Exodus 19:16-19

So it happened on the third day, when it was morning, that there were thunder and flashes of lightning, and a thick cloud was on the mountain, and a very loud blast was sounded on a ram’s horn, so that all the people who were in the camp trembled. Then Moses brought the people out of the camp to meet God, and they stood and presented themselves at the foot of the mountain. ORD Mount Sinai was wrapped in smoke because the LORD descended upon it in fire; its smoke ascended like the smoke of a furnace, and the whole mountain quaked violently. And it happened, as the blast of the ram’s horn grew louder and louder, Moses spoke and God answered him with [a voice of] thunder. [Deut 4:12]