Albert Barnes Commentary Numbers 10:5-6

"And when ye blow an alarm, the camps that lie on the east side shall take their journey. And when ye blow an alarm the second time, the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys." — Numbers 10:5-6 (ASV)

Blow an alarm - that is, a long continuous peal. Compare Numbers 10:7, ye shall blow, but not sound an alarm: that is, blow in short, sharp notes, not in a continuous peal. A third and a fourth alarm were probably blown as signals.