Deuteronomy 27:15-26 -

The curses to be pronounced were twelve in number, probably to correspond with the number of the tribes. The blessings are not here recorded; but when the injunction here given was fulfilled by Joshua, the blessing as well as the curse was pronounced ( Joshua 8:34 ). And probably, as the Jews report, each, the blessing and the curse, was pronounced alternately. It has sometimes been doubted whether any human voice could be audible over so wide a stretch as that between these two mountains; but this need be no longer matter of doubt, for the experiment has been repeatedly tried in recent times with success. In the clear atmosphere of the East sounds travel far. It is to be borne in mind also that it was not a single voice that had to make itself heard across the valley on this occasion, but a chorus of voices pro-seeding from a body of priests stationed apparently in the midst between the two companies (cf. Joshua 8:33 ), and chanting in unison the words of each blessing or curse.