On episode 18 of season 2, the Mythbusters showed this was true. They had a vocal instructor sing, unamplified, at the proper frequency and shattered a lead crystal glass.
Welcome to the Straight Dope Message Board! I assume you are refering to Cecil’s classic column Can opera singers shatter glass with their high notes?. We like to include a link to the column so others reading your post will know what you are discussing. 
The column was from 1990, and obviously should be updated to include the Mythbusters result. Just a minor note: it wasn’t a vocal instructor who did it, if I recall correctly, it was a rock singer. To me, that made it even more impressive. 
We had another thread where the Mythbusters episode was brought up and discussed just a couple of weeks ago:
It’s probably recent enough that you’d be safe bumping that thread, with discussion intact, instead of starting a new one. Or maybe the mods would be willing to merge them for you.