When using a phased array transducer, delay laws are applied to each channel to generate a beam with a given refraction angle and focal distance. The ultrasonic beam is generated by the constructive interference of each transducer element’s contribution in the desired direction.
In some cases, this interference can also be constructive in other directions. These lobes of energy emitted outside the electronically driven direction are called grating lobes.
These energy lobes can interact with the part to be inspected in the same way as the main beam, and thus generate echoes causing interference to the inspection. Therefore they have to be avoided as much as possible.
The angle of the position of grating lobes in relation to the main beam is given by the following formula