What is a side lobe of Ultrasonic Sensors?

Directionality can be depicted by a graph that shows the sound level in different directions with the center of the oscillator (maximum sound strength) as the reference. In general, the further away the angle from the reference, the smaller the sound level, but after a certain point it begins to increase again. This area is called the side lobe. If this side lobe reflects off a surrounding object, it may affect the detection characteristics of the Ultrasonic Sensor.