Albert Barnes Commentary Revelation 9:13

"And the sixth angel sounded, and I heard a voice from the horns of the golden altar which is before God," — Revelation 9:13 (ASV)

And the sixth angel sounded. See Barnes on Revelation 8:2-7 and following.

And I heard a voice from the four horns of the golden altar which is before God. In the language here used there is an allusion to the temple, but the scene is evidently set in heaven. The temple in its arrangements was designed, undoubtedly, to be in important respects a symbol of heaven, and this idea constantly occurs in the Scriptures.

Compare the Epistle to the Hebrews throughout. The golden altar stood in the holy place, between the table of showbread and the golden candlestick. See Barnes on Hebrews 9:1-2.

This altar, made of shittim or acacia wood, was ornamented at the four corners and overlaid throughout with thin plates of gold. Hence it was called "the golden altar," in contrast to the altar for sacrifice, which was made of stone. Compare Barnes on Matthew 21:12 and following.

On its four corners it had projections which are called horns (Exodus 30:2–3), which seem to have been intended mainly for ornaments. See Jahn, Arch. § 332; Josephus, Antiquities iii. 6, 8. When it is said that this was before God, the meaning is that it was directly before or in front of the symbol of the Divine presence in the most holy place.

This image, in the vision of John, is transformed to heaven. The voice seemed to come from the very presence of the Deity, from the place where offerings are made to God.