There is one knock sensor per bank. The ECU correlates the crank position with knock sensor activity, and thus gives each cylinder a "window" in time for which the knock voltage is valid. The two sensors are mounted roughly in the center of each bank, so they are closest to cylinders 2 and 5. This causes knock voltages to appear higher for those two cylinders. The knock sensors themselves are sensitive to a very narrow band of frequencies. The ECU does additional filtering before the knock voltages are used to determine ignition retard by the knock control system based on knock recognition.