The blind zone means the minimum detection distance from the surface of the ultrasonic transducer. The main factor affecting the blind zone is ringing. When the ultrasonic transducer is driven by a specific frequency of electronic driving power, the piezoelectric ceramic correspondingly generates vibration and emits ultrasonic waves simultaneously, but this vibration does not stop immediately when the driving power is cut off. It fades away gradually to a static state after the remaining vibration is absorbed completely by the back damping layer of the transducer. The duration between the end of vibration and the static state is called ringing. The longer the ringing, the longer the blind zone. If the blind zone overlaps with the echo signals, the actual ToF cannot be calculated and therefore cannot detect the real distance.