Active noise cancellation in autos-as postulated in Stone Song

"The use of active noise cancellation for fuel economy benefit on Terrain is among the first at GM," said Paul Beaker, program engineering manager for GMC Terrain. "It has strong potential for implementation on other four-cylinder vehicle programs."

When GM engineers set out to deliver segment-leading fuel economy on Terrain they chose to lower the 6-speed transmission's gear shift points to enable the Ecotec 2.4L four-cylinder engine to run at lower rpm torque. In this "Eco" mode, which the driver can activate with a click of a button on the console, the torque converter clutch engages at lower engine speeds to help save gas. While the engineering action improved fuel efficiency by up to one mpg, it also created an objectionable low-end frequency boom. To counteract that boom the engineers turned to active noise cancellation technology.

Terrain's noise cancellation system relies on two microphones embedded in the headliner to detect the hum and prompt an onboard frequency generator to create counteracting sound waves through the audio system's speakers and sub-woofer. The system also reduces higher rpm engine noise at highway cruising speeds to help keep the vehicle interior quiet.