Road noise kills

Australians still travel long distances by car, in contrast with drivers in other parts of the world. Our love of motoring and the freedom of the open road often comes at a cost though – sometimes life itself.

We often speak of 'white-line fever', but according to a noise, vibration and harshness specialist at Mercedes-Benz, it's the constant, unrelenting hum from the wheels of the car that's most likely to send you to sleep on a long trip.

And fatigue associated with driving long distances in a short space of time remains a significant contributing factor in the road toll, which is why state and federal governments give practically the same precedence to fatigue as they do alcohol and speed – at least they do for truck drivers.

Jorg-Stefan Möller, speaking with Australian journalists in Germany last week about Benz's new S-Class, explained that road noise generated by tyres on the sort of coarse-chip bitumen frequently laid on Australian country roads can affect driver's attention levels and focus in two ways.

"Our internal customer research centre... studied which things make us feel discomfort – which makes you tire. There are two main things [that] they found out: low-frequency noises in the car...  make the passengers tired over several hours.

"And the second thing is conversation – the ability to understand each other in conversations. We call it the quality of articulation. That's more the high-frequency [range]."

Herr Möller clarified how the quality of articulation can dull our situational awareness. It's not the act of shouting to be heard above the roar of the engine (or more often these days, the tyre noise); it's the protracted brain drain listening to passengers and ensuring you understand every word they articulate.

"If you cannot understand the passengers very well," he explained, "you see a loss of articulation; it's making you tired, because you have too much to concentrate on. Therefore we want to improve the quality of articulation – which is a high-frequency domain."

According to Möller, testing by Benz with participants monitored for heart rate and pulse revealed that the low-frequency droning from the tyres had a profound effect on drivers' reaction times after a while.

"They identified very clearly the low-frequency noises in the car makes people tired. They can't react very quickly after hours of driving,” he said.

Measures adopted for the next-gen S-Class to dampen and diminish noise encompass changes to the structure and underpinnings of the new model. Inside the new car noise has been reduced by as much as 2kHz, it's claimed.

Möller explained that he and his development team have focused on fatiguing noise across three frequency ranges: low frequency up to 400 Herz, mid-range frequency between 400 and 1000 Herz (1kHz), and high frequency from 1kHz up. Tyre noise, irrespective of road speed, tends to be around 200Hz.

Möller actually classifies different strata of frequencies by different names; so 'booming' takes place at 30Hz and 'droning' takes place between 150 and 200Hz.

Booming is also a consequence of car companies' obsession with saving fuel, with the automatic transmissions of some diesel-engined cars dropping into a higher gear and forcing the engine to run as low as 1000rpm. The torque is sufficient to maintain the car's momentum, but at such lower engine speeds the booming noise is quite apparent.

Möller offers two solutions for car company engineers: isolate the vibrating components of the drivetrain from the car body, with an insulating axle carrier for instance, or to refine the drivetrain itself.

Low frequency noise is typically "structure-borne" noise, whereas higher-frequency noise is "airborne". On that basis wind-generated noise can enervate the driver also. That is a lesser cause than striving for optimal fuel efficiency, but it is nonetheless another reason the next S-Class will be more aerodynamic than the current E-Class – and expected by the manufacturer to be class-leading.

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