A tympanum (plural, tympana) is the semi-circular or triangular decorative wall surface over an entrance, bounded by an arch. There are four tympana over the West Doors as follows.
The North Porch tympanum carries a sacrificial depiction of the Crucifixion of Our Lord,“He that loseth his life shall save it.”
This triumph of peace and righteousness is represented above the Central West Door by an enthroned Christ surrounded by saints, his right hand raised in blessing, his left holding the Book of Life, his foot treading upon the powers of darkness in the form of a winged serpent.
The South Porch depicts the Resurrection and victory over death, “O Grave where is thy Victory?”
Above and inside the West Door, there is a fine specimen of mosaic craft, depicting five angels traced on a gold background. The angel in the centre is playing the organ, with those on either side playing a trumpet and a violin. The outer angels are kneeling, with one singing and the other praying. This is an appropriate memorial to the choristers of the Cathedral, who were killed in the Great War -- 1914-1918.
The amazing detail in this mosaic can be appreciated in this close up of the central angel.