Making tailpipe music

  • 18-Aug-2009 01:15 EDT
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In Eberspächer's active sound system, a speaker in each rear muffler actively influences tailpipe sound. According to the supplier, the new muffler generation is extremely compact and offers packaging benefits on top of flexible sound engineering.

Engineering the tailpipe sound of an automobile has a long tradition. To date, mechanical devices such as flaps and/or alternate pathways in the exhaust system have served to give a car its brand-specific sound.

Eberspächer, an exhaust-system and heating-solution supplier, has now demonstrated an electronic way of actively engineering the tailpipe sound. Named active sound system, this solution can muffle the tailpipe noise by up to 20 dB, can make a diesel sound like a sporty gasoline engine, and could give a tiny, downsized gasoline engine the beefy sound of a much bigger engine. Or, it can make sure that a nearly silent all-electric vehicle can be heard by pedestrians.

Antiphase sound

Instead of flaps or other mechanic devices, the new Eberspächer system is based on a speaker that is integrated in each rear muffler. Controlled by a dedicated electronic control unit (ECU), the speaker emits antiphase sound waves that damp, eliminate, or add individual frequencies and vibration orders in the exhaust gas flow.

Computing of the frequencies and the intensity of the antiphase sound is based on load and engine speed information provided by the engine ECU. The underlying principle is very old; it was first patented by Paul Lueg in Berlin in 1933.

“However, only very recently, microcontrollers began to offer the computational speed for the complex mathematical algorithms needed to calculate the antiphase sound waves,” explained Mathias Keck, head of Eberspächer’s exhaust technology R&D. That is why the supplier could only truly advance the systems since 2000 after an initial development phase in the 1990s.

Test phase concluded

Currently the system works with a Bosch ECU and uses the CAN (controller area network) bus for networking the active sound system and the engine control. “However, a FlexRay bus will offer ideal conditions,” said Dr. Jan Krüger, Eberspächer’s Director of Acoustics Exhaust Technology. “Given the computing power of the next engine ECU generation, we will only need 1-2% of the available processing power. So the active sound system’s control level could be fully integrated into the engine ECU.”

Full integration would make it easier for the OEM to offer the system as an optional feature. If the customer wants it, the system will be integrated in the reserved “slot” of the ECU.

While the active sound system can aid in the marketing of a car, it also has substantial implications on many technical and economic levels. Looking back at testing the system in various cars with four-, six-, and eight-cylinder engines since 2004, Eberspächer has found many potential benefits.

“Equipped with active sound, the rear muffler, for instance, can be reduced by up to 60% in size and may have up to 40% less weight,” according to Keck. More importantly, he added, “by adding the possibility to engineer the tailpipe sound via software, the exhaust system components could be standardized to a much higher amount than is possible today.” According to Keck, the resulting scale effects would be very tangible.

“With our active sound system, adapting an exhaust system to an individual car and engine will be a matter of resetting software-controlled parameters," Keck explained. "The current need for using different exhaust system hardware for every single powertrain configuration would be massively reduced. By eliminating complex exhaust gas flow paths, the exhaust backpressure can be lowered. We have found that the backpressure can be reduced by up to 150 mbar. If we add the weight and backpressure effects up, a car could save 3 g of CO2 emission per km just by using the active sound system."

This is why the system makes economic sense, Keck is convinced. Of course, a rear muffler with integrated speaker and the added electronic control is more expensive than a simple metal muffler. However, as Keck explained, “the bottom line makes sense on the vehicle level.”

Cultivating the diesel

“The tailpipe noise is an essential part of a car’s fingerprint,” Keck continued. Campbell diagrams reveal just how clearly different engine types are distinguishable.

Analyzing the original tailpipe sound is part of defining the approach to sound engineering. With diesel engines, the active sound system adds the missing dominant engine frequencies. In contrast, the system muffles overly dominant frequencies of the second and fourth order in four-cylinder gasoline engines and of the third order in six-cylinder gasoline engines.

Eberspächer have been working closely with several OEMs on the active sound system since 2005 and has equipped Audi demonstrator cars powered with diesel and gasoline engines.

The audible result during a technical briefing and test driving was convincing; yes, a diesel can sound like a gasoline V8. Combined with the diesel’s massive low-end torque, this might help to convince U.S. car buyers to consider buying a diesel, according to Krüger.

Currently a fleet of five test vehicles is in use at the supplier. “By 2010, the system will be ready for diesel applications; by 2011, the gasoline option will be readily available,” said Krüger. Eberspächer “is very close to signing a contract for a first diesel series application,” Keck added.

No more silent running

While sound engineering always has a psychological level to it in combustion-engine cars, it can be an element of safety in electric vehicles. Installed in the 2009 Rinspeed concept car iChange, the active sound system gives the vehicle an audible “engine” sound that will help make people aware of the approaching car.

Eberspächer is also in contact with OEMs seeking a solution for masking the sound difference between conventional engine operation and the all-electric driving mode of hybrid-electric vehicles.

“Again this can either be solved by muffling the combustion engine’s audible noise or by generating combustion engine noise during engine-off,” said Krüger. In both cases, the active sound system would be used to avoid unwanted feedback to the driver.

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