Is it recommended or an innovation to convey the Imam's voice from behind him?

"Repeating the Imam's Takbir" refers to a worshipper raising his voice with the Takbir (Allahu Akbar) to make it heard by the rest of the worshippers. This is permissible when there is a need, such as the Imam's voice being weak or the mosque being very large. However, if it is done without need, it is a reprehensible innovation (bid'ah makruhah) by the agreement of the Imams. Scholars have differed regarding the invalidity of the prayer of the one who repeats the Takbir in such a case.