comfort, the HVAC systems are designed with lower pressure drop which helps reduced low frequency broad-band noise but does not mask the BPF tone and its harmonics.

Air-conditioning (A/C) induced moan is a very commonly observed phenomenon in automotive refrigerant systems. Since most of the automotive A/C systems cycle ON/OFF four to six times every minute, the A/C induced moan is quite readily audible under engine idle and even while driving, especially under lower engine/vehicle speeds. It is not unusual for an A/C compressor to moan or not, on some vehicle/s under certain operating conditions. Most of the OEMs resolve or suppress the A/C moan potential to barely audible levels. However, under some unique and extreme operating conditions, A/C moan is quite readily induced and often results in customer complaints. This paper discusses A/C moan related root-causes, sources and paths of propagation. A systematic diagnostic test-procedure is also described to diagnose and develop the needed most cost-effective design-fixes. Finally, based on this case-study - some objective targets are recommended to suppress the A/C moan to acceptable levels.