Quiet revolution under the hood

One of the primary considerations among car buyers these days is fuel consumption, and that generally means smaller engines.

One of the primary considerations among car buyers these days is fuel consumption, and that generally means smaller engines. That can lead to another problem: people also want quiet cars, but four-cylinder engines can be noisy. General Motors has tackled the problem with its new Active Noise Cancellation, or ANC.

Standard on the 2010 four-cylinder Chevrolet Equinox and GMC Terrain, it uses technology similar to that of noise-cancelling headphones, sending out a sound wave to counteract engine noise.

“It calculates (engine) frequencies and there are two microphones in the headliner, one over the driver and the second in the rear seat,” says Jim Vallance, noise & vibration development engineer. “It knows the wave form, what its phase is, and it inverts it and sends out a mirror image of the wave. They cancel each other out.”

Engine speed is measured in revolutions per minute, or r.p.m., which is how fast the crankshaft is spinning. A lower r.p.m. means better fuel economy, but it’s not always pleasant to hear.

“There’s an ‘eco’ button that takes the r.p.m.s down to 1,125 from 1,350,” Vallance says. “That’s an area that’s just nasty for noise and vibration. It sounds like a manual car trying to take off in fifth gear, and the boom level resulting from that was overwhelming. We couldn’t put enough damper or any type of noise treatment on the vehicle beyond what we had that could solve the problem. We tried all the traditional things first, before we went to this.”

The sound waves come out of the front door stereo speakers and the rear subwoofer. The system has separate circuitry in the amplifier, so even if the stereo’s off or the music’s cranked, the ANC still works.

­“It sends out a wave and then uses the microphones as feedback to see what happened when it did that,” Vallance says.

“It happens very quickly, in less than three-tenths of a second. Very rarely do you sit at one r.p.m. when you’re driving. You’re constantly changing r.p.m., so the target frequency is constantly changing. It’s always working, always keeping up with the r.p.m. and cancelling it out, no matter where you’re sitting.”

Along with ANC, the Equinox also contains traditional sound-deadening technology, including laminated glass and absorbent material in the dash, making the vehicle exceptionally quiet. “ANC is transparent to the driver,” Vallance says. “You don’t turn it on or off. It’s something that allows us to get the fuel economy numbers, but the driver is still comfortable.”