Hi jovan,
Density comparisons between different types of foam materials (polyfoam, memory foam, or latex foam) aren’t particularly meaningful or useful because each type of foam material comes in very different density ranges that can’t be compared to each other and the effect of density is different with each type of material.
With polyfoam … density is a “quality spec” and has little to nothing to do with the firmness of the foam. The density of polyfoam is the single most important factor in its durability but any density of polyfoam can be made in a wide range of firmness levels so you could have low density firm polyfoam or soft polyfoam or higher density firm polyfoam or soft polyfoam. The specs that will have the biggest effect on the firmness of a polyfoam layer are the ILD/IFD of the foam (which would be the “simplest” method of comparison), the compression modulus of the foam, and the thickness of the foam.
Latex on the other hand is very different and with latex density is a “comfort spec” which has a direct effect on the firmness/softness of the latex. Even here though density comparisons aren’t meaningful if you are comparing different types and blends of latex and are only useful if you are comparing the same type and blend of latex. Talalay latex will tend to have a lower density than Dunlop in the same firmness level and synthetic latex will also tend to have a lower density at the same firmness level than natural latex. If you are comparing the same type and blend of latex then higher densities will be firmer (see post #6 here).
Phoenix