Commentary Par XIV 1-9

A simile, with the formal markers of the trope suppressed (e.g., 'just as,' 'so,' 'like') but with reference to its literary kind embedded in it (similitudine [a hapax: [Par XIV 7]]), this comparison of the sounds of Thomas's voice, at the circumference of the smaller circle of saints, and of Beatrice's, issuing from near Dante at the center of that circle, draws attention to the mind of its maker, a witness of such celestial phenomena.  The meaning is clear enough, if some have stumbled over the question of how water in the center of a bowl may be struck (answer: by something falling from above [like the thought that drops into Dante's mind -- see the last part of C.Par.XIV.7-9]).
For the frequency of figurative language in Paradiso, see Vanossi (Vano.1993.1).