Sound insulating material, barrier systems

Can a good sound insulator be a poor absorber?

Yes. Sound insulation depends on the mass and airtightness of a material, which reflect sound. Sound absorption depends on porosity, which lets sound enter and lose energy. A dense concrete or brick wall is an excellent insulator (high transmission loss), but its hard, smooth surface reflects almost all sound back into the room, so its absorption coefficient is very low (about 0.02 to 0.05). On the other hand, a mineral wool blanket absorbs well but, being light and porous, insulates poorly. So the two properties are not the same and may even be opposite.

Systems of sound insulation

Sound travels as airborne sound (voice, TV) and as impact or structure-borne sound (footsteps). Insulation is provided by barriers.

Vertical barrier systems (walls and partitions)

1. Heavy, dense walls: thick brick or concrete (mass law: doubling the mass gives about 5 to 6 dB more).
2. Cavity (double) walls with a gap of 50 mm or more, no rigid ties, and the gap filled with sound-absorbing wool.
3. Staggered-stud or double-stud partitions with separate frames.
4. Plastering on both faces to seal pores and gaps; sealing of all cracks and service holes.
5. Acoustic doors, and double-glazed windows with panes of different thicknesses.

Horizontal barrier systems (floors and ceilings)

1. Heavy RCC slab (for airborne sound).
2. Floating floor: a screed and floor finish laid on a resilient layer (rubber, cork, mineral wool) over the slab, and separated from the walls by a gap or edge strip, so that it does not touch the structure.
3. Resilient floor finishes: carpet, rubber or cork tiles to reduce impact noise.
4. Suspended (false) ceiling hung on resilient hangers, with absorbent material above it.

 Floating floor:
   finish
   screed
   resilient layer (wool)  <- edge strip at wall
   RCC slab