476. When and How to Answer Amen

Brachos 1:13

When a Jew hears another Jew recite a bracha, he is required to answer amen even if he didn’t hear the entire bracha and even if he was not himself obligated in that bracha. One does not answer amen to the bracha of a non-Jew, an apostate, a Samaritan, a child who is studying his brachos, or an adult who changes the text of a bracha.

Brachos 1:14

When answering amen, one must not recite a hurried amen, a “cut-off” amen, a short amen or a long amen; one should pronounce a medium amen. One should not reply amen louder than the other person recited the bracha. If one did not hear a bracha in which he is obligated, he should not reply amen with the other people.