You could hear a pin drop
Definition 1 of 1
Pro Tip 1/3Tense Expectation
Suggests a moment of high suspense when everyone is silently waiting for something to happen. 
Right before the verdict was announced, you could hear a pin drop.
Pro Tip 2/3Surprise Reaction
Implies that an event was so surprising or shocking that people became completely silent. 
After the unexpected win, you could hear a pin drop in the stadium.
Pro Tip 3/3Seriousness Indicator
Indicates a very serious atmosphere where silence is a response to gravity of situation. 
As the CEO discussed layoffs, you could hear a pin drop in the boardroom.