Quiet please!

MAN´s acousticians like it best when it´s really quiet in their laboratory. Then they can accurately hear and analyze the fine nuances of the vehicle noises. A visit to the MAN Truck & Bus acoustics laboratory.


From a distance, muffled hammering and the whirring of electric screwdrivers faintly penetrate the building located at the Munich plant. While the hum of machinery and tools resounds inside the production buildings, it is quiet here. The acoustics laboratory operated by MAN Truck & Bus is not a room full of loud sounds, but a place of tranquility.

At first glance, the laboratory looks like a normal workroom with a conference table, filing cabinets and a desk. What make it different are the loudspeakers and the headphones with over-dimensional black ear pads attached to the computer. "The headphones are some of our most important tools," explains Stefan Heuer, new Head of the Acoustics Laboratory. Before joining MAN in September 2013, Mr. Heuer worked for an engineering service provider and was responsible for acoustics in various sectors, including, for instance, automobiles. Mr. Heuer now finds it challenging to be focusing solely on noises generated by trucks and buses. "Compared with cars, the dimensions are simply larger here and that impresses me. I´m looking forward to taking new steps in terms of our products´ acoustics," says Mr. Heuer.

Stefan Heuer
Stefan Heuer, Head of the Acoustics Laboratory at MAN Truck & Bus

Commercial vehicles have become noticeably quieter

It is the pauses between the tones that acousticians find most interesting. Then there is complete silence. Silence is something one does not immediately associate with commercial vehicles, but in the case of trucks and buses, a lot has been done to reduce noise levels over the past few decades. In this context, acousticians consider many different factors which influence exterior noise, namely engines and gears, intake and exhaust systems, cooling fans and also the road noise created by tires. As Mr. Heuer explains, trucks have become about eleven decibels quieter since 1970, which is a noticeable reduction. In technical terms, a decrease of even three decibels results in a fifty-percent reduction in noise. MAN´s commercial vehicles have always ranked among the quietest in the industry. However, apart from exterior noise, our acousticians also address interior noise. What do the window winders sound like? Maybe the windshield wipers squeak? Is the sound of the door slamming acceptable? And above all, how well does the cab cut out noise when the driver wants to take a break at a loud rest area? The acousticians´ aim is clear – the noises made by a vehicle must sound acceptable and agreeable, both inside and out.

New regulation calls for additional noise reduction

One of the many tasks facing MAN´s acousticians is ongoing improvement of the pass-by noise generated by trucks and buses. On the way to creating quieter vehicles, a great deal of patience is required on the part of manufacturers and acousticians. Since 1995, various committees have, for instance, been negotiating at a European level on the details to be incorporated into a new regulation on pass-by noise. It is expected that the ECE-R51.03 regulation will come into force in 2014. This will limit the exterior pass-by noise under acceleration on a European scale. The new method of measurement simulates "real-world urban driving". This means that measurements are taken under operating conditions which occur every day in every town, based on the fact that, especially in towns, many people are affected by the noise of daily traffic more than they are along highways for instance. This is why every effort is being made to achieve further optimization in this sector. While being measured, the vehicles are accelerated to 35 km/h to attain both a high engine speed and high engine load. This represents a speed that is normal in towns. MAN´s acousticians have already done their homework for this regulation. "Developing a measurement method to meet this new standard has taken five years to make sure that it reliably fulfills all the technical and statistical requirements," says Mr. Heuer.

MAN wind channel test

Acoustics can be optimized everywhere

In the face of new developments and new products, acousticians are never short of work. "There´s always optimization potential," confirms Mr. Heuer. For instance, one assignment is to determine target values for the sounds created by new vehicle parts or even by completely new vehicles. Or, in the course of interacting with the technical development team, it can be that undesirable noises have to be localized, reduced or eliminated, either in the case of new developments or vehicles already in series production. "First of all, we try to find out where to begin. Does the noise come from only one source or are we trying to come to grips with a number of sources that also interactively affect each other." In Mr. Heuer´s opinion, it is precisely this ability to think broadly and explore various approaches that makes a good acoustician. But what exactly are good acoustics? Ultimately, a definition is always subjective, but based on surveys and interviews, a common understanding can be reached. For instance, MAN Truck & Bus questions truck and bus drivers at highway service areas and bus stations on how they rate their cab insulation or which sounds, such as the clicking of the indicator, could in their opinion be optimized even further.

Changing, encapsulating, insulating

Mr. Heuer works closely together with other areas on a daily basis, yet sometimes, he and his colleagues have to firstly bring their terminology into line. "The design engineers determine whether or not a part holds. We say that something rattles, peeps or sounds inharmonic during a specific rotation. Everyone hears differently. First of all, we have to agree on what we´re talking about," explains Mr. Heuer. "Only then can we consider, together with the design engineers, whether a component should be made in a different way or changed, or whether it will have to be encapsulated or insulated."

Acousticians live with the fact that their results cannot be presented visually and are often scarcely audible. But this doesn´t bother Mr. Heuer. For him, it´s important that drivers arrive at their destination as relaxed as possible, thanks to the agreeable volume and tone of the noises on board, and that the pass-by noise level is as low as possible for third parties.

acoustic camera

To identify noises in engines, gears, exhaust systems or mufflers that are undesirable or too loud, Mr. Heuer and his colleagues make use of an acoustic camera. This can be used to measure exterior noise and, by assembling it differently, also to measure sounds inside vehicles.

acoustic camera

In this example, Mr. Heuer and his team are analyzing pass-by noise. In order to accurately determine the noise created by a vehicle, the acoustic camera has to be used in a quiet setting. This ensures that the sound waves reach the microphones, which are positioned at a distance from each other, within different periods of time and via different routes. All the data is immediately assimilated on a computer.

picture made by the acoustic camera

In this way, an acoustic colored picture of the vehicle is created, similar to that produced by a thermal imaging camera. The acousticians can see at once where the noise originates from, their picture showing particularly loud components in red, while areas generating a particularly low level of noise are colored blue.

Stefan Heuer

The acousticians working with Stefan Heuer are then able to analyze the results – and together with their colleagues, reduce the noise.

Share this page
share Share on Facebook
tweet Share on Twitter
Share on LinkedIn