Vibration measurements for active noise control in a HVAC system with a passive absorbers

A hybrid active-passive noise control system for a HVAC duct combines both a physical noise absorber and an active system. Due to the presence of the passive component, requirements for the active system can be relaxed, removing the need for detecting and suppressing noise in the frequency range already covered by the passive elements. A typical noise measurement system adapted to working in airflow usually uses a microphone with a housing designed to reduce the noise generated due to local turbulent flow introduced by the housing itself. Alternatively, there are microphones specifically designed to work in the airflow. During work on the hybrid active-passive noise control system both a microphone designed for airflow and a microphone with special housing were tested. While these solutions can be used for research, both have issues making them impractical when designing a commercial product. This, along with the required narrow frequency range motivated the authors to consider vibration measurements performed by appropriately installed accelerometers. An audio signal is then synthesized using those measurements and it is confronted with signals obtained at the same time with microphones. In the paper the proposed method is presented and validated with a laboratory HVAC installation of a large cross-section and an originally designed passive absorber. Obtained results encourage further research.