GM engineers strapped a six-speed automatic to a 2.4-liter direct-injection four-cylinder engine and equipped the vehicle with an eco mode that lowers the transmission's shift points while improving the gas mileage. However, when eco mode was activated, the powertrain offered up an audible hum.
To counter the buzz, engineers installed an active noise canceling system that relies on two headliner-mounted microphones to detect the hum and a frequency generator that emits an "antinoise" signal through the vehicle's speakers to cancel out the engine's boom. And that's how headphone technology, in a roundabout kinda way, cranked up the Terrain's fuel efficiency.
[Source: GM via Autopia]