1 Corinthians 14:8

For if the trumpet give an uncertain sound - The trumpet was used commonly in war. It is a well-known wind instrument, and was made of brass, silver, etc. It was used for various purposes in war - to summon the soldiers; to animate them in their march; to call them forth to battle; to sound a retreat; and to signify to them what they were to do in battle, whether to charge, advance, or retreat, etc. It therefore employed a “language” which was intelligible to an army. An uncertain sound was one in which none of these things were indicated, or in which it could not be determined what was required.

Who shall prepare himself ... - The apostle selects a single instance of what was indicated by the trumpet, as an illustration of what he meant. The idea is, that foreign tongues spoken in their assembly would be just as useless in regard to their duty, their comfort, and edification, as would be the sound of a trumpet when it gave one of the usual and intelligible sounds by which it was known what the soldiers were required to do. Just as we would say, that the mere beating on a drum would he useless, unless some tune was played by which it was known that the soldiers were summoned to the parade, to advance, or to retreat.