This study shows that a concurrent active noise control strategy for engine harmonics and road noise has a complementary effect. In particular, we found that engine booming noise is additionally attenuated when road noise control is concurrently used with engine harmonics control; an additional attenuation of 2.08 dB and 1.25 dB for the C1.5 and C2.0 orders, respectively, was achieved. A parallel multichannel feedforward controller for non-stationary narrowband engine harmonics and broadband road noise was designed and implemented to reduce noise in all four seats. Two control signals were considered independent because the reference signals, engine revolution speed for the engine harmonic controller, and acceleration signal for the road noise controller are uncorrelated. However, if the reference sensor for the road noise controller is installed along the overlapping transfer path between the engine noise and road noise, the engine noise may also be suppressed by the control signal for the road noise attenuation. Based on transfer path analyses for both engine harmonics and road noise, the optimal positions for the reference sensors were selected. In addition, we identified several overlapping transfer paths between the engine booming noise and road noise. A practical active noise control system combined with a remote microphone technique was implemented for a large six-cylinder sedan using a vehicle audio system to evaluate the noise attenuation performance. The experiments showed that the interior noise from the engine and road excitation was effectively suppressed by the proposed concurrent control strategy..

Komatsu Forrest AB have a demand for a media solution where the user can select between different input sources as well as output channels. The input sources are Radio, Bluetooth, AUX, two microphones and PC. The system shall also be able to mix sound from an ANC unit (Active Noise Control). Due to the demands from the company, the system must be able to be digitally controlled. The goal is to present a concept that offers the solution for these demands.To specify the needs from the user a dialog with employees with more knowledge about the user’s use have resulted in a table. From this a technical design has been created and resulted in a technical design requirement. With the design structure as a base, simulations have been done. After the structure has been verified by simulations a prototype with the basic functions have been designed and programmed and verified to clarify the function.Simulations and measurements have shown that the design works as desired. The result shows that the unit can meet the requirements set by the company in a user-friendly manner. The project has resulted in different solutions and these evaluated for pros and cons compared to the chosen solution. / Komatsu Forest AB behöver en multimedialösning för att uppfylla specifika behov exempelvis simultant användande av två mobiltelefoner och två mikrofoner. Enheten ska klara av att hantera ljud från flertalet ljudkällor samt två mikrofoner och aktiv bullerreducering. Systemet ska styras digitalt från till exempel en PC. Målet är att presentera ett koncept som erbjuder en lösning för dessa problem.Utformningen av enheten har gjorts genom att utvärdera användarens bruksområden tillsammans med den vision som legat till grund för projektet. Detta har genom samtal med anställda på företaget medfört en sammanställning över förarens användnings-områden. Utifrån denna sammanställning av användningsområdena har en teknisk skiss för enhetens uppbyggnad möjliggjorts. Efter att funktionen för skissen har verifierats med simuleringar har ett prototypkort för enhetens grundstomme konstrueras, programmerats samt verifierat konstruktionen.Simuleringar och mätningar verifierar att konstruktionen fungerar som önskat. Resultatet visar att enheten skulle uppfylla de uppsatta önskemålen på ett bra och användarvänligt sätt. Då flera lösningar har tagits fram under projektets gång diskuteras för och nackdelar med dessa samt motiveras varför den valda lösningen är bäst anpassad.

Recent experimental achievements in active noise control (ANC) for cooling fans have used near-field error sensors whose locations are determined according to a theoretical condition of minimized sound power. A theoretical point source model, based on the condition previously stated, reveals the location of near-field pressure nulls that may be used to optimize error sensor placement. The actual locations of these near-field pressure nulls for both an axial cooling fan and a monopole loudspeaker were measured over a two-dimensional grid with a linear array of microphones. The achieved global attenuation for each case is measured over a hemisphere located in the acoustic far field of the ANC system. The experimental results are compared to the theoretical pressure null locations in order to determine the efficacy of the point source model. The results closely matched the point source model with a loudspeaker as the primary source, and the sound power reduction was greatly reduced when error sensors were placed in non-ideal locations. A weakness of the current near-field modeling process is that a point monopole source is used to characterize the acoustic noise from an axial cooling fan, which may have multipole characteristics. A more complete characterization of fan noise may be obtained using a procedure based on the work of Martin and Roure [J. Sound Vib. 201 (5), 577--593 (1997)]. Pressure values are obtained over a hemisphere in the far field of a primary source and the contributions from point source distributions up to the second order, centered at the primary source, may be calculated using a multipole expansion. The source information is then used in the aforementioned theoretical near-field calculation of pressure. The error sensors are positioned using the complete fan characterization. The global far-field attenuation for the multipole expansion model of fan noise is compared to that of previous experiments. Results show that the multipole expansion model yields a more accurate representation the near field, but is not successful in achieving greater sound power reductions in the far field.

<p>In many practical multi-channel active sound control (ASC) applications, such as active noise control (ANC), various constraints need to be satisfied, such as the robust stability constraint, noise amplification constraint, controller output power constraints, etc. One way to enforce these constraints is to add a regularization term to the Wiener filter formulation, which, by tuning only a single parameter, can over-satisfy many constraints and degrade the ANC performance. Another approach for non-adaptive ANC filter design that can produce better ANC performance is to directly solve the constrained optimization problem formulated based on the <em>H</em>₂/<em>H</em>_inf control framework. However, such a formulation does not result in a convex optimization problem and its practicality can be limited by the significant computation time required in the solving process. In this dissertation, the traditional <em>H</em>₂/<em>H</em>_inf formulation is convexified and a global minimum is guaranteed. It is then further reformulated into a cone programming formulation and simplified by exploiting the problem structure in its dual form to obtain a more numerically efficient and stable formulation. A warmstarting strategy is also proposed to further reduce the required iterations. Results show that, compared with the traditional methods, the proposed method is more reliable and the computation time can be reduced from the order of days to seconds. When the acoustic feedback path is not strong enough to cause instability, then only constraints that prevent noise amplification outside the desired noise control band are needed. A singular vector filtering method is proposed to maintain satisfactory noise control performance in the desired noise reduction bands while mitigating noise amplification.</p> <p><br></p> <p>The proposed convex conic formulation can be used for a wide range of ASC applications. For example, the improvement in numerical efficiency and stability makes it possible to apply the proposed method to adaptive ANC filter design. Results also show that compared with the conventional constrained adaptive ANC method (leaky FxLMS), the proposed method can achieve a faster convergence rate and better steady-state noise control performance. The proposed conic method can also be used to design the room equalization filter for sound field reproduction and the hear-through filter design for earphones.</p> <p><br></p> <p>Besides efficient filter design methods, efficient filter implementation methods are also developed to reduce real-time computations in implementing designed control filters. A polyphase-structure-based filter design and implementation method is developed for ANC systems that can reduce the computation load for high sampling rate real-time filter implementation but does not introduce an additional time delay. Results show that, compared with various traditional low sampling rate implementations, the proposed method can significantly improve the noise control performance. Compared with the non-polyphase high-sampling rate method, the real-time computations that increase with the sampling rate are improved from quadratically to linearly. Another efficient filter implementation method is to use the infinite impulse response (IIR) filter structure instead of the finite impulse response (FIR) filter structure. A stable IIR filter design approach that does not need the computation and relocation of poles is improved to be applicable in the ANC applications. The result demonstrated that the proposed method can achieve better fitting accuracy and noise control performance in high-order applications.</p>

A wind tunnel from the 1950s located in Bromma, Stockholm, once used for military research is today used for commercial activities. Today the tunnel is used for indoor wingsuit flight and the facility has an interest in reducing the overall noise generated by the tunnel. Acoustic measurements indicate noise problems in the 50 Hz range. A 3D and a 2D model was structured with physical measured dimensions of the wind tunnel for simulations.Results indicate that a 37 dB decrease of the second higher order azimuthal spinning mode was achievable with the same number of monopole anti-sources as fan blades with each individual monopole modeling an enclosed loudspeakers. This acoustic mode was identified as the most problematic due to the cut-on frequency for the geometry coinciding with the fundamental blade pass frequency of the fan source during normal operating conditions. / På 1950-talet byggdes en vindtunnel i Bromma, Stockholm kallad "LT1". Vindtunneln användes då för militär utveckling för bland annat flygförsvaret. Idag drivs den efter en period utan användning kommersiellt för flygning av ekorrdräkt så kallad "Wingsuit" inomhus. Området kring tunneln har kommersialierats under tiden den var ur drift och omgivande verksamheter påverkas av ljud från vindtunnelns verksamhet varför tunnelns verksamhet söker förbättringsmöjligheter avseende bullerdämpning. Akustiska mätningar i och utanför tunneln indikerar ett problemområde kring frekvensen 50 Hz som härstammar från den stora fläkt som skapar flödet i tunneln. Vindtunneln modellerades i 3D och 2D med hjälp av fysiskt uppmätta dimensioner på plats. Modellerna användes sedan för att kunna numeriskt simulera och beräkna möjligheten att dämpa ljudet från fläkten med hjälp av aktiv kontroll. Resultaten indikerar att en 37 dB ljudreduktion är möjlig av andra ordningens högre akustiska snurrande mod. Detta var möjligt att uppnå med lika många anti-källor modellerade som monopoler vilket motsvarar slutna högtalare, som fläktblad. Denna akustiska mod identifierades som den mest problematiska eftersom cut-on frekvensen för tunnelns geometri sammanfaller med den fundamentala bladpass-frekvensen för fläkten under normal drift.