A trumpet. You can read the account in Judges 7:8-22.
But this was not a brass choir playing the Olympics theme. The trumpets here were shofars made from ram's horns. They were of different lengths and used for both religious purposes and as a call to battle. One is pictured above. With 300 of those things going off and each man breaking a jar at the same time, it's no wonder the other army took to its heels and ran!
Click here to listen to the sound of a shofar. See what I mean?
