Kara Gordon and an aachen head. Image Courtesy GM

How GM makes a car sound like what a car is supposed to sound like

Every car has a soundscape—even noise is designed.

During the car sales pitch, there is a moment when you first get behind the wheel. You hit the ignition, the engine turns over, and the car comes to life. You are bathed in the sound of countless mechanical and electrical parts as they spin, twist, push, pull, and rub against one another. It’s Kara Gordon’s job to make sure you like what you hear.

Gordon is a noise and vibration performance engineer with GM. She’s been doing this since 1999 and is part of the team that designed and tested the soundscape of the new Chevy Impala. GM is pushing the luxury car as their quietest full-size sedan, ever. While they’ve clearly put a great deal of effort into isolating and sealing the cabin, what’s more interesting is the effort they put into the sounds you do hear.

There may have been a time when the noises a car made were simply a byproduct of its mechanical function, but that time is long past. Like every other part of a contemporary car, the audio experience is carefully designed and tweaked.

It’s not enough for the car to run well, it needs to sound like it’s running well too. Sound is part of the user experience, and it’s also a critical part of the user interface.

It takes a surprisingly large team of people and, throughout most of the period between drawings and full production (this takes a couple years), those engineers will work exclusively on a single car. There is a group that worries only about what noises the engines make. Another person works on accessories. Alternators, says Gordon, are notoriously whiny. Then another person to handle HVAC, another for all the chassis work, and so on. Gordon works on isolating the car using sound insulating materials and customer actuated sound. Two other engineers work on sealing, and squeak & rattle; annoying sounds of no particular origin. “It could be anything from the suspension to plastic panels touching,” she says.

“My joke to my mother is I drive around and listen to things,” says Gordon. She spends a great deal of her time driving prototype vehicles in all sorts of conditions, listening for strange sounds. She is often accompanied by a specialized recording device called an Aachen HEAD which is modelled after a human head, with microphones in place of ears. Back at the lab, engineers can put on headphones and experience what the Aachen HEAD heard, using EQ filters to locate and isolate noises to figure out where they’re coming from.

“I don’t just care about loudness, I care what it sounds like,” says Gordon. “Like a whistle in a crowd, a tone can be 10db below the noise level and still be really audible.” For car owners, an unexpected noise is often the first sign that it’s time to visit a mechanic; it wouldn’t do to roll off the assembly line with an annoying rattle.

“We worry about sounds that sound unpleasant or like it’s breaking,” says Gordon. This is especially true for noises originating from ‘user-actuated events’, moments when the car makes a sound because of something you did, like clicking buttons, moving levers, or opening and closing doors. “The Impala has a wonderful [dashboard] display that moves out of the way,” she says. “We spent a good deal of time tuning that noise to sound just right.”

‘Just right’ changes from model to model. A sports car might have less insulation between the engine and the driver so they can hear and feel the rumble. A family sedan might be more concerned with making sure the passengers can hear each other and the entertainment system.

In the case of the Impala, it’s a luxury car, so GM was going for ‘comfort’ and best-in-class quietness. That said, they didn’t want to completely silence the engine. “We want you to still know that you’ve got a v6 underneath the hood,” says Gordon. “We want people to say, ‘Wow that engine sounds powerful but far away, and it sounds like it’s doing its job like it’s supposed to.’”

Strangely, to give people the impression that the Impala’s engine was working as intended, GM had to partially mask its real sound.

In general, to improve fuel economy in a car, you want to reduce the engine’s RPM. Over the past few decades, the auto industry has been doing that. In the 90s, says Gordon, a 4c engine might be cruising at 3,400 RPM. Today, it’s below 2,000.

But as you reduce the speed that the drive shaft is rotating, you lower the frequency of the sound it’s making. There comes a lower limit where the engine is making what Gordon calls “groan-y and moan-y” noises which people find unpleasant. The car sounds broken. So cars had to keep the engine’s RPM above a certain level, hurting their fuel efficiency, or risk alienating customers.

GM’s solution was to implement active noise cancellation, the same technique used in some headphones to quiet ambient noise. Microphones in the body listen to the ambient sounds the car and engine are making, and the car plays the opposite of that over the vehicle’s speakers. The sound waves from the engine are cancelled out by the sound from the entertainment system, netting a quieter ride that can be more fuel efficient without being so bothersome.

Some sounds in the car are completely artificial. The telltale clicking of a turn signal was once an artifact of the mechanical process that turned the light on and off. But that mechanism has long since been replaced by an electronic circuit that operates silently. Still, audible feedback is valuable so the car plays an MP3 file of a turn signal over the speakers.

“It could sound like anything,” says Gordon. “We asked, ‘What if we wanted it to sound like birds?’ They said no.”

These questions will become more important in the next generation of cars as more switch to electric motors, which are nearly completely silent. The sound of an approaching car’s engine is an important safety feature for nearby pedestrians, especially people with low vision. Future cars will have to make some kind of noise. What that sound will be is uncertain, says Gordon. “There are regulations coming to talk about making the electric cars make noise… We don’t know what the government will make it sound like. Will it be an annoying tone? Will it have to sound like a car revving? It’s fun to think about the possibilities.”

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