The amount of ultrasonic energy reflected back to the sensor from the object depends largely on the object surface area and its distance from the sensor. If an object is positioned, for example, 200 mm (8") from the sensor, the received echo is approximately 4 times stronger than if the object is at 400 mm (16"). Thus, it is possible that the echo strength from an object with a small surface area, placed at the maximum sensing distance, may be too weak to detect.