Blow an alarm - i. e. along continuous peal. Compare Numbers 10:7, ye shall blow, but not sound an alarm: i. e. blow in short, sharp notes, not in a continuous peal. A third and a fourth alarm were probably blown as signals.
Related Commentaries of Numbers 10:5
Numbers 10:5-6
5 When ye blow an alarm, then the camps that lie on the east parts shall go forward.
6 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.