Foam Density is frequently specified as a foam product requirement. In reality, foam density has little to do with performance. Density is helpful when trying to determine a specific part weight.
One of the most important specifications for foam is compression deflection, which will give you a value to judge the softness or firmness of the required foam material.
A lower density foam can be made to feel firmer by altering the chemistry, as well, a higher density foam can be made to feel soft by changing the formulation. Additionally, a closed cell foam will have more compression resistance for a given density then an open cell foam at that same density, which will appear softer. In providing a consistent product. Specifying a compression deflection value will help