Vom Impedanzmessrohr in den Transmissionsprüfstand - Maßgeschneiderte Akustiklösungen

Warum „klingt“ ein Schlafzimmer anders als ein Bad und warum kann man sich in manchen Räumen so schlecht unterhalten? Die Antwort ist: Es hängt von den Dämpfungseigenschaften der Materialien in diesen Räumen ab. Die akustische Untersuchung neuer Materialien und die Entwicklung von Konzepten zur Schalldämpfung erfordern viel Technik und Know-how. Dies gilt insbesondere für Leichtbaustrukturen und -systeme, weil deren Masse und Volumen auf ein Minimum reduziert werden müssen. Das unten dargestellte Impedanzmessrohr ist ein etabliertes Hilfsmittel, mit dem sich Materialien akustisch untersuchen lassen. Neue ultraleichte Materialien wie Aerogele können damit schnell und zuverlässig geprüft und mit herkömmlichen Dämmstoffen wie Schäumen oder Mineralwolle verglichen werden. Vielversprechende neue Materialien werden anschließend im Schalltransmissionsprüfstand des Instituts weiteren Tests unterzogen. Diese erfolgen in einem größeren Maßstab und unter realistischeren Randbedingungen. Auf diese Weise entstehen innovative Schallschutzlösungen, die eine hohe Schalldämpfung mit niedrigem Gewicht und Volumen in sich vereinen

Noise reduction results of the ACASIAS Active Lining Panel

Advanced concepts for aero-structures with multifunctional capabilities are investigated within the EU-project ACASIAS. In work package 3 of ACASIAS, components of an active noise reduction system are structurally integrated into a curved sandwich panel by means of 3D printed inserts. This so-called smart lining is intended for application in aircraft as a modular and lightweight interior noise treatment in propeller-driven aircraft. The broad application scenario of smart linings ranges from retro-fitting of current regional aircraft such as ATR 42, ATR 72, DHC-8 Q400 to the application in new short-range aircraft with energy efficient counter rotating open rotor (CROR) engines or with distributed electric propellers. A key feature of the smart lining with integrated active components is its modularity, facilitating a flexible application in the aircraft cabin. This requires a fully self-contained sensing mechanism based on structurally integrated accelerometers. Using the normal surface vibration data from the integrated sensors, the smart lining is able to predict the sound field in front of it. The so-called virtual microphone method with remote sensors and observer filter allows to get rid of real microphones and wiring in the aircraft cabin. This makes retro-fitting easier because it reduces wiring effort and costs which is beneficial for future aircraft as well. However, the use of virtual instead of real microphones might deteriorate the performance or even the stability of the active noise reduction system because it relies on accurate plant models. Laboratory experiments in a sound transmission loss facility are conducted to assess the behavior of the smart lining with virtual microphones and compare it to a smart lining with real microphones. The sensitivity of the smart lining to environmental changes and the noise reduction performance and control system stability are investigated in this study

EXPERIMENTAL RESULTS OF AN ACTIVE SIDEWALL PANEL WITH VIRTUAL MICROPHONES FOR AIRCRAFT INTERIOR NOISE REDUCTION

This work focuses on the reduction of aircraft interior noise by means of actively controlled sidewall panels (smart linings). It was shown in prior work that considerable reductions of interior sound pressure level can be achieved using structural actuators on the lining and microphones distributed in the seat area in front of the linings. Simulation results suggest that the physical microphones in front of the linings can be replaced by so-called virtual microphones. The signals of these virtual microphones are obtained from filtering the normal surface vibrations of the lining through an observer filter. Accelerometers are mounted on the lining structure to obtain the vibration signals. Simulation results of a smart lining with virtual microphones show a mean sound pressure level (SPL) reduction of 10 dB and 5.9 dB(A) in front of the lining. These results must be verified by laboratory experiments applying real-time control because the effects of time variances and imperfect path models will deteriorate the performance of the smart lining. Therefore, the present contribution describes an experimental realization of a smart lining with remote sensors and virtual microphones in a realistic laboratory setup. A double panel system consisting of a primary carbon fiber reinforced plastic (CFRP) structure (fuselage) and a coupled smart lining is installed in the opening of a transmission loss facility. The real-time behavior of the smart lining is tested and the influence of using virtual instead of physical microphones on the SPL reduction is quantified. One focus is on evaluating the robustness of the smart lining under changing environmental conditions

On the noise reduction of active sidewall aircraft panels using feedforward control with embedded systems

Keeping aircraft interior noise on an acceptable level is an important aspect for the passenger comfort and health in modern aircraft design. Generally there is a trade-off in the achievable transmission loss of aircraft and the additional weight of the insulation provision, especially at low frequencies below 500 Hz. In this frequency range, passive methods for sound insulation usually have high weight and volume requirements for suitable performance in aircraft. The present work focuses on aircraft with rotor engines having characteristic low frequency narrow banded noise sources due to the rotational speed of the rotors. A major transmission path of the noise is given by the linings of the aircraft cabin, which are coupled to the fuselage. These large sound emmitting surfaces radiate the noise directly into the passenger zone. Active control is a promising method to reduce at least the low-frequency noise radiated by the panels. These so-called smart linings, augmented with suitable actuators and sensors, are also usable for additional tasks, such as e.g. passenger announcements and noise masking. The feasibility of active feedforward control to reduce narrow banded frequencies below 500 Hz has already been tested successfully at DLR on test panels and in a fully equipped test aircraft on ground, which are excited externally by a loudspeaker array. This paper is dedicated to a discussion of the recent activities to simplify the underlying control strategies in a way that they can be implemented on small embedded systems with limited performance. In the future, such microprocessor units enable the integration of the actuators, sensors and control algorithms directly into smart lining modules, having a positive effect on maintainability and weight footprint of the smart Panels

DESIGN OF AN ACTIVE NOISE CONTROL SYSTEM FOR A BUSINESS JET WITH TURBOFAN ENGINES

An active noise vibration control (ANVC) system is designed for a Dassault Falcon 2000LX business jet. Measurement data from ground tests and recordings from cruise flight reveal narrowband and tonal cabin noise in the bandwidth from 50 Hz to 500 Hz. In both conditions the sound pressure level (SPL) of the tonal components is up to 10 dB above the cabin noise floor. The designed ANVC system is expected to reduce the tonal components to the noise floor. The system is realized on the ceiling panel in the aft of the cabin. A frequency domain filtered-x least mean squares algorithm (FxLMS) is used for control. The main design task is the definition of actuator (inertial exciter) and error sensor (microphone) locations on the ceiling panel and in the cabin fluid. measurement data from a ground vibration test of DLR test aircraft iSTAR is used for the identification of suitable actuator and sensor locations. The system performance is predicted with numerical simulations using sampled sound pressures and identified frequency response functions (FRF). The sound pressures are recorded at 312 locations in the volume of the aft cabin. FRF from 48 actuator locations to 312 microphone locations are available for optimization. The system will be realized in the iSTAR in July 2023. Ground tests will be performed with engine excitation up to 80% N1 (rotational speed of low the pressure compressor shaft)

New Sound Transmission Loss Test Facility for Acoustic Evaluation of Smart Lightweight Panels

The activities of the Institute of Composite Structures and Adaptive Systems in the field of ASAC are focused on the development of smart lightweight structures with improved acoustic properties, especially in the challenging low-frequency domain. The new sound transmission loss test facility will provide the experimental conditions necessary for the Investigation and validation of numerically designed and optimized ASAC systems

Active Reduction of Car Interior Noise

Active Structural Acoustic Control (ASAC) is an effective measure to reduce the windshield-vibration-induced interior noise in an automobile passenger compartment. Different control strategies for the active reduction of windshield-vibration-induced car interior noise are developed, experimentally validated, and now available. The comparison of the vibration levels in open and closed loop show a global reduction of 5dB to 7dB in the acoustically relevant frequency band containing the second and third eigenmode of the windshield system (100 Hz to 150 Hz). The acoustic effects are reflected in a reduction of up to 15dB in SPL at 145Hz

Active Sidewall Panels with Virtual Microphones for Aircraft Interior Noise Reduction

This work deals with the reduction of aircraft interior noise using active sidewall panels (linings). Research work done in the past showed that considerable reductions of the sound pressure level (SPL) in the cabin are possible using structural actuators mounted on the lining and error microphones distributed in front of the linings. However, microphones are undesirable for error sensing because they are not suitable for the realisation of an integrated and autonomous active lining (smart lining module). Therefore, the goal of the present work is the replacement of the microphones by structural sensors. Using the structural sensors as remote sensors in combination with an acoustic filter, virtual microphones can be defined. The present study relies on experimental data of a double-walled fuselage system which is mounted in a sound transmission loss facility. Simulation results based on measured time data and identified frequency response functions are provided. Different configurations of virtual microphones are investigated regarding the SPL reduction and the induced vibration of the lining panel

Leicht und leise - weniger Lärm in der Flugzeugkabine

Gitterversteifte Rumpfstrukturen (Gridstrukturen) sind potenziell leichter als ihre konventionellen Pendants in Stringer-Spant-Bauweise. Die Gewichtsvorteile gehen jedoch zu Lasten der Akustik, weil leichte und hochsteife Rumpfstrukturen den Schall sehr effizient leiten und somit hohe Störpegel in der Kabine hervorrufen können. Während Lärmreduktionsmaßnahmen für konventionelle Rumpfstrukturen seit Längerem am DLR erforscht werden (vgl. Algermissen, Haase, Unruh, Misol, Aktive Lärmreduktion im aeroakustischen Windkanal-Experiment, Innovationsbericht 2016, S. 57), sind die vibroakustischen Eigenschaften von Gridstrukturen wenig erforscht. Es ist jedoch zu erwarten, dass die geänderten geometrischen und dynamischen Eigenschaften von Gridstrukturen auch neue Akustikmaßnahmen erfordern. Im Rahmen des Sonderforschungsbereichs (SFB) 880 forscht das DLR seit 2016 an einer Kombination aus aktiven und passiven (aktiv-passiv-hybriden) Lärmreduktionsmaßnahmen für Rumpfstrukturen zukünftiger Flugzeuge. Im Fokus stehen innovative Konzepte zur leichtbaukonformen Reduktion von tieffrequentem Kabinenlärm