The Gemara (Sukka 51b), while discussing the magnificent architecture of the “Yipolostin” of Mitzraim, mentions that, “Anyone who had never seen [the Yipolostin] structure while it was filled to capacity had never seen the honor of Israel”. After making this unbelievable and sensational statement that acts as a testament to the splendor of this structure, the Talmud proceeds to explain the buildings layout, and the extravagant furnishings that were displayed inside.
While describing the manner in which services were handled in this “basilica”. The Gemara mentions that at times, the congregation was so large, that there were 1.2 million people in attendance! Due to the myriads of congregants in the crowd, many of them could not hear the chazzan at the front of the pulpit.
Being that in those times it was almost impossible to innovate a voice projection system (microphone); a shamesh would stand in front of the crowd and raise a huge cloth when it was appropriate for the congregation to say Amen.
Tosafos D”H V’Keivan, raises an interesting question. If the congregation was so large that they could not hear the words which the Chazzan was saying, then wouldn’t their Amen be an “Amen Yisoma” (only heard others saying Amen but did not hear the bracha itself) (Brachos 47a)? In which case, the Gemara in Brachos says that the punishment for one who says an Amen Yisoma is that he himself will have yisomim (orphans)!
Tosafos answers both in Sukka (51a) and in Brachos (47a), firstly that there is a difference between saying Amen during Torah reading than during Tefilla. Secondly he adds that one need only be afraid of saying an Amen Yisoma if he does not know which bracha he is saying, amen to. But if he knows which Bracha the Chazzan is saying then, the Amen that he says is not considered to be an Amen Yisoma.