1 Corinthians 14:7-9

7Even things without life, whether flute or harp, when they make a sound, unless they make a distinction in the sounds, how will it be known what is piped or played?

8For if the trumpet makes an uncertain sound, who will prepare for battle?

9So likewise you, unless you utter by the tongue words easy to understand, how will it be known what is spoken? For you will be speaking into the air.