Originally Posted by Pete4

Noise cancellation works on the principle of sending a wave with exactly opposite polarity, so timing is crucial. Easy to do on the headphones, but much more difficult with speakers, why? Because sound travels with speed of around 360 meters per second (give or take depending on altitude, temp., etc.) Lets say rear speakers are about 2 meters away from your head so by the time the sound reaches your ear, it is delayed by 1/180 sec. Assuming everything else is perfectly tuned and there is no delay in electronics, perfectly tuned opposite wave coming from headphones will work but delayed by 1/180sec wave coming from speaker will be most likely be out of phase, unless you put your speakers at the same position as the noise (tires), rather impossible, or you can adjust timing of the wave which your headphones probably don't have. BTW, I found noise canceling headphones waste of money. The noise is diminished by the fact of having the headphones covering my ears much more than the noise cancellation technology itself IMO. This technology could have big benefit for example for a jet mechanic, where jet engine noise could be canceled, so he can hear somebody talking to him, but I still see them using hand signals instead. As for passenger on the plane for example, I rather have heavy noise reducing padding on my headphones instead of noise cancellation electronics. Maybe it was just the headphone set I have that's not working too well, but so far I'm not impressed. Also low roll resistance tires should generate lower noise, me think, so I would start there first, with generating lower noise to begin with.

Originally Posted by wwest

You can now buy noise cancellation headphones that also have heavy noise insulative foam around the earpiece.

Originally Posted by wwest

Noise cancellation technology only works for continuous/repetive sounds, sound sources. Random, discontinuous, sounds still come through. Sorry, you will still hear that back seat driver.