Η παρούσα διπλωματική εργασία αποτελεί βιβλιογραφική έρευνα στο επιστημονικό πεδίο της προσαρμοστικής ακύρωσης θορύβου (ANC). Αρχικά γίνεται μια αναφορά στη γενική φιλοσοφία της ANC, παρατίθεται μια σύντομη ιστορική αναδρομή και αναφέρονται τα πεδία εφαρμογής της ANC. Γίνεται ανάλυση των βασικών κατηγοριών προσαρμοστικής ακύρωσης θορύβου βασισμένες στον εμπροσθοτροφοδοτούμενο και στον ανατροφοδοτούμενο έλεγχο. Πρώτα συζητείται η δομή του ευρυζωνικού προσαρμοστικού εμπροσθοτροφοδοτούμενου ελέγχου, με την εξαγωγή και την ανάλυση του αλγόριθμου FXLMS. Στη συνέχεια αναλύονται τα στενής ζώνης εμπροσθοτροφοδοτούμενα συστήματα, εισάγοντας τη μέθοδο σύνθεσης κυματομορφής, τα προσαρμοστικά φίλτρα αποκοπής και την ANC πολλαπλών συχνοτήτων. Έπειτα αναπτύσσεται η έννοια της προσαρμοστικής ακύρωσης μέσω ανατροφοδοτούμενου έλεγχου από τη σκοπιά των σημάτων αναφοράς προσδίδοντας μια συσχέτιση με τα συστήματα εμπροσθοτροφοδότησης. Εν συνεχεία, η ανάλυση των μονοκαναλικών συστημάτων επεκτείνεται στα πολυκαναλικά συστήματα ANC. Παρουσιάζονται διάφορες online τεχνικές μοντελοποίησης δευτερεύουσας διαδρομής. Επιπρόσθετα, παρουσιάζονται διάφοροι ειδικοί ANC αλγόριθμοι όπως δικτυωτή ANC, ANC στο πεδίο της συχνότητας, ANC υποζώνης και ο αναδρομικός αλγόριθμος ελαχίστων τετραγώνων (RLS). Τέλος παρουσιάζονται αναλυτικά οι εφαρμογές της προσαρμοστικής ακύρωσης σε πρακτικό και πειραματικό επίπεδο, ενώ δίνεται μια ποσοτική συνεισφορά στη μείωση θορύβου. / This thesis is a literature research in the scientific field of adaptive noise cancellation (ANC). Originally becomes a reference to the general philosophy of the ANC, given a brief historical overview and a reference to the fields of application of ANC . An analysis of the main categories of adaptive noise cancellation based on feed-forward and feedback control. First a discussion is made, of the structure of broadband adaptive feed-forward control by extracting and analyzing the FXLMS algorithm. Then an analysis of narrowband feedforward systems is given, introducing the waveform synthesis method, adaptive notch filters and multiple frequency ANC. Then we develope the concept of adaptive cancellation via feedback control from the perspective of reference signals giving a correlation to feedforward systems. Subsequently, the analysis of single channel systems extends in multi-channel ANC. Various online secondary path modeling techniques are presented. Additionally, several special ANC algorithms are presented, such as lattice ANC, ANC in the frequency domain, subband ANC and the recursive least squares algorithm (RLS). Finally applications of adaptive cancellation are presented in detail, on practical and experimental level, given a qualitative contribution to noise reduction.

Noise pollution from road traffic is one of the greatest environmental issues in modern day, and the social cost for road traffic noise was estimated to over 16 billion SEK per year in Sweden in2014. Passive or active control methods can be used to reduce the noise. Active control methods or active noise control is more suitable for attenuating noise in lower frequencies. Active noise control reduces noise by eliminating the noise with a secondary source. There are different control strategies to construct an active noise control system, where the update of the secondary sourceis controlled by an algorithm. There are several different algorithms that are possible to use, and one option is to use a Feedforward Filtered-X Least-Mean-Square (FXLMS) algorithm. It uses control positions where the noise is meant to be reduced and reference signals that measure the noise upstream prior the secondary source. FXLMS also uses a model of the secondary source path to the control position in order to ensure convergence of the algorithm. Although the use of multiple reference signals increases the accuracy of the algorithm, it also increases the convergence time and the practical cost of such an installation. Unfortunately, it can require many reference signals to obtain a sufficient noise reduction when the unwanted noise source is complex and has multiple propagation paths.This study investigates the possibility of producing a new, reduced set of reference signals with a linear combination of the original reference signals that still contain the majority of information needed for suficient noise reduction. This new set of reference signals are sometimes called virtual reference signals. Three different methods of virtual reference signals are analysed; first a constant method using singular-value decomposition on the covariance of the reference signals, second another constant method using singular-value decomposition on the covariance of response estimate from each corresponding reference signal, third an adaptive algorithm updating the linear combination to adapt for incoming data. The different strategies are tested on road test measurements at three different constant speeds, 40km=h; 80km=h and 120km=h, and on data generated from a numerical vehicle model in COMSOL.The results from the analysis indicates that the virtual reference signals could sufficiently reproduce information from the original reference signals to obtain a similar noise reduction with fewer reference signals. However, the virtual reference signals with the adaptive algorithm could not manage to track a transient system where the signal amplitudes are varying over time. Further work is needed to analyse the limits and requirements to obtain virtual reference signals that can represent and track a system even for transient events.

Universidade Federal do Triângulo Mineiro / This work is dedicated to the study of the problem of active noise control, evaluating some numerical and experimental methodologies. The analysis is restricted to the case of noises in ducts, in which the acoustic propagation phenomena are modeled. Four approaches for this type of models are presented. The first one is formulated by using the basic equations of the acoustics. This procedure generates an infinite dimension model of the duct. In the second approach, the infinite model is truncated by using Taylor s series. The third approach performs a modal expansion using the poles of the infinite dimension model, and, in the fourth, it is also considered a modal expansion, but in this case, by taking into account zeros and poles of the infinite dimension model. The four models studied are discussed and compared in the present contribution. A second part of this work is concerned with active noise control techniques. Monochannel (which uses only a loud speaker and a microphone) and multi-channel (which uses several loud speakers and microphones) controllers are studied. The studied active noise controllers use LMS adaptive algorithms. The noise signals are filtered using X-LMS techniques. These types of controller are usually simple and robust. The coefficients of the controller (modeled as a digital filter) are determined by using an online adaptive procedure looking for minimizing the noise levels. The control methodologies are tested numerically by using the mathematical model of the acoustic duct proposed. With the aim of validating experimentally these controllers a test rig instrumented with loud speakers and microphones was built, and the algorithms were implemented using a personal computer. At the remaining, the numerical and experimental results are discussed and some suggestions are presented in order to continue future works. / Este trabalho formula e discorre sobre o problema de controle ativo de ruído e avalia algumas metodologias de controle tanto numérica como experimentalmente. A análise é restrita ao caso de ruídos em dutos, onde o fenômeno da propagação acústico é analiticamente modelado. Apresentam-se quatro abordagens para tal modelagem. A primeira, formulada a partir das equações fundamentais da acústica, gera um modelo de dimensão infinita para o duto. A segunda aproxima o modelo infinito por uma série truncada de Taylor. A terceira formulação realiza uma expansão modal, a partir dos pólos do modelo de dimensão infinita e a quarta, também realiza uma expansão modal, mas considera tanto os pólos como os zeros do modelo infinito dimensional. No trabalho são discutidos e comparados os quatro modelos numéricos propostos. Numa segunda parte este trabalho discorre-se sobre diversas técnicas de controle ativo de ruído em dutos. São estudados controladores do tipo mono canal, que utilizam um sensor e um atuador apenas e controladores do tipo multicanal, com vários sensores e atuadores. Todos os controladores ativos de ruído (CAR) estudados utilizam algoritmos adaptativos do tipo LMS (Least Mean Square) e técnicas de filtragem-X LMS. Este tipo de controlador tem como características marcantes à simplicidade e a robustez. Os coeficientes do controlador, modelado como um filtro digital, são adaptados on-line segundo uma estratégia que busca minimizar os ruídos não desejados. Estas metodologias de controle são testadas numericamente a partir do modelo matemático proposto para o duto acústico. Para avaliar também experimentalmente tais controladores, montou-se uma bancada de testes constituída por um duto de PVC instrumentada com alto falantes e microfones sendo os algoritmos de controle implementados em um microcomputador pessoal devidamente configurado. O trabalho encerra discutindo os resultados numéricos e experimentais obtidos e sugerindo desdobramentos a serem investigados no futuro. / Doutor em Engenharia Mecânica

Les travaux présentés proposent une nouvelle approche du contrôle actif du bruit à l'intérieur des hélicoptères. Les stratégies de contrôle s'attachaient à réduire le bruit moyennes fréquences en grande partie produit par la boite de transmission principale. Nous proposons d'utiliser le contrôle actif pour traiter les basses fréquences dans la cabine. Les niveaux mis en jeu dans ces gammes de fréquences sont tels qu'un contrôle acoustique nécessite une attention particulière dans la confection des sources antibruits.<br />Nous avons montré l'intérêt de scinder le problème en deux parties. Un premier dispositif est conçu pour traiter la fréquence produite par le rotor principal de l'autogire, et un second dispositif est destiné à traiter la partie large bande du bruit basses fréquences. Nous proposons une méthode spécifique pour la conception des sources antibruits.<br />Compte tenu de la faible disponibilité des aéronefs pour les essais du système, nous avons eu recours à une maquette à l'échelle 1. Nous avons proposé une méthode originale permettant d'y produire un champ primaire perceptivement similaire à celui présent sur l'hélicoptère.<br />Nous avons validé notre stratégie de contrôle par des simulations d'algorithmes FXLMS et IMC-FXLMS. Ces simulations, basées sur des mesures issues d'une campagne d'essaisl, ont pu être validées par des essais du dispositif sur la maquette mettant en œuvre un contrôleur et les sources que nous avons développé. Au cours de l'étude, nous avons participé à la mise au point du matériel embarquable (contrôleur, ampli. et sources). Ce matériel nous a permis de réaliser des essais en vol en fin de thèse qui ont montré l'efficacité du dispositif.

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