Arrange the 30 foam blocks into three groups of 10. You will be cutting the blocks into three different-sized wedges. By placing the largest wedges at one end of the wall, the middle-sized wedge in the middle and the smallest wedge at the other end of the wall, you can approximate an angled wall. As well as approximating an angle, the wedges will create a peaked surface.
Slice 10 of the blocks diagonally from the left top edge to the right bottom edge.
Slice 1 inch from the top the second 10 foam blocks, and then slice them into wedges so you have 20 slightly smaller wedges.
Slice 2 inches from the remaining 10 blocks, and then slice them into wedges. You now have 60 wedges in three sizes.
Spray each wedge in fire-retardant spray. It is essential to spray them after you have cut them. Spraying the wedges and then cutting them is dangerous because only the outside of the foam blocks is fire-retardant. Once you slice them into wedges, they become a fire hazard as the inner foam has not been treated.
Attach old bed sheets to the wall with drawing pins so that the entire wall is covered.
Glue the bottom of a large wedge and stick it to the top left of the wall, so the slope points right. Glue a second large wedge and attach it to the wall directly below the first. Repeat this process until all of the large wedges are on the wall.
Glue on the middle-sized wedges next to the large wedges. If you do not have sufficient wedges to cover the whole wall, leave space in between the large and medium wedges. As long as the flat surface of the wall is converted to an irregular surface and the majority of the wall is not parallel with the opposite wall, this will achieve a good level of muffling.
Glue on the smaller wedges to the right of the medium-sized wedges.