Jamieson, Fausset and Brown on Exodus 28:34

34. a golden bell and a pomegranate--The bells were hung between the pomegranates, which were said to have amounted to seventy-two, and the use of them seems to have been to announce to the people when the high priest entered the most holy place, that they might accompany him with their prayers, and also to remind himself to be attired in his official dress, to minister without which was death.