Anti horn horn

We need an active noise cancelling horn that automatically detects a horn and emits a suitable anti-phase version intended to make it quieter for the car occupants.

It would be noisier outside the car, probably.

Once everyone's got one, we could have loads of fun assembling many cars in a confined space. Honk once and retire to a safe distance while observing the effect of all the cars trying to cancel each other in a cacophonic chain reaction.