A.T. Robertson Commentary 1 Corinthians 14:9

"So also ye, unless ye utter by the tongue speech easy to understood, how shall it be known what is spoken? for ye will be speaking into the air." — 1 Corinthians 14:9 (ASV)

Unless ye utter speech easy to be understood (εαν μη ευσημον λογον δωτε). Condition of third class again (εαν and aorist subjunctive). Ευσημον (ευ, well, σημα, sign) is old word, here only in N.T., well-marked, distinct, clear. Good enunciation, a hint for speakers.

Ye will be speaking into the air (εσεσθε εις αερα λαλουντες). Periphrastic future indicative (linear action). Cf. αερα δερων (beating the air) in 9:26. Cf. our talking to the wind. This was before the days of radio.