Noise cancellation is a huge problem. Many auto manufacturers have attempted something like this. It is a very sensative process. The biggest problem is you have to monitor the placement of your ears (not really as necesary in a car) to adjust the phases for every frequency across the spectrum. Another problem is a cancelled noise for the person in the front seat may be doubled in amplitude for a person in another seat. These problems have no effect for noise cancelling headphones, this is why they work as well as they do.

Alex Lear