Dr. Wayne Grudem from his Systematic Theology:

Tongues are “words of prayer or praise spoken to God. This definition indicates that speaking in tongues is primarily speech directed toward God (that is, prayer or praise). Therefore it is unlike the gift of prophecy, which frequently consists of messages directed from God toward people in the church. Paul says, “one who speaks in a tongue speaks not to men but to God” (1 Cor 14:2), and if there is no interpreter present at the church service, Paul says that someone who has the gift of speaking in tongues should “keep silence in the church and speak to himself and to God.” (1 Cor 14:28).”

Dr. Grudem lectures on this subject (lecture number 57) at this link.