Some great engineering here, pretty cool.:The new 2010 Chevy Equinox that was unveiled at the Detroit Auto Show has an interesting new bit of technology that aids fuel efficiency even though most people wouldn't think of it that way. When the engineers were working on the calibrations for the new 2.4-liter direct injected four cylinder engine, they were able to get some significant fuel economy improvements by lowering the idle speed and by lowering the speeds at which the torque converter locks up. Unfortunately there were some downsides. The new calibrations resulted in some very annoying low frequency (33-100 Hz) noise that would be unacceptable to most customers. In the past, the engineers might have avoided those kinds of calibrations and just taken the hit on efficiency. This time however, they got approval to employ active noise cancellation. A pair of microphones embedded in the headliner listen for those frequencies and then generate an opposite wave sound from the audio system speakers to cancel out the offending noise. By using this technology to offset the negative NVH effects, they were able to get a one mpg improvement in economy for the four cylinder Equinox.http://www.autobloggreen.com/2009/06/10/noise-cancelling-speakers-help-2010-chevy-equinox-get-1-mpg/
Heard about this. Pretty clever. I wonder if the system still works if you have the windows rolled down.