Author(s): Zorzo Artur , Fonseca William , Brandão Eric , Mareze Paulo, Fengler Bárbara
Active noise control can be a powerful tool when dealing with problematic low-frequency tones or general broadband noise. The primary goal of this paper is to discuss design methods, analyze their performance in measurements and to compare analog and digital Active Noise Control (ANC) systems for headphones. The systems were implemented for an AKG K44 headphone. The algorithm used for the digital system was the FXLMS (Filtered Least Mean Squared), which is a variation of the Least Mean Square algorithm (LMS). As for the analog version, filtering and inversion circuits were designed based on a precision operational amplifier. The same microphone preamplifier was used in both systems. The performances were measured using a head and torso simulator (HATS) and the external noise was generated by a dodecahedral omnidirectional sound source (in a reverberation room). Some results were impaired by the poor coupling between the headphone and the pinna of the HATS utilized. For this reason, the complete measurement set was carried out with and without the artificial pinna (outcomes are discussed throughout this work). Although the digital system was designed to attenuate broadband noise, great results were achieved for both: broadband and tonal noise. The results for the tonal noise were not as expressive in the analog system as they were for the digital version. Nevertheless, similar results were accomplished for the broadband noise. For this paper, only a few values of the algorithm's convergence parameters were tested. Therefore, better results may still be obtained for the digital system with the correct adjustment of these values. Moreover, it is possible to improve the sound attenuation (for both systems) with a more refined design for the preamplifier and analog filters.
Name: Mr Artur Zorzo