Silent noise

How should an electric vehicle sound? We get the answers from the sound engineers for the e-Golf.

Text Christian Buck Photos Kerim Delikan

Obviously, a car is coming. Or is it?

Somehow, the car’s sound resembles an engine — yet it also sounds like something entirely different. That’s many people’s reaction after hearing the e-Golf 1 for the first time. They immediately recognize that a car is heading toward them, but they also notice that it can’t be any ordinary vehicle.

The creator of this dual impression is Gordan Matkovic of Volkswagen AG and his colleagues from vehicle development. Several years ago they tackled something that at first glance may seem rather paradoxical: to produce a distinctive silent sound. To be more precise, they began to create a virtually silent car sound for the e-Golf. Under its hood, an electric motor revs but is virtually silent. It really is fascinating. While this could be good news for city dwellers, it could also cause a problem. “Electric vehicles are so quiet that pedestrians often don’t hear them coming,” explains Matkovic. “This could be dangerous in traffic.”

Gordan Matkovic and his colleagues create the noises for the e-Golf. Using a variety of tests, they determined how the silent engine ought to sound.
Gordan Matkovic and his colleagues create the noises for the e-Golf. Using a variety of tests, they determined how the silent engine ought to sound.

Especially for the visually impaired, the well-known engine sound is an important guide as they walk through town. Interest groups pointed out the problem with quiet electric vehicles soon after they were introduced. The U.S. has already reacted: The National Highway Traffic Safety Administration (NHTSA) has proposed that, by 2018, all electric cars must be acoustically noticeable. Volkswagen also recognized the problem and took initiative to begin working on sound effects for electric vehicles.

But what form should the silent noise take? This is where Gordan Matkovic and his colleagues got involved. Together with representatives from Volkswagen AG Development, Distribution and Board of Directors, they searched for the perfect sound for the e-Golf. Thanks to modern technology, they could have given the car practically any conceivable sound — that of a horse and carriage, a helicopter, or a circular saw.

This made the initial discussions quite lively. “Of course tastes differ, and that’s why there was a wide range of suggestions in the beginning,” says Matkovic. “Some people wanted the classic sound of a combustion engine, others were in favor of an approaching metro train, and others wanted something completely new.” What they easily agreed on, though, was that the e-Golf shouldn’t announce its presence by anything resembling a ringtone or the beeping of a reversing truck. These sounds were simply too annoying.

The e-Golf should always sound like an e-Golf and not like anything else.

If possible, the sound system in the engine compartment should be subtly audible inside the car.
If possible, the sound system in the engine compartment should be subtly audible inside the car.

After various tests, they reached an acoustic compromise: The e-Golf should sound like a cross between a combustion engine and an electric machine. “The sound of a classic engine is pretty well known and also still fairly audible in a loud environment,” explains Matkovic. “The electric part makes us stand out and causes people to realize this is a new technology.” Because it would take some getting used to, they decided against a purely electric sound. The human brain recognizes the familiar engine sound immediately because we have internalized it since childhood as a warning signal in traffic. “There’s no reason not to gradually increase the electric sound in the future once we have become more accustomed to the new sound of electric mobility,” he says.

Once the basic decision was made, the acoustic experts went back to the studio and recorded both sounds. For the electric machine sound, they filtered out unpleasant whistling sounds and added deep frequencies. The result is a rich sound with an even frequency that resembles a metro train leaving the station. The sound of the combustion engine was developed using a four-cylinder engine, but the acoustic experts eliminated the sounds of the generator and belt drive. “Of course, we could also have used a 16-cylinder engine,” says Matkovic. “But that wouldn’t have matched the car — the e-Golf isn’t supposed to sound like a sports car.”

The e-Golf sound works thanks to Volker Wehrmeyer and his colleagues from the electronics development. They developed the acoustic system that brings the synthetic sound to life. Their new control device resembles a miniature stereo system. A small aluminium case holds an amplifier and houses the memory for the sound file and a computer responsible for accurate playback. The control device for the system is installed in the radio shaft behind the display. The developers are particularly proud of the loudspeaker, which works in the already full engine compartment. “We had to generate a good sound with compact casing,” reports Wehrmeyer. “In addition, the loudspeaker has to be very robust. After all, it is exposed to moisture and high temperatures.” Together with the Belgian hi-fi specialist D+M®, they designed a loudspeaker with a water-repellent membrane and magnet that can withstand temperatures up to 248 degrees Fahrenheit. Its heat-resistant plastic casing is also custom made and fits perfectly in the small space behind the radiator grille on the right side behind the fog light.

To optimize the driving noise of the e-Golf, the experts performed extensive tests in an acoustics hall.

Exactly how loud the car sound is depends on the car’s speed. When the car is still, it’s silent. As the car moves faster, it becomes more audible. In addition to the volume, the electronics also vary the speed of playback: the faster the e-Golf is moving, the higher the pitch.

The technology is capable of more. For example, it would be no problem to give the car a different sound to suit the occasion or mood. “But this is not an option for us,” says Wehrmeyer. “We don’t want to orchestrate anything with the sound. The e-Golf should always sound like an e-Golf and not like anything else.”