Carpet underlay type determines whether flooring absorbs sound at all

Evidence

The floor is covered with wall-to-wall clipped-pile carpet on felt underlay that is as thick as is practical. Felt acts as a fibrous absorber; most carpet underlays do not. The carpet needs to be porous, e.g. jute backed, not rubber backed. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1440

Categorization

• CANDY: Nice concrete gotcha: the backing/underlay, not the carpet itself, decides whether your floor is an absorber or a mirror.
• DEFINE: Clarifies that felt, not foam/rubber, is what makes underlay acoustically fibrous.