Step sound and acoustics in floor constructions
From floating subfloor to elastic interlayer — how impact sound is damped
When the neighbor upstairs walks across the floor, sound travels through the construction as footfall noise. It's one of the most annoying forms of noise in apartment buildings precisely because it comes from below and above and can't be shut out by closing the window. Floor construction is the single most important factor for how much footfall noise gets through, so acoustics is part of the floor layer's craft.
§Step sound and air sound
- 01Step sound — structure-borne sound from footsteps moved furniture and falling objects propagating through floor itself
- 02Airborne noise — airborne sound such as speech and music that penetrates through the construction
- 03Flanking transmission — sound that travels around the floor via adjacent walls and structures
§The floating floor dampens best
The principle in good stair sound reduction is to break the fixed connection between the floor surface and the load-bearing deck. This is done with a floating floor, where the floor rests on an elastic interlayer rather than being fixed to the deck. The greatest combined damping is typically achieved with a heavy floating underfloor — for example of concrete — laid on a sound-absorbing mat and with a fully glued, soft covering on top. Mass and elasticity work together here.
§Avoid sound bridges
A single rigid connection between the floor surface and the ceiling or wall can destroy the entire damping. A screw that runs through the elastic layer a glob of mortar or a skirting that clamps the floor to both floor and wall forms a sound bridge which sound travels through bypassing the soft structure. So the floor must be kept clear of walls with a gap and penetrations should be avoided or made with soft collars.
| Løsning | Power on footfall |
|---|---|
| Fixed floor directly on deck. | Poor damping, sound propagates |
| Floating floor on elastic mat | Good damping, breaks the connection |
| Heavy floating subfloor + soft covering | Best overall damping |
| Sound bridge (screw, mortar, wedged strip) | Damages damping locally |
§The role of the coating
The top surface also matters. A soft, fully bonded textile or elastic surface reduces high frequencies in footstep noise better than a hard wooden floor or tiles. Often it is the combination that counts: a floating subfloor takes the low frequencies and structural noise, while the soft surface takes the sharp footsteps. The floor layer must understand the interaction to advise and execute correctly.
Acoustics are invisible, but noticeable. A customer doesn't judge a floor on how nicely it's laid, but on whether there's quiet in the apartment below. Therefore, understanding impact noise and the right floor construction is one of the skills that distinguishes the professional floor layer from an amateur.