Title: Engine Noise Reduction Using Narrowband Feedback Control
text: A narrowband feedback control is developed to cancel periodic noise. The method consists of digital bandpass filters centered at a reference frequency and its harmonics. Each of these bandpass filters is then followed by a finite impulse response (FIR) filter which changes the phase of the control signal to achieve the 180° phase shift in the open loop system. A gain is applied to the control signal to adjust the overall noise reduction, considering reasonable stability margins in the system. In a set of experiments the proposed technique was compared with the filtered-x adaptive feedforward control method. The periodic noise, introduced by a noise speaker, inside a reverberant box was controlled (reduced) using both techniques. With the gain properly adjusted, the amount of noise reduction at the microphone was slightly less than that achieved by the filtered-x adaptive control, but the stability characteristics of the feedback control system was considerably better. The feedback system was able to maintain a gain margin of about 6 dB's and a phase margin of 45° over the entire frequency range of interest while the feedforward controlled system became unstable at a few frequencies even with very slow adaptation rates. The proposed feedback technique was also used to attenuate the interior noise, due to the firing of the engine, in an automobile. The reduction of up to 8 dB's in sound pressure level at the driver's head was achieved.