Active noise cancellation is a technology often found in headphones that is now starting to be installed in cars as well. It is the opposite of passive noise cancellation systems like the car's muffler and insulation, and uses microphones in the cabin to monitor the noise and send it to a digital signal processor. The signal processor generates a signal with inverted phase which is then played over the vehicle's speakers. This inverted signal collides with the noise and results in phase cancellation, effectively nullifying the sound. The systems generally work whether the vehicle's sound system are activated or not. Active noise cancellation works particularly well with low frequency noise, especially engine, wind, and tire noise.