This paper presents the study of the intervention for the acoustic correction of a Medieval church, the cathedral of Carinola in Italy. The investigated church built on site where a Paleochristian church existed, was completed between the XIV and the XV century. The hard materials, including a marble floor and hard plastered walls, make its reverberation time of almost 5 sec. at low frequency and over 3 sec. at mid frequencies, resulting in poor speech comprehension. As common in ancient worship heritage, the acoustic improvement interventions were challenged by the denial of using conventional sound-absorbing materials due to aesthetic and architectural reasons. The intervention aimed to shortening the reverberation time by substituting the plaster with sound absorbing plaster. Acoustic simulation results are reported, and the homogeneity of acoustic parameters across the floor plan is discussed.