1 Corinthians 14:9

So also ye, unless ye utter by the tongue speech easy to be understood, how shall it be known what is spoken? for ye will be speaking into the air.

The force of 1 Corinthians 14:7-9 is that the false tongues of Corinth were unintelligible nonsense, having no meaning whatever, being nothing more than jabberings of orgiastic demonstrators; and here was the delicate part of the whole situation, the UNINTERPRETED manifestations of the genuine gift itself resembled the false tongues so perfectly that no one on earth could have told any difference! It was a master stroke of the devil that he had prevailed upon some who had the true gift to bring it into the public worship; and therefore, when Paul condemned the false, his care not to discourage the true variety of tongues resulted in an occasion of misunderstanding of this subject for centuries afterward. What Paul said here is applicable to both varieties of tongues, both kinds being forbidden in public worship, the true kind because it was not interpreted and had no business in the public worship to start with, and the false kind because it was nothing but pure nonsense anyway.

The essential thing to see is the close likeness in appearance of the two kinds of tongues; and this is paramount as an indication that the true tongues of Corinth were unlike those of Pentecost.