In car audio systems with active feedback built in, the sensor mikes are usually built in (in pairs) in the driver's headrest. Lotus did this with their early systems, and then went to a multi-mike system for their active noise cancelling stuff that used the amplifier to inject anti-phase noise. Very successful, never once implemented by commercial manufacturers (the plonkers).

Maybe that's a better idea - instead of active level compensation, where you're always fighting ambient, just inject an anti-phase component from the sensed environment. Then you just cancel it out without affecting your audio!

(You would have to consider patent issues if anyone tried this commercially, but I suspect they may well be coming up to public domain soon)

One of the few remaining Mk1 owners... #00015