Our Fields of Research


Control of Structure-Borne and Airborne Sound

Machine acoustics is an engineering science dealing with noise caused by vibrating machine structures and their creation. The fundamental equation of machine acoustics provides a model representation for the creation of radiated sound power of a structure due to force excitement. With that, it clearly shows possible weak points for the acoustic changes of structures. The core approaches for influencing sound radiation are the so called “primary measures”, which constructively change the structure to such an extent, that its structure-borne sound is reduced before it can be radiated into the air. The model representation of the fundamental equation of machine acoustics considers structure-borne sound propagation, however, as a “black box” and therefore provides no scientific approach for analysing structure-borne sound transfer. Due to new methods and tools are required for efficient and effective simulations of structure-borne and airborne sound because of rising acoustic requirements and shorter development cycles, this means that the primary measures are usually based on empirical data (results and experiments).


Structural Intensity (STI)

A powerful alternative to using the fundamental equation of machine acoustics is using the sound’s energy flows (intensities) in order to create a model representation of the sound propagation. In a solid body, for example, one can use structural intensity (STI) to describe the flow of structure-borne sound energy from a energy source to a energy sink.

The following topics in the field of STI are currently being researched at our research group:

  • Numerical analysis of the structural intensity distribution – STI can be calculated numerically, e.g. with an FE-program, using the system’s stress resultants. Analysing STI depicts, among other things, the location of the critical paths of energy on a structure. Therefore, using STI-Analysis can greatly expand our understanding of structural behaviour. With this knowledge, local starting points can be found to create impedance discontinuities using constructive measures in order to reduce vibration.
  • Numerical structure optimization by means of structural intensity – the input or dissipation of energy in defined structural areas can be calculated based on STI. For example, it is possible to use STI to calculate the optimal distribution of damping patches on a structure so that maximal energy is dissipated.
  • Investigations of the factors that influence structural intensity – in order to reduce the sound radiation of a vibrating structure, the concept of redirecting energy can be chosen as an alternative to the dissipation of vibrational energy. This means that an attempt is being made to change the structure, e.g. the structure’s geometry, so that most of the energy flows into non-critical areas. As a result, hardly any sound is radiated. Our current research investigates possibilities that can be used for this purpose.



Determination of Transmission, Reflection, and Absorption

Nowadays, energy-FE methods or the statistical energy analysis (SEA) are increasingly being used for the simulation of structure-borne propagation and airborne radiation of sound, in and from large structures (e.g. in shipbuilding). A component of these methods is the decomposition of an entire system into sub-structures. Between each substructure lies an interface where structure-borne sound, and therefore the structure-borne sound energy flow, is being exchanged. Therefore, it is necessary to understand the transfer of structure-borne sound at these interfaces. The borders between substructures are set at places where discontinuities in the structure exist. These places can be, for example, joints, such as welded joints, or changes in the cross-section of the geometry. At such discontinuities (impedance discontinuities) structure-borne sound is both reflected and transmitted. Coupling factors that describe the proportions of reflected and transmitted energy at these interfaces are thus necessary in order to assemble the substructures into an overall system. Within a research project at the Research Group SAM, we investigate the impact of various interfaces on the structure-borne sound propagation by using a test stand (being continuosly developed) that measures the reflection and transmission of structure-borne sound at coupling points. Up to this point, current research has shown that the well-known measurement methods cause large uncertainty that result in a lot of variation in the measured coupling coefficients. Therefore, a more robust measurement method is being researched. The measured coupling coefficients for these different coupling points are used in the context of a research project (EPES) – in cooperation with multiple universities and industry partners – for the validation of the project’s simulation tool.

Similar uncertainties also occur with the measurement of damping materials in the Kundt’s tube. In a current research project, the Research Group SAM is investigating processes (e.g. the mounting of samples) that yield the highest uncertainty and how they can be controlled.



Integration of Active Components

Through the use of active components (actuators, sensors, as well as regulators), a system’s functionality can be increased. The active influence of vibrational behaviour offers, for example, the possibility of active sound reduction or auditory masking. Active noise cancellation (ANC) and active structural acoustic control (ASAC) are research fields used for active noise compensation, which is achieved through the insertion of artificially generated sound (structure-borne and/or airborne sound) by means of active components.

Currently at the Research Group SAM, we are researching to further develop the following processes as well as their relation to human listening behaviour:

  • Multiple use of active transducers for additional noise generation – existing active systems offer the opportunity to use already available actuators and sensors for further purposes. For example, it is imaginable to dampen structural resonances with an active system in the low frequency domain (primary goal) while simultaneously generating structural vibrations (masking noise) in the high frequency domain. This allows the system to appear more pleasant.
  • Consideration of human hearing on the control of active systems – the acoustic behaviour of mechanical structures can be customized by means of active components. The ‘sound quality’ of products – a measure of how customers rate product noise – is becoming an increasingly more important consideration. Successfully achieving the goal of producing simple, quiet products can lead people to notice annoying noises in quiet, noise-controlled environments that otherwise would not be obvious. The goal of our research project is to extend the existing methods for active noise reduction using our knowledge of human hearing. The error signal should be weighed in terms of the functionality of human hearing as well as the effect of noise on people in order to obtain a result that is fitted to humans. The focus is primarily on using active components to make the acoustic behaviour more pleasant and not on reducing the sound radiation.



Psychoacoustic Optimization of Gear Noise

In a collective research project with the Applied Cognitive Psychology Unit of the Institute of Psychology, our Research Group is investigating the psychoacoustic effect of gear noise. These conventional analysis routines are matched with psychoacoustic metrics and auditory experiments. Structure-borne sound signals are also being considered in the process. While the primary objective is to be able to predict customer acceptance, the secondary goal is to be able to detect gear damage by means of state of the art procedures.



Inequidistant gearing used to reduce gear noise (INVerz)

A strong tonal noise excitation occurs when meshing conventional spur gears through the strictly regular arrangement of the teeth. This noise is referred to as gear whine. Gear whine is perceived as an unpleasant noise, which often leads to customer complaints and negatively influences the product’s quality.

In order to prevent gear whine in gear trains, an innovative approach is investigated by the research group System Reliability, Adaptive Structures, and Machine Acoustics SAM of the Technische Universität Darmstadt. The principle of strictly regular (equidistant) pitch of the teeth along the circumference of spur gears is replaced. Through an optimized variation of individual tooth positioning and tooth thicknesses, an irregular gearing is developed – the inequidistant gearing.

Sound example: conventional equidistant gearing (measured at 1500 rpm)
Sound example: innovative inequidistant gearing (measured at 1500 rpm))

  • Reduced annoyance of the meshing noise – Through the irregular arrangement of the teeth, the periodicity of the tonal noise excitation is reduced. The meshing orders no longer stand out dominantly, rather are lost in the newly emerging sidebands. The annoying tonal noise character is shifted towards a more pleasant, white noise-like character. Furthermore, the sound character is moved to lower frequencies, which also leads to a reduction in annoyance.
  • Reduced loudness and reduced total sound pressure level – In the illustration to the side, the experimentally investigated results for the loudness and the sound pressure level (SPL) at increasing rotational speeds are depicted. Significantly lower values over the entire rotational speed range are achieved for the loudness (top diagram) with inequidistant toothing (red line). The noise is perceived subjectively less loud by humans and thus is felt to be less annoying. The tendency that the inequidistant gearing is quieter can also be seen for the sound pressure level (SPL, bottom diagram). Thus, the inequidistant gearing also leads objectively to less noise.
  • Reduced excitation of structural resonances – The dominant orders of the meshing frequency are less excited. Instead, a broad mixture of frequencies, each with smaller amplitude, is excited. As a result, the resonances of adjacent structures (e.g. gearbox housing) are less excited, which in turn leads to reduced sound radiation.



Noise reduction of electrical machines and actuators

  • Investigation of control effects on the acoustic behaviour of a force-feedback-actuator – as part of a ZIM project (funded by the BMWi), the Research Group is working with a medium-sized industry partner to investigate the effect of various types of control used in managing the sound radiation of a force-feedback-actuator. Here, the main focus of research is on the inverter-fed, synchronous motor of the actuator which depicts a significant source of vibration excitation.
  • Acoustic behaviour of bearings/special gears – the forces and deformations in elliptically deformed balls are being investigated, like those used in strain-wave gear. Therefore, a numerical simulation model is being developed and synchronized with measurement data. The goal is to specify the ball forces, the shaft’s restoring force, and accelerations that act on the surface of the bearing (for diagnostic purposes) with respect to construction parameters.



Scaling laws for analyzing and assessing the acoustics of size ranges

Scaling laws can be used in machine acoustics to scale-up measurement results of a small-scaled prototype to those of the original structure. Furthermore, scaling laws allow for analyzing noise and vibration responses of size ranges such as a size range of gear boxes. The research focuses on the derivation of scaling laws of vibrating and noise radiating structures from virtual simulations (e.g., finite element simulations). Therefore, the methods of similitude analysis are combined with sensitivity analyses. This allows for directly obtaining scaling laws in the post-processing of virtual simulations. The scaling laws can be used to scale-up results from numerical and experimental simulations. Besides scaling the geometry parameters the scaling of the material properties is also investigated, e.g., to allow for predicting noise and vibrations of structures based on additively manufactured prototypes. This can contribute to the analysis and optimization of noise and vibrations during early stages of the product design process. The research group SAM collaborates with the university Federico II in Naples (Italy) on this research.



Development, Modelling, Evaluation and Use of Smart Structural Components and Systems

The development of smart systems – taking into account newer actuators and sensors – makes it possible to monitor and improve the mechanical properties of products. Generally, actuators characterize elements that convert different outputs so that an action or effect is produced. In addition to their application in actuator technology, the energy-converted properties (electrical into mechanical or mechanical into electrical energy) of many multifunctional materials are also allowed to be utilized in sensor technology.

At the Research Group SAM, we deal with the system design of smart systems. Here, the focus lies in the further development of simulation methods and the component design.


Development of high sensitivity piezoelectric sensors

In a collaborative project with the Research Group Electroacoustics at TU Darmstadt, high sensitivity sensors that use piezoelectret films are being developed and investigated. These films are particularly suitable for the production of flat sensors (accelerometers and microphones) with high sensitivity.



Multi-physical overall system simulation (numerical and experimental)

Smart systems are interdisciplinary technologies that implement autonomous structural systems, which can independently adjust to changing boundary conditions. All mechanical engineering constructions can be used as application scenarios, for which their vibrational behaviour, sound radiation, contour and geometric properties, or even damage tolerances should be actively manipulated. To control the high functioning of smart systems, a detailed numerical system characterization is necessary in the earlier design phase. Our research goal is to improve and expand the current methods for multi-physical, overall system simulation. On one hand, we are investigating how the complex relationship (e.g. the coupling of electrical and mechanical domains) of a smart system can be displayed in a more numerically efficient manner, e.g. through use of a metamodel. On the other hand, we are researching the use of acoustic target functions (e.g. the calculation of the radiated sound power from structural vibrations) in the design of smart systems.



Stochastic Sensitivity Analysis and Statistical Design of Experiments

An active or smart system usually has a complex structure with multiple actuating and sensing functional elements. Interactions between the individual components and their impacts on the functionality and reliability of the overall system are hardly researched and rarely predictable. Therefore, at the Research Group SAM we are searching for new experimental and numerical methods of sensitivity analysis for the detection and influence of main effects and interactions. Current research projects are:

  • implementation of different methods for global sensitivity analysis of complex systems,
  • numerical simulation and analysis of these various methods for academic reference systems,
  • evaluation of the convergence behaviour of these methods and the quality of the results,
  • qualitative experimental validation of the results of the numerical simulation by means of statistical design of experiments,
  • generation of a general methodology for the sensitivity analysis for complex systems and their transfer to industry-oriented systems.



Aktive Minderung des Wolftons am Cello

The „wolf in the cello” is an acoustical phenomenon that arises from the interaction of string and body. Practically every cello is affected by the wolf, while it occurs in an individual frequency for each cello. The wolf note is characterized by strong beats in the radiated air-borne sound, reminding the howl of a wolf, which is considered to be annoying. The wolf note can be eliminated by the combination of smart materials and active systems. The development of real-time-algorithms enables the detection and the effective elimination of the wolf notes while being able to adapt itself to changing environmental conditions.


Characterization, Evaluation and Control of the Reliability of Mechanical Systems

In addition to the construction of smart systems, the Research Group SAM also research the technical reliability of mechanical and smart systems. The increasing complexity of systems, due to the higher functionality and the larger number of installed components, complicates the assessment of system reliability. New methods take into consideration the interactions between the different components of smart systems, and make it possible to guarantee that mechanical and smart systems are reliable.


Inspektionsfrei – Erhöhung der Korrosionsschwingfestigkeit von Radsatzwellen aus Stahl durch induktive Randschichthärtung


Control of Uncertainty in Load-Carrying Structures of Mechanical Engineering

The Research Group SAM is researching the methods of characterizing, evaluating, and controlling uncertainty in load-carrying structures in mechanical engineering within the framework of the Collaborative Research Centre SFB 805.

  • Load monitoring: In subproject C1 of the SFB 805, the algorithms for determining the loads and the changes to system properties are being expanded. These algorithms are for the applications already in place, and structurally characterize, evaluate, and finally control arising uncertainty.
  • Active stabilising technologies: Subproject C2 deals with controlling uncertainty in the use of actively stabilized structures. The focus is to prevent a buckling failure of an axially compressed column by using active technologies and to, therefore, control the uncertainty in the load.
  • Assessment of structure-property-relations in active systems: In subproject C3, the uncertainty in the property’s variances of an entire, load-carrying system is controlled. A method is being developed for quantifying and characterizing the uncertainty found in the interaction between the passive and active system elements.
  • Variable processes and application of methods and technologies for uncertainty control in load-carrying systems: In subproject C5, the uncertainty that occurs in the development, production, and usage of load-carrying systems is described and assessed as a result of both a variable process modelling and application through use of the SFB-Demonstrator.
  • Structural health control of load-carrying mechanical systems: In subproject C7, the uncertainty in the predicted lifetime, caused by collapse or impairment, of mechanical systems is controlled, on one hand, through autonomous approaches of Structural Health Monitoring (SHM) and, on the other hand, through the autonomous dynamic intervention of Structural Health Control (SHC).





Damage mechanisms of elastomer components

The influence of temperature on the material properties and durability of elastomers is being investigated by the Research Group SAM as part of the BMWi-funded project “Elasto-Opt II” – capture, simulation, and evaluation of the thermomechanical damage mechanisms of elastomer components under dynamic mechanical loads.

In the case of higher-frequency loadings, it can be seen that the material heats up quickly. This has an effect on the material properties and, consequently, the life under oscillating loads. In the previous project “Elasto-Opt“ (IGF project no. 400 ZN) a model made of thermo-viscoelastic material was initially developed. This model is bidirectionally coupled, and reproduces mechanical and thermal material behavior. With this model, the self-heating of the material can be calculated through loads depicted in a Finite-Element-Analysis, by which the mechanical behavior of the material can change depending on the predominant temperature.

The tools developed so far in the follow-up project need to be refined and prepared for practical use. The concept for temperature-dependent lifetime analysis proposed in ”Elasto-Opt" will be tested on other natural rubber based elastomer compounds and each module will be further modified if necessary. Finally, the possibilities and the limitations of the concept will be determined and presented, from which a recommendation for application will be developed.