Experimental tests of a multichannel active noise control system with single and multiple reference signals applied to the cabin of a tractor

A feedforward active noise control system represents a valid solution whenever it is necessary to attenuate a disturbing noise at low frequencies. One example of application is the case of tractors, in which the agricultural operators are exposed for a prolonged time to loud low-frequency noise, mainly generated by the engine. Generally, the feedforward systems make use of one reference sensor to measure the noise produced by the main source. However, this way of proceeding could be inadequate to describe the disturbing noise, especially when the noise consists of both structureborne and air-borne components. In this article, a comparison between the use of single and multiple reference signals, processed in a multichannel active noise control system applied to the cabin of a motionless tractor, is proposed. In particular, an accelerometer positioned on the turbocharger is used to sense the vibration generated by the engine, while a microphone placed near the cooling fan is used to sense the air-borne noise. The active system implements the filtered-X least mean squares (FXLMS) algorithm to produce the anti-noise signals. The results show that the use of two simultaneously acquired reference signals generates only small benefits in the performance of the active system.