I suspect public speakers who are used to talking in very large rooms also already have this skill, since they pretty much always hear the slapback echo as they

I suspect public speakers who are used to talking in very large rooms also already have this skill, since they pretty much always hear the slapback echo as they speak. You can tell a new speaker by when they start to speak, they suddenly stop for a moment before continuing (he-hello!), because the slap back is really jarring when you aren't expecting it.