One of Dex's favorite things is "his" foghorn. I borrow it from him sometimes when I want to go kayaking on a misty day. Anyway, he has trained me to use it to call him when I am out of earshot. If I blow two blasts in the same rhythm as "DEX,,,COME!" he will understand that I want to give him a bone-us biscuit just now, and so he will come running even if he is a long way off (which he knows he shouldn't be). If we go out walking together, I usually show him the horn and the biscuits before we depart, just to remind him of the game. So I don't actually have to use the horn very often.
A few notes:
The horn is made of bright orange plastic, so no animals have been sacrificed in its creation, and not really too much oil hope.
The horn is non electic so, since I regularly breathe in and out, its use does not materially increase my carbon footprint.
As for his delicate hearing (which is excellent, due the quiet environment in which we mostly live), the horn is much less loud than his own bark, and never so close to his ears.
MD