Chat and music kept inside the breakfast room of this hotel

This is the picture I took from the breakfast room in the hotel in Mechelen this morning. Do you also notice the weird curves in the glass windows? I tried to find out what kind of material that is. It is not classic double isolated glazing. It looked like thick single-layered glass. Some minutes before I was standing at the other side of the glass, when I left my room and entered the hallway. I looked into the breakfast room and was surprised that I could not hear any sound from the crowded breakfast room. I saw people moving around, eating, and chatting. And yet, I did not hear any sound. I noticed the weird curves in the glass already then, and I wondered what flat substance could stop the sound from the breakfast room and keep it out of the hallway and the nearby bed rooms. When I was seated in the breakfast room myself, I was surprised that there was also some light music on. And yet, I did not hear any sound in the hallway.

Any idea what that material could be?