Active Noise Control Systems Employing the Loudspeaker as a Combined Source and Detector

This project has shown that it is possible to attenuate low frequency noise in a duct using the loudspeaker as a combined source and detector. The most important results and conclusions from the project will be presented in the following.
It is possible to achieve active noise attenuation at low frequencies, using the loudspeaker as a combined source and detector. The analogue control system gave attenuation at single frequencies between 60 Hz and 170 Hz of 30 dB to 50 dB. The parallel digital control system gave attenuation at single frequencies between 110 Hz and 150 Hz of 9.5 dB to 23.8 dB, while the series digital control system gave attenuation between 110 Hz and 200 Hz of 6.8 dB to 22.8 dB.
When the duct is closed at the end opposite to the loudspeaker, it acts as a A/4-wave resonator at low frequencies and as a A/2-wave resonator at high-frequencies. While the duct is open at the end opposite to the loudspeaker, it acts as a 4/2-wave resonator at low frequencies and as a A/4-wave resonator at high frequencies.
When measuring the mechanical parameters of the loudspeaker, dynamic measurement methods are better and more reliable than static methods. The circuit used in the analogue attenuating system suffered from instability and caused the attenuation to vary. When one distinct frequency was attenuated, sometimes a higher harmonic of the attenuated frequency was amplified at the same time.
The digital control system did not work as hoped for, because the software used was not specifically programmed for this purpose.