Here are some examples of open cell spray foam. In open cell spray foam, the tiny cells of the foam are not completely closed. The trapped air causes the R-value per inch to be lower than closed-cell foam, but it also makes it an amazing sound attenuator!  In fact, open-cell spray foam is one of the highest quality and most effective sound attenuators on the market.They are broken and air fills all of the “open” space inside the material. This makes the foam feel softer than closed-cell foam.