Whisper Dishes

Stand and speak, and only one other person can hear you, and they're forty feet away

Two parabolic dishes face each other 40 feet apart. When you whisper at the focal point of one dish your friends can hear you loud and clear as they listen at the other dish. It's because the shape of your dish focuses the sound waves from your voice and allow them to travel much further than they normally would, and because the shape of your friend's dish collects the sound and bounces it to their ear.