Board Thread:False Info and Speculative Discussion/@comment-13965604-20150721190839/@comment-4091815-20150722030646

Volume, proximity and direction determines the impact of sound waves on solid objects.

A digital recorder's is capable of boosting the sound volume significantly. A cup directs the sound waves into a single spot.

The glass did not shatter, it cracked in a single spot. Once the glass cracked, it's structure broke down much like a small crack in a car's windshield will spread over time.