This thesis covers the working principles, design aspects and usage possibilities of portable noise cancelling devices. It proposes a novel, acoustically open design. It reviews the acoustic and signal processing problems arising from the special environmental conditions and offers appropriate solutions.
The general principle of active noise cancelling is to generate anti-noise using electronic equipment that will interfere with the noise at a given location and thus eliminate it. Solutions utilizing digital signal processing are usually based upon some variation of the FxLMS algorithm. Personal noise cancelling devices in Hungary range from $90 to $500 and come in the form of headphones. According to my measurements the low end ANC headphones can achieve 10 dBA suppression of periodic noises and 5 dBA of stochastic noises in a narrow frequency band. Wearing headphones over a sustained period can become uncomfortable due to the pressure exerted around the ears. The ergonomic qualities of such devices can be enhanced by using an open design, but that introduces acoustic problems not studied in depth before. I have created computer simulations displaying the suppression field of small-sized noise cancelling designs. The suppression region is limited to a narrow ring around the speakers. The results have been confirmed by practical measurements. Accommodating to these results I have created an open design using one speaker and one noise microphone next to each ear and one reference microphone for the whole system. These devices have been attached to a construction helmet. Noise cancelling has been realized by a DSP developer environment running NFxLMS algorithm in a feedforward structure.
Executing a set of measurements I have identified the optimal parameters for the system. I have measured the efficiency of noise cancelling with different sampling frequencies, different reference microphone positions, and cross-paths taken into account or being neglected.
The final configuration of the noise cancelling device has a reference microphone fixed on the construction helmet itself. It is capable of 10 dBA suppression of periodic noises in a wide frequency band and 5 dBA suppression of stochastic noises.
I have identified the necessary development steps to enable the device to successfully provide noise cancelling among industrial conditions.