I've experienced silence and something weirder. My boss sent me and a coworker to a "science & technology expo". It was a quite large area with more than 1000 people milling about talking and so on. The ambient noise level was very significant.
At opposite ends (about 200 yards apart) of the expo were 2 elevated platforms with parabolic dishes aimed at each other. Each dish was about 12 feet tall at the opening, and the focal point was marked on the floor of the platform.
I climbed the ladder of one and headed to the focal point to wait for my coworker to get into the other one. The ambient noise dropped quickly as I entered the dish and it was dead silent while I stood on the focal point. That was weird enough. But...
When my coworker reached the same point in the other dish, we could each whisper very quietly to the other and hear what was said like they were right at your ear. That was the coolest thing we saw that day.