Method of Salaah

Sunnah method of dua after fardh Salaah

Q: What is the correct manner of reading the dua after Salaah with regard to reading it loud or soft. Should the dua be made audible (loud) or inaudible (soft)? In our musjid the Imaam starts the duaa loud and reads the first part loud then remains silent and reads the duaa silently, thereafter he reads the ending of the duaa loudly again. Is this correct or is it a bid'a?