Mechatronik

One of the great challenges in modern society is the reduction of noise and vibration. High noise levels can occur in urban environments or industrial applications. Whether the goal is to protect humans from the negative effects of noise, enable communication in extreme environments, or acoustically disguise vehicles, traditional passive dampening is not always the best option. The concept of active noise and vibration control has been in existence for several decades, yet it has only been adopted on a widespread basis in certain applications, such as headphones. When larger acoustic volumes need to be controlled, active control systems with many components must be applied. In these systems, the computational effort of the algorithms, coupled with low latency requirements, presents a significant challenge. Hardware-based active noise control systems can be a solution due to their high level of parallel computing and low latency. In other areas of signal processing, they have already become the industry standard for the same reasons. In this thesis, acoustic applications of adaptive filters implemented on Field Programmable Gate Arrays will be presented and evaluated. The developed systems in this work include well-known setups, such as Kundt’s Tube or an active headrest, for validation purposes. The more advanced solutions incorporate a helicopter headset with additional speech enhancement capabilities and active impedance control of an underwater surface. Additionally, an overview of the theoretical background is given, and state-of-the-art related work is discussed. The results of the measurements show that hardware-based adaptive filters not only match their software counterparts, but they can enhance noise reduction performance. In the case of the helicopter headset, the additional computational headroom of the chosen platform is used to include Wiener filter-based speech enhancement. This system produces impressive results in the evaluated speech quality, in addition to the active noise reduction of the headset. Active control of underwater sound bears additional challenges when it comes to latency requirements due to the high speed of sound. The proposed hardware-based system in this thesis is capable of effectively minimizing reflected sound waves from an underwater surface. This outstanding feature could be used in future applications to conceal underwater vehicles from active sonar. Overall, the findings in this thesis provide compelling arguments for hardware-based adaptive filters for acoustic applications

This thesis presents a multidisciplinary research framework for the design, analysis, optimization, and experimental validation of both passive and Active Noise Control (ANC) strategies applied to the HafenCity window system. The increasing urban density and the associated rise in environmental noise call for novel and effective noise mitigation strategies in building façades. Addressing this challenge, the present work combines acoustics, mechanical engineering, signal processing, and adaptive control to develop an integrated, modular, and scalable system capable of reducing sound transmission through complex window structures. The core of the study is a laboratory demonstrator which serves as a flexible test environment for the iterative development of both passive and active measures. The initial part of the work is dedicated to a review of state-of-the-art technologies in sound reduction and ANC systems, highlighting their respective limitations and research gaps. Building upon these insights, a laboratory demonstrator integrated into a transmission test rig is constructed to enable controlled and repeatable investigations of acoustic behaviour under variable conditions. The demonstrator incorporates modular absorber components, actuators, and microphones, allowing for flexible adaptation to different configurations. Acoustic system characterization is conducted using several complementary measurement techniques, including sound reduction index determination according to standardized procedures, modal analysis of the transmission test rig, different system identification methods, and vibration measurements of the window structure. Advanced signal processing techniques, as the generation and deconvolution of exponential sine sweep signals, are employed to extract detailed information on the system's impulse response, transfer paths, and non-linearities. Additionally, metrics such as Reverberation Time (RT), spatial diffuseness and Total Harmonic Distortion (THD) are evaluated to quantify the acoustic performance and dynamic properties of the system. Subsequent chapters focus on the design, optimization, and experimental validation of passive absorber elements within the window cavity. Methods such as sound field mapping are used to visualize standing wave patterns and optimize absorber placement and geometry. Sensor and actuator positions are optimized using both numerical methods and heuristic algorithms, targeting improved controllability and performance of the ANC system. A validation strategy is presented that includes both simulation-based predictions and experimental testing, confirming the robustness of the optimization approach. Another contribution of this work is the implementation of adaptive control algorithms, specifically FeedForward (FF) and FeedBack (FB) variants, for real-time noise cancellation. These algorithms are tested under various boundary conditions, including varying delays, non-stationary excitation, and secondary path variability. Performance parameters such as quiet zone formation, real-time attenuation, spatial resolution and latency are analysed. Results show that reductions in transmitted noise levels are achievable confirming the applicability of ANC in façade-integrated systems. Last the development of an ANC prototype incorporates hardware components selected for embedded operation, including Micro-Electro-Mechanical-Systems (MEMS) microphones, miniaturized amplifiers, loudspeakers, and two real-time processing platforms, a Digital Signal Processor (DSP) and a Field-Programmable-Gate-Array (FPGA). Both platforms are benchmarked with respect to latency, computational power, implementation complexity, and scalability. Comprehensive experimental evaluations demonstrate that the integrated system achieves improvements in broadband noise attenuation, particularly within the lower frequency range where passive elements are less effective. In conclusion, the research demonstrates that window systems with embedded ANC functionality offer a viable path forward in mitigating environmental noise in urban architecture.

Diese Studienarbeit behandelt die Anwendung der statistisch optimierten akustischen Nahfeldholografie (SONAH) zur inversen Rekonstruktion der Schallschnelle auf der Oberfläche eines Kolbenstrahlers in einer unendlich ausgedehnten schallharten Wand. Die Eingangsgrößen der Rekonstruktion des Schalldruckfeldes lieferte das Rayleigh-Integral. Das Schalldruckfeld wurde mit Hilfe des Rayleigh-Integral simuliert und mit Näherungslösungen im Fern- und Nahfeld validiert. Die Implementierung der SONAH erfolgte unter Verwendung ebener Wellenfunktionen und wurde durch Tests hinsichtlich ihrer Genauigkeit beurteilt. Erste Rekonstruktionsergebnisse zeigten realistische Druck- und Schnelleverteilungen, jedoch bestehen noch Abweichungen, deren Ursachen analysiert und in Hinblick auf eine spätere Anwendung bewertet wurden.

Urbanization and the resulting problems are a major challenge for a large part of the inhabitants of cities and furthermore for the planet. The population of many cities around the world such as Beijing, China or Hamburg, Germany have been steadily increasing for several years. In addition to increasing CO2 emissions through rising numbers of private transport, the increase of sound pollution in urban areas contributes to environmental pollution. High noise levels have been shown to have a negative impact on health, especially cardiovascular diseases. As a conclusion, there are limit values to be observed in many countries in order to reduce noise, for example in the construction industry. In Hamburg there is a so-called ”Hafen-City-Clause”, which ensures that a noise level of 30dB(A) is not exceeded at night when the windows in bedrooms are partially open. Standard windows often do not achieve a sufficient reduction in sound transmission to be able to comply with this limit value. The sound transmission can be minimized by the use of passive measures, but this is accompanied by a restriction of the ventilation through the partially open window. In order to enable unrestricted air circulation, active systems, so-called Active-Noise-Control (ANC) systems, can be used. These systems can be placed in the sound transmission path and reduce noise through destructive interference without restricting airflow of the window. To design an effective active system it is necessary to examine the influence of sound transmission paths on transmitted noise. For further investigation of sound transmission, the complex geometry of a partially open window is simplified to a parameterizable two-dimensional gap. The transmission of noise is calculated analytically in a first step with a substitute model, from which requirements for an active system are drawn. In order to get a deeper understanding of the influence of a sound path on sound transmission, the influence of parameters of a small gap in relation to the wavelength and the influence of a control signal of an anti-noise source on sound transmission are investigated. As a result of the conducted analytical and numerical studies, an alternative approach of an algorithm for adaptive intensity-based noise reduction is introduced. In this approach, only sensors placed in the gap are used to reduce the transmission of sound. The presented approach is finally implemented in a rapid control prototyping system and the functionality is proven by an experiment.

The use of inertial actuators to control the radiated sound pressure of a steel ship model at a lake measurement facility is examined. Therefore, methods of active vibration control as well as active control of target sound fields are applied using a fixed configuration of twelve accelerometers, eight control actuators, and five hydrophones. A narrowband feedforward active control system is used to manipulate the sound pressure at hydrophone positions, focusing not only on reducing but also on adding spectral lines in the radiated signature. The performance is assessed using measured data by additional accelerometers inside the ship model as well as by hydrophones surrounding the measurement facility. It is found that less control effort is necessary for the generation of additional tones compared to the control of a present disturbance at hydrophones. In the frequency range considered (below 500 Hz), the actively induced change in the mean structural velocity is not necessarily proportional to the change in the radiated sound pressure. In contrast to the vibration velocity, no unwanted amplification of the sound pressure is found for the frequencies observed.

Sources for underwater sound with large dimensions l, such as a coating on a boat structure, can be suitable for generating low-frequency underwater sound. In this context, low-frequency refers to wavelength λ in water that fall within the range of 40 l>λ>1,5 l . Such low-frequency sources can be applicable for a wide variety of long-range applications, such as communication, navigation and sonar. In this study, an array of 18 circular piezoelectric actuators embedded in a potting compound create an active surface for generating underwater sound. The coating is attached to a plane plate of glass-fiber reinforced plastic, typical for maritime applications. The acoustic propagation is investigated in an underwater test range with free field conditions, specifically for low frequencies. This study focuses on sound radiation and beam steering behaviour (in terms of propagation and vibroacoustics). On the one hand, the test results show the strongly frequency-dependent performance of the coating as an underwater sound source. On the other hand, a significant influence of the vibroacoustic behavior on the sound radiation can be recognized. The beam steering experiment reveals the phase shifts, up to which the coating still emits sound without major losses.

To test methods for active noise and vibration compensation, the mathematical model of a rectangular simply supported plate in a rigid baffled is reproduced experimentally in a laboratory. In order to verify the accuracy of this replication, the assumed boundary conditions are examined by using harmonic signals for excitation. The investigated frequency range comprises the first 20 natural frequencies of the plate and ranges in the first quartile below the coincidence frequency. The acoustic boundary conditions are tested by comparing the calculated sound field of the circular piston radiator in a rigid baffle with the measured sound field generated by a loudspeaker of the same size mounted in a large wooden wall. The structural boundary conditions are verified by comparing the normal vibration velocity calculated using the model of the simply supported plate and the velocity field measured on a plate using a laser vibrometer.

Reducing the sound reflected by objects under water and thus their ”visibility” is a design challenge and requires detailed information about the geometry- and material-dependent backscattering behavior of the structure. Using various numerical methods and measurements, the radiation-relevant areas of objects are to be determined using generic model examples, taking into account different types of sound sources. The contribution briefly presents the methods used (boundary element method, ray tracing and hotspot detection), analyzes the results of numerical simulations and compares them with corresponding measurements.

From a strategic point of view, it is necessary to protect underwater vehicles from reconnaissance. Modern sonar systems are capable of determining the exact position of those vehicles. They do this by emitting signals that reflect off the surface of the vehicles, which are then detected and evaluated. At low frequencies, reflection can be reduced by using an active system. The incoming sound waves are measured in front of the surface and then processed by a control system. The system drives an actuator applied to the surface that minimizes the reflection. Three different actuator concepts are compared in this study: First, a 3-3 mode piezoceramic patch transducer attached to steel; second, a bending actuator embedded in foam; and third, a multilayer structure made of Poly(vinylidenfluorid-Trifluorethylen) (PVDF-TrFE) film. Measurements in laboratory tests show the different characteristics of the actuators under the influence of sonar-like signals and under hydrostatic pressure from 1 to 40 bar.

Layered media, in form of both fluid layers and solid structures, play an important role in underwater acoustics. Typical applications are acoustic windows for sonar systems or absorption layers for submarines. However, the either desired total absorption, total reflection or total transmission is not achieved in practice. The vibro-acoustic behaviour of an object plays a particularly important role in the resulting target echo strength. Active surfaces, for example made of piezo-ceramics, increase the possibilities of influencing the target echo strength beyond the geometry, material properties and material composition of the layer.\nThis study compares the target strength behaviour of different layered media including active surfaces for underwater applications. An analytical model is used to estimate the reflection and transmission behaviour. When the reflection of a surface is minimized by an active coating, the transmission increases significantly. This is a particular problem in layered media, as the transmitted sound waves can cause subsequent layers and structures to vibrate and thus increase the total target echo strength. Various countermeasures are being investigated to prevent this.