I take it you've never had a conversation on a dance floor, across a street busy with traffic, or without any risk of waking the baby (or your enemies, I guess)

I take it you've never had a conversation on a dance floor, across a street busy with traffic, or without any risk of waking the baby (or your enemies, I guess), or behind somebody's (highly proximate) back. Sign languages are for humans. Even those of us with sharp hearing can learn to take greater advantage of the bandwidth available to us.