An experimental study on the upper limit frequency of global active noise control in car cabins

In this case study, the upper limit frequency for global noise control in car cabins is investigated with numerical simulations, and experimental results of global control are presented for the first time. To simulate the complicated noise field in a running car, a 1.90 m × 1.50 m × 1.10 m rectangular enclosure, which is approximately the size of a typical car is used with point monopoles evenly distributed near the boundaries generating the primary sound field. Besides verifying the fact that the upper limit frequency of effective control increases with the number of secondary sources, a formula is proposed in this paper to estimate the number of secondary sources required to achieve global control up to a certain frequency. The simulations are carried out in the time domain based on the transfer functions measured between the secondary loudspeakers and 40 evaluation points evenly distributed in a typical car, which show that the upper limit frequency of 3 dB global control is about 200 Hz while using 4 secondary sources and it can be improved to 315 Hz with 7 secondary sources. An active noise control system is realized with 16 accelerometers picking up vibration reference signals and 4 or 7 loudspeakers as secondary sources, and the real time test results in the car running at 50 km/h on a typical asphalt road demonstrated the validity of the findings in numerical simulations.