Striking the right note

“Acoustic research in the Rhine-Main region will be able to take a real leap forward thanks to our new laboratories”, says Tobias Melz, Professor of System Reliability, Adaptive Structures, and Machine Acoustics (SAM) at TU Darmstadt and Head of the Fraunhofer Institute for Structural Durability and System Reliability (LBF). It will now be possible for experimental analyses of individual components and complete machines, including vehicles, to be carried out in future at the Lichtwiese Campus. In cooperation with partners from industry and research, the researchers want to develop new processes and methods to understand better the mechanisms of sound generation and propagation so that they can specifically influence machine acoustics.

In the automotive industry, for example, despite advances in car acoustic design, it is still challenging to control precisely the acoustic properties of such complex mechanical engineering systems during early product development process. If irritating sounds are identified on a product prototype, it will require expensive and time-consuming improvements to the design. At the same time, a brand-specific acoustic experience that drivers are already familiar with is highly desirable and also a crucial factor in their purchasing decision. In order to satisfy these contrasting requirements: “our work focuses on a design for acoustics at the earliest possible stage, beginning in the best case scenario directly with the source of the noise”, says Christian Adams, a researcher in the SAM Group.

Three physical variables are important for influencing noise: mass, stiffness, and damping. “To design vehicle components based on these variables, we need a versatile set of methods”, says Adams. This is because it is not only important that the components in the vehicle are correctly designed but also that we pay attention to all of the interactions between the different materials and production processes. This is relevant, for example, in future electric mobility systems. An electric motor is quieter than a combustion engine so that the motor will no longer mask undesired noise. Electric vehicles also have to be as light as possible to increase their range. However, lightweight designs and low-vibration solutions with an optimal acoustic design are often not compatible with one another. “This needs to be understood and anticipated at an early stage”, says Melz.

Innovative methods and processes are thus required to enable early stage design for acoustics. In the new acoustic laboratories: “We work in a completely noise-decoupled environment, which allows us to focus on those sounds that are relevant for a particular experiment and block out all others”, explains Adams.

The researchers initially focus on structure-borne sound. When a car starts up, the engine generates vibrations that propagate throughout the structure of the vehicle as structure-borne sound. At the surface of the structure, the structure-borne sound of the engine radiates into the interior of the vehicle, for example, as airborne sound, which can be perceived by passengers. The trick is to ensure that as little noise as possible radiates into the interior of the vehicle, while also traffic noise does not pollute the environment outside.

The team at TU Darmstadt has identified two effective levers for correctly balancing the structure-borne and airborne sound. One of these levers is the structural intensity – a measure that shows how the structure-borne sound energy propagates throughout the structure. The researchers want to understand exactly the paths taken by structure-borne sound energy so that they can control it in a targeted manner.

The second lever is so-called “low-tone gearing” of transmission mechanisms. The teeth of gears are irregularly arranged along the circumference. This is an innovation in engineering that should ensure that structure-borne sound is evenly distributed over many frequencies so that unpleasant, individual tones become less prominent.