Case foam

Case foam is a layer or block of foam fitted inside a keyboard's case, below the plate and PCB, to absorb sound energy that would otherwise bounce around the empty case cavity. Without it, many keyboards produce a hollow, metallic ping on certain keys, especially larger stabilized ones, because the case acts like a small resonating chamber.

Different foam thicknesses and densities produce different results, from a subtle reduction in ping to a much deeper, muted sound depending on how much of the empty space is filled. Some builders combine case foam with separate plate foam or PE foam layers closer to the switches for an even more controlled sound profile.

When adding case foam, check that it does not interfere with the PCB, standoffs, or any daughterboard cables inside the case, since foam cut too thick can create pressure points or prevent the case from closing fully. A common mistake is assuming more foam always sounds better, when some cases actually sound best with a thin layer rather than a dense, tightly packed fill.