It wouldn't be very useful, because anyone would be able to shoulder surf the combination through a window. It would have to be done audibly, using tones as you turn the knob, it will elicit the sound of a dog barking, a hand clapping, a rocket shop, a firecracker, and you have to select 4 sounds in combination to unlock the unit. Of course, anyone in the car would be able to ear-surf the combination. :(

So at best it would be a toy, so given a "4 microphone array" how about a simple voice print?

Calvin