Quiet and low-vibration machines or vehicles make for a pleasant work atmosphere or convey a feeling of comfort and luxury. Plus, they contribute to safety because they reduce distractions. In addition, vibrations that often accompany noise can considerably impair the service life of products and, for example, lead to significant costs during the operation of production machines. Consequently, there are good reasons why manufacturers invest a major effort into continually optimizing their products in terms of acoustics and vibrations.

At automotive and industrial supplier Schaeffler, more than 200 people work in the field of NVH. This abbreviation stands for noise and vibration, and harshness as their subjective perception.

NVH measurements are complex and require special conditions. That’s why Schaeffler has integrated into its development processes testing and analysis methods using high-tech acoustic laboratories that are some of the quietest places in the world – providing the basis for delivering precise and reproducible results.

For their complex NVH tests, acoustics specialists at Schaeffler can draw on more than a dozen of these anechoic rooms worldwide, five of them at Schaeffler’s headquarters in Herzogenaurach. How quiet are these facilities in reality? Can a dropping pin be heard there? “The dropping of the pin probably not, but the impact can surely be heard,” says Professor Pecher, who heads the research and development department at Schaeffler’s Competence Center NVH and Chassis System Analysis, with a smile. “And that doesn’t even require our sensitive sensors.” People standing in such an anechoic room can even hear their blood rushing through their bodies.

The expert

© Thomas Welker/Schaeffler

Schaeffler’s employee Prof. Dr.-Ing. Alfred Pecher is an acknowledged expert in the field of Noise, Vibration and Harshness and was instrumental in establishing Schaeffler’s competence center for Acoustics in Herzogenaurach. In addition, he’s an adjunct professor at the Department of Mechanical Engineering at Technische Hochschule Würzburg-Schweinfurt and advisory board chairman of Fraunhofer Institute for Digital Media Technology IDMT.

Talking about sensitivity: While the tests are taking place, disturbing noise is off limits! It must be possible to turn off the fans on computers during the measurements and wrist watches may not be worn. Even normal light sources cause vibrations that we humans cannot hear but that the sensitive measuring technology would record.

High technical complexity

To make acoustic rooms like those at ­Schaeffler nearly anechoic entails efforts of high technical complexity. Specialty absorbers consisting of open-pore foam materials on ceilings and walls absorb the sound waves occurring there. The entire room structure, i.e., the cast floor as well as the cast ceiling and the brick walls are made of high-density material to prevent structural transmissions of vibrations. Consequently, even external vibrations cannot distort the measurements.

To investigate the acoustics of products in motion such as bearings, transmissions, or hybrid models, the drive units as potential noise sources are located in another sound-insulated room and connected to the component under test via a shaft.

Isolated from the rest of the building

Such isolation is relevant also for the second measurement domain, vibrations. To avoid solid­borne noise, some of the measuring rooms are isolated from the building structure. They have no hard contact with the foundation of the building and rest on springs that are impermeable to the relevant frequencies. The walls and ceilings of rooms like that are isolated from the other parts of the building as well, by a large air gap. “That’s how we prevent solid-borne noise entering or the transmission of vibrations from the outside into the room, which would considerably distort the measurements and, in the worst case, make them useless for analysis and evaluation,” says Professor Pecher.

Precision NVH measurements mean that even the minutest sounds and vibrations can be recorded and analyzed. The accuracy of such measurements can be in the range of micro-Pascal (µPa) for sound levels and micro-G (µg) for vibrations. That enables the beginning of crack formations in materials to be proven. Professor Pecher and his team measure various parameters including vibration levels (air-borne and solid-borne noise), frequency spectrums, and psychoacoustic parameters. Highly sensitive microphones capture even softest sounds and those whose frequencies the human ear cannot hear.

A look at the recorded frequency range enables the acoustics specialists to dissect the tone of a noise. What frequencies are dominant, which ones can be ignored? That’s important for developing targeted noise reduction measures but Pecher and his colleagues are more than noise reducers. They are – if you will – noise composers as well. “We’re not focused strictly on the volume of noises but also on their quality,” says the expert. Low-pitched sounds, for instance, are more pleasant to human hearing than high-pitched or treble sounds.

Making sound and vibrations visible

While highly sensitive microphones prick up their ears to pick up sounds Schaeffler’s NVH pros use so-called laser Doppler vibrometers (LDVs) to detect and analyze vibrations. These instruments are extremely sensitive as well, enabling resolutions of nanometer amplitudes, i.e., vibrations whose dimensions not even match the diameter of the coronavirus. Another advantage is the fact that laser light as a sensor does not affect the object under measurement, i.e., it is non-reactive, so enabling measurements on smallest and lightest-weight structures. As written above: those instruments are extremely sensitive.

Schaeffler’s experts make sources of noise visible as well, using an acoustic camera that shows where noises come from. NVH engineers use this technology for systematically identifying sources of noise.

Expert know-how is key, AI helps

The best equipment and technology are useless, though, if the setup and performance of tests and the subsequent analysis of frequency spectrums, sound levels, and vibrations are flawed. Professor Pecher knows “that performing and analyzing acoustic measurements calls for an in-depth understanding of the physics and technology of sound and vibrations.”

In addition to their own know-how, Pecher and his colleagues increasingly use artificial intelligence (AI). AI algorithms can analyze large data volumes, recognize patterns, and make predictions with greater speed and precision than any NVH pro, no matter how highly trained they are – to make the world of Professor Pecher and his team even quieter.