Impact sound insulation (Ln,w)
The ability of a floor to limit noise created by the structure being struck directly, by footfall or moved furniture. Measured as L’n,w in decibels, where lower is better.
What is impact sound and how does it differ from airborne sound?
Impact sound arises when the structure is excited directly: footfall, a dropped object, a chair pushed across the floor. The energy does not travel through the air first. It enters the floor and moves through the structure until it radiates back into the air in the room below, which is why it is heard in a place remote from where it was made.
Airborne sound insulation deals with the opposite case, speech and a television arriving at the construction as pressure waves, and there mass helps. That is precisely why impact sound cannot be solved with mass. A heavy reinforced concrete slab is excellent against a conversation next door yet passes footfall almost undiminished. Impact performance is measured as the weighted normalised impact sound pressure level, written L’n,w and expressed in decibels, where lower is better.
| Question | Impact sound | Airborne sound |
|---|---|---|
| How the energy enters | Direct excitation of the structure | Pressure waves in the air |
| Typical source | Footfall, furniture, dropped objects | Speech, television, music |
| What helps | Decoupling, floating layers, resilient bearings | Mass, multiple leaves, cavity absorption |
| What does not help | Adding mass alone | A resilient underlay alone |
| Measured as | L’n,w, lower is better | A weighted sound reduction index, higher is better |
| Governing Slovak standard | STN 73 0532 | STN 73 0532 |
What does the Slovak requirement ask for?
Building acoustics requirements in Slovakia are set by STN 73 0532, and for floors between two dwellings the commonly cited limit is 55 dB or less. The figure is verified by measurement on the completed building rather than by calculation alone, which has a practical consequence that designers sometimes miss: the design has to carry a margin, because the measured value includes flanking transmission and every site imperfection that the calculation assumed away.
The related thermal standard, STN 73 0540, governs a different set of properties entirely, so a floor build-up has to satisfy two independent sets of requirements that pull in different directions: thickness and mass for acoustics, thermal resistance and heat output for comfort. Resolving them together is a design task, not a site one.
What is a floating floor and what makes it work?
The proven approach is a floating floor: a screed cast on an elastic layer that fully separates it from the structural slab. The screed becomes an independent plate which flexes under footfall and passes only a weakened share of the energy into the slab beneath.
The condition is rigorous separation from everything else, meaning not only from the slab but from every wall and every pipe penetration. One rigid contact, a sound bridge, undoes the whole layer. A screed touching the plaster at a single point leads footfall into the wall and from there into the adjoining space, and the measured value then comes out several decibels worse than the calculation promised. The failure is invisible after the floor covering goes down, which is why the inspection has to happen before it does.
How is a sound bridge avoided on site?
- Turn the elastic layer up around the entire perimeter above screed level and trim it only after the floor covering is laid.
- Run pipes and conduits below the elastic layer rather than through it, and sleeve any unavoidable penetration in a resilient collar.
- Detail door thresholds and stair landings as breaks in the screed, not as a continuous plate running from room to room.
- Fix skirtings to the wall only, leaving a gap to the floor covering that is closed with a flexible sealant.
- Keep mortar droppings and offcuts off the elastic layer before the screed is poured, because a hardened lump is a bridge.
None of these details is expensive and none of them is difficult. All of them are routinely lost when the floor is laid by a trade that was briefed on levels and falls but not on why the perimeter strip is there.
How does underfloor heating change the floor build-up?
Where the floor is heated, the same screed has to both store heat and float. That means a greater thickness, movement joints set out by the heating design, and an elastic layer that stays dimensionally stable through temperature cycles and under sustained load. The elastic layer is usually a high-density mineral wool board made specifically for use under screed, or an elastified polystyrene product; what decides the acoustic result is the declared dynamic behaviour of the board together with the mass of the screed above it.
The build-up is therefore designed by the heating engineer and the acoustician together rather than in sequence, because each of them can invalidate the other's assumptions. Floor covering choice matters too: a hard covering on a heated floor is chosen for its thermal resistance, and that choice interacts with the finish compatibility question long before anyone thinks about footfall.
Where does impact sound insulation most often go wrong?
| Location | What happens | Remedy |
|---|---|---|
| Perimeter strip | Trimmed too early, screed touches plaster | Trim after the covering is laid, never before |
| Pipe penetrations | Rigid contact through the elastic layer | Services below the layer, resilient collars where unavoidable |
| Stair flights | Cast into the party wall, footfall goes straight through | Elastomeric bearings, decoupled flights and landings |
| Thresholds | Screed continuous between rooms and out to the hall | Deliberate breaks at door lines |
| Laminate underlays | Sold as a substitute for a floating screed | Treated as an improvement, not as compliance |
The second typical problem area is staircases and party walls in semi-detached and terraced houses. A stair flight cast into a party wall carries footfall straight into the neighbour's bedroom, and no amount of floor treatment on either side will fix it afterwards. Where the plot also has an external noise source, the noise study and the floor acoustics belong in the same conversation, because a house can comfortably satisfy one and fail the other.
Frequently asked questions
- What is the difference between impact sound and airborne sound?
- Impact sound is generated by the structure being struck directly: footfall, a dropped object, a chair pushed across a floor. Airborne sound arrives at the construction as pressure waves from speech or a television. The distinction matters because mass helps against airborne sound and does very little against impact sound.
- What impact sound level is required between two dwellings in Slovakia?
- Building acoustics requirements are set by STN 73 0532, and for floors between two dwellings the commonly cited limit is a weighted normalised impact sound pressure level of 55 dB or less. The figure is verified by measurement on the completed building, so the design has to carry a margin rather than aim exactly at the limit.
- Why does a heavy concrete slab not solve footfall noise?
- Because the energy never travels through the air first. Footfall enters the slab directly and moves through the structure until it radiates back into the air in the room below. A heavy slab is excellent against a conversation next door and passes footfall almost undiminished, which is why the remedy is decoupling rather than mass.
- What is a sound bridge?
- Any rigid contact between the floating screed and the surrounding structure: screed touching plaster at one point, a pipe passing rigidly through the elastic layer, a skirting screwed to both wall and floor. One such contact leads footfall straight into the structure and can cost several decibels against the calculated value.
- Do underlays beneath laminate flooring help?
- They are a partial measure. A resilient underlay improves the perceived sound in the room being walked on but does far less for the dwelling below than a full floating screed. Between two dwellings they are usually not enough on their own, and they should not be presented to a client as an equivalent solution.