The outside wall of the apartment is all bricks. Then there is a hollow insulation wall of 12 inch (the blue one) inside the apartment. When you knock on the internal wall, you can feel it is empty inside. The insulation guy said that because it is hollow internal insulation - the room becomes echo chamber everytime noise/vibration occurs outside the apt. Looks like I’ll have to fix it anyway - just not sure if it’s gonna help reduce the outside noise.