Knock Sensor
WARNING: This page is about a different car, the 2011 Nissan Quest. However, it is still accessible from the selected car via links, so may be relevant.
The knock sensor is attached to the cylinder block. It senses engine knocking using a piezoelectric element. A knocking vibration from the cylinder block is sensed as vibrational pressure. This pressure is converted into a voltage signal and sent to the ECM.