How the Perfect Car Door Sound Is Made For BMW

BMW has partnered with Medium’s design hub, re:form, to explain how its design process works, and bring you an unrivaled insider’s look at the evolution of BMW’s iconic driving machines.

When acoustic engineer Robert Berens advised the city of Boston on construction noise abatement in the late 1970s, jackhammers topped officials’ list of complaints. The bane of city peace and quiet, jackhammers blast out 110 decibels from two meters away, just slightly less than the crack of your average thunderclap.

To Berens, the solution seemed simple: mufflers could be installed to reduce the machines’ noise. But it wasn’t quite so easy.

“The DPW guys down to a man thought those [muffled] jackhammers didn’t work right,” recalls Berens, now a principal consultant at Acentech in Cambridge, Mass.

They worked just fine, of course, but with the tooth-rattling noise reduced, the workers suspected the jackhammers’ power had been reduced, too. They’d lost the indicator they’d relied on to tell them everything was running as it should.

For most people, sound is more than a set of vibrations; it carries complex social and cultural signals. This point has not been lost on designers, and thanks to new digital tools and the insights of big data, our soundscapes are being crafted with extreme precision. Engineers can manipulate the sounds our products and gadgets make so that they do more than please our ears — they influence our perception.

Some of the most advanced uses of these techniques today are found in the high-end automotive industry, where engineers and designers now routinely lavish attention on every acoustic detail, from the sound of the engine to cabin noise to the squeak of the windshield wipers to the reassuring k-thunk of a car door closing — a subtle but remarkably influential sound when it comes to a potential customer’s decision to buy a new car, experts say.

“The customer may not really know this is important for him, but this is something that really affects his decision to buy a car,” says Florian Frank, Specialist for Noise, Vibration and Harshness who works on acoustical design for BMW and is responsible for perfecting the sound profile of new car designs.

“You can imagine that certain cars would have a sound that gives the customer the feeling of safety, of protection, of cocooning; other cars should give the customer the sensational quality of precision and control; and again other cars just want to say high value,” he adds.

For example, the new BMW 4 Series Gran Coupe is “a sporty car, so the door isn’t supposed to sound too heavy. It can’t sound too light, because a light door wouldn’t convey the right aspects of quality and safety. But it’s not supposed to sound too heavy, either. It should have a precise sound.”

By contrast, “the BMW 7 Series would be a bit softer maybe, a bit darker in the sound as it’s our flagship sedan.”

That’s the actual sound of a door closing on the new BWM 4 Series Gran Coupe. The moment the door connects with the frame, the metal parts of the latch collide creating a low but audible click. A thunk follows more clearly as the dampers and seals compress, decelerating the door nearly instantly as it locks in place, deadening the sound at once to create a crisp and precise effect.

That’s not how it sounded when the first BMW 4 Series Gran Coupe prototype showed up for acoustic testing at the BMW design facility in Munich, Germany.

Listen closely. Can you hear the difference?

In a battery of audiophile-quality recordings, BMW’s acoustic engineers did. They detected a slight imbalance between low and high frequencies, resulting in a “thin” or “light” sound. That was a problem, because consumers subconsciously tend to interpret imbalance as structural weakness, not protective strength.

You can actually see the error in an audio fingerprint BMW made that visually captures the sound signature of the door.

Notice the lack of symmetry between the heavy presence of red in the top left part of the graph, and the very thin streaks of turquoise in the bottom right.

“Let me compare the whole thing with an orchestra,” says Alexander Ziemann, Head of BMW Aeroacoustics, Function Generated Acoustics and Watertightness. “When you want to design a sound you have to conduct a lot of instruments that play their role. And we have a lot of instruments in this orchestra starting from the latch over the hinges, some buffers, the metal panels, and all the door seals.

“In the BMW 4 Series Gran Coupe, especially, we had a task, because more people are sitting in the back, and we have four doors without a frame around the window. So the big task was to make the system from outside and from inside sound good enough.”

They began as usual, optimizing the dampening material on the side panels and some smaller things on the latch. When that didn’t work, they had to dig deeper.

“It turned out the lack of an upper frame over the window created complications that were ultimately addressed by optimizing the dampening of the roof — it is very unusual to have to take that kind of step,” he added.

The corrected audio fingerprint looks like this:

In this case the red in the upper left is more evenly matched by the ratio of green in the lower right portion of the graph.

Imbalance is one of a several common acoustical problems in door designs that engineers seek to find and eliminate. Others include the presence of “high-frequent impulses,” which sound like something has come loose inside the door.

Another common error comes from “narrow-banded tones that last longer than the main impulse,” which make a door sound hollow.

Although most of this design work is done using computer simulations, acoustical engineers at BMW might get up to three prototypes to test on the way to the finished product. With these they might make up to 100 separate highly controlled recordings of a specific feature to isolate various sound qualities and repair imperfections.

Equally important to the physical challenge of building a specific and well-defined sound is deciding what sound to make.

BMW has been perfecting its acoustic engineering techniques for the past 15 years, understanding what effects various choices in panels and dampeners have on the final sound of the car.

More recently, it has begun to do intensive market research with consumers to zero in on what sounds its customers consider most appealing. In one study, researchers interviewed over 800 subjects, collecting feedback on dozens of sounds, each one captured as a visual audio fingerprint and organized into a database with the various responses. Using this database, designers can customize different sounds for different classes of cars, and in the future, perhaps different markets.

Such intense focus on sound design is rapidly expanding into new territory as our tools for predicting acoustical effects improves. Acentech in Cambridge, Mass. is just one of a growing number of acoustic design firms seeking to reimagine noisy products, from washing machines to vacuums, and for some the ambitions are reaching even further.

Rudolph Stefanich, an industrial designer in Shanghai, earlier this year announced the Sono, a gadget that might one day allow us to customize indoor soundscapes by amplifying or dampening the noises that filter inside. As Megan Garber reported earlier this year in The Atlantic, with Sono you can ostensibly “amplify the songs of birds outside. And you can drown out the noise of the traffic on the street below.”

Futuristic stuff, perhaps. Sono is still in the concept phase. But such leaps of imagination are not complete fantasies. They’re grounded in real world applications that have improved by orders of magnitude since the arrival of digital acoustic analysis techniques some three decades ago.

“Before computers you used to take a Nagra recorder out in the field. Every reporter who ever did enthomusicology in Africa used one,” cracks Berens. “It was analog and we had sound level meters and analyzers and we could see spectrum. But with computers we could suddenly do digital analysis and an awful lot of math and figure out why things actually behave the way they do.”

As we deepen our understanding of how people respond to sound, our newfound acoustic powers are quietly but profoundly altering our relationship to the worlds we inhabit, and create.