Daimler Trucks and the Search for the Right Acoustics

In legislation or when establishing norms and regulations, threshold values help immensely because they provide measurable quantities that must be adhered to, they serve as a means of orientation for everyone involved. There is an abundance of these rules for manufacturers of commercial vehicles. They specify what sort of engine emissions are allowed, for example, and in what quantities. Or they indicate how heavy, how long or how loud a vehicle can be.

Compliance with noise thresholds, in particular, is as important as ever. To make sure their trucks and buses fulfil these requirements, the acoustics experts at Daimler Trucks routinely go sleuthing in the world of sound waves. At the new, ISO-certified acoustics measurement site for commercial vehicles in Münsingen, they check whether new models or new engine and exhaust variants of Actros vehicles and other Daimler brands satisfy the noise emission limits prescribed by law. And for that, one thing is especially important: silence.

Testing in the tranquil Jura Mountains

A light breeze flows through the yellow autumn leaves in the treetops at a former military training area of the Bundeswehr. The only thing that interrupts the silence is the occasional bleating of sheep, of which there are roughly 25,000 grazing here and there at this almost 6,500-hectare site in the Swabian Jura region. “The background noise here is about 35 to 40 dB(A), so it’s generally quiet enough for measurements of electric trucks,” remarks Kamal Idrisi, team leader of the Exterior Noises unit at Daimler Trucks, as the last test drives of the year 2020 are completed.

These are perfect conditions for the Daimler engineers. At the moment. Soon, though, the weather will be too wet, cold and changeable. “At temperatures below five degrees Celsius, acoustic measurements aren’t allowed, and we’re almost at that point here at the testing site, which is about 750 meters above sea level,” says Uwe Bickel. A workshop supervisor at Daimler Trucks, Bickel monitors the special measuring systems on the trucks when they make their rounds on the recently opened acoustic measurement track.

The new site consists of an approximately 500-meter-long, partially heatable asphalt road with two turning circles, a further 250 meters of run-up track, a parking area and a measuring station. The asphalt is certified according to ISO 10844 with a quality check occurring every two years. In ducts below the road, there are kilometres of connection cables for links to weather stations, photoelectric sensors, microphones and other measuring components. “This is a big advantage compared to our previous track, where we always had to set up all the equipment again before any test run. Here, everything is pre-installed. And with the heating system for the road, we can always perform our measurements on a dry surface that satisfies the legal specifications,” says Idrisi, who worked for Daimler Trucks in China for many years and helped set up a test centre there at the joint venture partner Beijing Foton Daimler Automotive (BFDA).

Even quieter in phase three

In the approval tests for combustion-engine vehicles according to UN/ECE 51.03, two highly sensitive microphones are positioned at a distance of seven and a half meters to the left and right of the driving track in the middle of the measurement area. At these points, the measured sound must not exceed the maximum permissible value, a quantity that varies according to the vehicle model and engine power. The ECE standard that must be adhered to has been harmonized worldwide for the most part – there are less stringent limits in isolated cases. The standard has been in effect since 2016 and is divided into three phases: In the current phase, phase two, heavy-duty vehicles rated at over 250 kW must not be louder than 81 decibels when they pass by. For smaller trucks rated up to 135 kW, the threshold is currently 75 dB(A). In phase three, which is scheduled to begin in 2026, no more than 79 or 74 dB(A) would be allowed. “If these limits are not adhered to in the inspection by certification authority TÜV, then we are not allowed to sell the truck,” says Idrisi, spelling out the business repercussions.

In accordance with the ECE standard, the engineers measure an “accelerated passing”. A loaded truck – simulated in tests with a weight on the rear axle – is driven at constant engine and vehicle speed toward the measurement zone. Starting at a certain point, the trained driver accelerates for 20 meters plus one vehicle length. When it leaves the measurement zone, the truck must achieve the legally required speed of approximately 35 km/h. “The manoeuvre isn’t the easiest thing in the world. When they’re going in, the drivers have to be able to accurately estimate what their speed will be when they exit,” says Bickel.

The test scenario is supposed to represent an ‘urban noise setting’, where pedestrians and residents are exposed to many sources of noise. In addition to the accelerated passing, engineers also measure other features, such as the noise generated by the engine brake or the pneumatic system. All of the TÜV inspections take place under the open sky in Münsingen.

Measuring station becomes listening booth

Loudspeakers are used to transmit the vehicle noise into the measuring station, which has a front window at a flat angle to the measuring track to minimize any potential interference from reflected sounds. This is also a requirement of the standard: No reflections are allowed within a 50-meter radius of the microphones. Before the data undergo a digital analysis, the mechanical engineer Christian Richter first pays close attention to the sounds at the measurement station, which serves as a sort of listening booth. “This is a job that trains your sense of hearing. If there’s a rattling sound somewhere, I usually already have some idea where the source of the noise could be. Or you can tell based on the frequencies. A structure-borne noise from a mechanical connection sounds different from a resonance or a flow-generated noise.”

On board in every phase of a project

Just before a TÜV inspection, the music heard in the ‘listening booth’ consists of nothing more than soft and subtle intermediate tones. But the measurement site in Münsingen is also used when the orchestra is still rehearsing. Each vehicle is monitored by acoustics engineers in the course of its development. “We’re on board during every project phase, checking whether each stage of development satisfies the regulatory requirements,” says Idrisi, who earned a doctorate studying interior noises in aircraft for the U.S. space agency NASA.

If a vehicle exceeds a limit during the tests, engineers use techniques like frequency analyses, sound location or enclosures of secondary noise sources. “This way, we can filter out whether the drive train, for example, is the source of the noise,” says Idrisi. The results are then passed on to the relevant development unit, which works with the sound engineers to develop possible solutions.

Visualizing sounds

And even after the vehicle models have been approved by TÜV, the trucks still have to be tested periodically on the acoustics measurement track. They are tested again, for example, when new engine variants or exhaust systems are introduced, or when they undergo styling changes. Even changes in control software can lead to different sound values. “In these cases, we usually test with models that are likely to be closest to relevant thresholds, such as a semi-trailer tractor with a short wheel base and short driver’s cabin, which has a greater tendency to be problematic acoustically than the standard model,” says Bickel.

When trying to locate sources of sound, engineers use what is called an ‘acoustic array’ instead of only two microphones. The array is a sound location system with 42 microphones arranged in a semicircle. When a truck drives past the array, software translates the signals of the individual microphones into a digital 3D model. To the layman, the result is a complex picture, but sound experts can use it to quickly pinpoint the location of a noise. “With the sound location system, we can track down noise with great precision,” says Christian Richter.

Louder!

When it comes to electric vehicles, engineers have recently been faced with the opposite problem. In the case of electric trucks, the important thing is to make sure that these vehicles are loud enough to be noticed in time by pedestrians and other road users. Electric vehicles must therefore make more than a certain minimum level of noise. The thresholds are specified in UN/ECE Regulation 138 on “Quiet Road Transport Vehicles”: At 10 km/h, the minimum is 53 decibels, and at 20 km/h, vehicles must reach 59(A) decibels. Higher speeds are not correlated with minimum thresholds, because tire and wind noises are dominant at such speeds. In addition, engineers measure the sound of an electric vehicle moving in reverse at 6 km/h. In this case, at least 50 dB(A) is required. “At the TÜV inspections for electric vehicles, the microphones are located only two meters away from the driving lane. It’s not all that easy to drive through them in reverse,” says workshop supervisor Bickel. Furthermore, these vehicles are measured at constant speeds and not under acceleration, because in the case of electric trucks, the focus is not on noise pollution but on road safety.

If an electric vehicle fails to reach the values specified in the regulation, it must be equipped with an acoustic vehicle alerting system, or AVAS. The regulation specifies in great detail what an AVAS should sound like, and what it should not sound like. This applies, for instance, to the minimum and maximum volume of sounds, as well as to certain sound components.

“The acoustics of our vehicles are hugely important for how they’re perceived,” says Kamal Idrisi, referring to both the exterior and interior noises. “This is often underestimated, but a truck driver who is sitting in a quiet cabin feels better, experiences less stress, is more relaxed – and drives more safely.”