What part of the church building is the tympanum where is it located why was this feature added?

The tympanum is part decorative semi-circle or triangle, usually a relief. It is located above the entrance. It is located there as it is part people naturally look at.

Explanation:

The tympanum is a decorative part above the entrance. It is usually made in semi-circular or triangle form, bounded by the arch. The decoration presented is most often done in a form of relief, a low-cut sculpture bound to its background. It can show some imaginary or ornaments, depending on the function of its building. It is very often a part of Christian architecture, and it includes religious scenes.

The tympanum is added because the part above the door is visible to all, and people often naturally observe it before entering. It is a very practical place to put sculpture or relief decoration, and this is why it usually portrays the central monumental piece of art on the facade of the building.

i'm not too sure - if you could attach the picture