2 Samuel 18:16

Make thee two trumpets of silver; of a whole piece shalt thou make them: that thou mayest use them for the calling of the assembly, and for the journeying of the camps. And when they shall blow with them, all the assembly shall assemble themselves to thee at the door of the tabernacle of the congregation. And if they blow but with one trumpet, then the princes, which are heads of the thousands of Israel, shall gather themselves unto thee. When ye blow an alarm, then the camps that lie on the east parts shall go forward. When ye blow an alarm the second time, then the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys. But when the congregation is to be gathered together, ye shall blow, but ye shall not sound an alarm. And the sons of Aaron, the priests, shall blow with the trumpets; and they shall be to you for an ordinance for ever throughout your generations. And if ye go to war in your land against the enemy that oppresseth you, then ye shall blow an alarm with the trumpets; and ye shall be remembered before the LORD your God, and ye shall be saved from your enemies. Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, ye shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your God: I am the LORD your God.

So Joab blew a trumpet, and all the people stood still, and pursued after Israel no more, neither fought they any more.

Then the woman went unto all the people in her wisdom. And they cut off the head of Sheba the son of Bichri, and cast it out to Joab. And he blew a trumpet, and they retired from the city, every man to his tent. And Joab returned to Jerusalem unto the king.

For if the trumpet give an uncertain sound, who shall prepare himself to the battle?