The complete guide to sound insulation, STC and IIC ratings, impact noise, and practical solutions for concrete floor acoustics
Understand how concrete floors transmit and block sound, what STC and IIC ratings mean, why concrete excels at airborne noise but struggles with impact sound, and exactly what treatment options are available to meet building codes and achieve acoustic comfort in 2026.
A complete reference for engineers, architects, builders, and building owners on the acoustic performance of concrete floors in 2026
Concrete floors transmit sound via two fundamentally different mechanisms: airborne sound and structure-borne (impact) sound. Understanding the distinction between these two pathways is the starting point for any acoustic assessment or treatment of a concrete floor, because each type requires a different solution strategy in 2026.
Airborne sound originates as pressure waves in the air — voices, music, television, and mechanical equipment noise. These waves strike the concrete surface, cause it to vibrate, and the vibration is radiated as sound on the other side of the slab. The more massive and dense the concrete, the harder it is for airborne pressure waves to set it vibrating, and the better the airborne sound insulation. This is why concrete is generally a very good airborne sound barrier compared to lightweight timber construction.
Structure-borne (impact) sound is generated by direct mechanical contact with the floor surface — footsteps, dropped objects, chair legs scraping, and mechanical vibration from equipment. The energy is injected directly into the concrete structure and travels through the solid material, bypassing the air entirely. Because concrete is rigid and highly efficient at conducting mechanical energy, it is paradoxically a poor insulator of impact sound in its bare state — even though it is an excellent airborne sound barrier.
Sound originates in the air (speech, music, TV) → strikes the floor surface → sets the concrete slab vibrating → vibration radiates sound into the room below. The key metric is STC (Sound Transmission Class). Concrete mass is the primary defence — heavier slabs vibrate less.
Mechanical force strikes the floor directly (footstep, dropped object) → energy enters the slab as vibration → travels through the structure → radiates as sound below. The key metric is IIC (Impact Isolation Class). Resilience and decoupling are the primary defences — bare concrete performs poorly.
Sound travels indirectly — not through the floor itself, but along connected structural elements (walls, beams, columns). Flanking can undermine otherwise excellent floor acoustic performance. It is especially problematic in concrete frame buildings where floors, walls, and columns are monolithically connected.
Two primary laboratory-measured ratings are used globally to describe the acoustic performance of floor-ceiling assemblies in 2026. Both are single-number ratings derived from weighted averages of sound transmission loss across a range of frequencies, and both use a scale where a higher number = better acoustic performance.
The STC scale is a single-number rating derived from measuring sound transmission loss (in decibels) at 16 one-third octave frequency bands from 125 Hz to 4,000 Hz. A standard contour curve is then fitted to the measured data following ASTM E413. The rating is the transmission loss in decibels at 500 Hz on the fitted contour. A difference of 10 STC points represents a roughly doubling of perceived loudness — so an STC 50 wall allows through approximately half the sound energy of an STC 40 wall.
| STC Rating | What Can Be Heard Below | Typical Application | Performance Level |
|---|---|---|---|
| 25–30 | Normal speech clearly intelligible | Single-family residential (basic) | Poor |
| 35–40 | Loud speech audible but not fully clear | Basic timber floor systems | Below Average |
| 42–47 | Loud speech faintly audible | Minimum code (some jurisdictions) | Minimum |
| 50–54 | Loud speech not audible; music faintly audible | Bare 150 mm concrete slab | Good |
| 55–59 | Very loud sounds faintly audible | Treated concrete floor assembly | Very Good |
| 60+ | Most sounds inaudible | High-spec residential / recording studios | Excellent |
The IIC rating is measured using a standardised tapping machine (ISO 717-2 / ASTM E492) that strikes the floor surface with calibrated hammer blows. The sound pressure level in the receiving room is measured at multiple frequencies, and a single-number rating is derived by fitting a standard contour. Unlike STC, where less transmission is better (higher STC = higher transmission loss), the IIC measures the reduction of impact noise — a higher IIC means less impact noise reaches the room below.
| IIC Rating | Impact Noise Experience Below | Typical Assembly | Performance Level |
|---|---|---|---|
| 25–35 | Heavy footsteps clearly audible; impacts very loud | Bare concrete slab, no underlayment | Very Poor |
| 36–44 | Footsteps audible; impacts noticeable | Concrete + hard tile (no underlay) | Poor |
| 45–49 | Footsteps faintly audible | Below minimum building code | Below Code |
| 50–54 | Footsteps barely audible | Code minimum (most jurisdictions) | Code Minimum |
| 55–64 | Normal footsteps not audible | Concrete + acoustic mat + hard floor | Good |
| 65–75 | Footsteps inaudible; excellent isolation | Concrete slab + carpet + underlay | Excellent |
A bare concrete slab without any additional treatments or floor finishes presents a classic acoustic paradox: high STC but low IIC. It is an excellent barrier to airborne sound by virtue of its mass, but a poor isolator of impact sound because of its rigidity and high structural conductivity. This is the fundamental challenge of concrete floor acoustics in residential and commercial construction in 2026.
A solid 100 mm (4 inch) reinforced concrete slab typically achieves approximately STC 44–46 for airborne sound. This is below the minimum code requirement of STC 50 in most multi-residential jurisdictions. Its IIC rating without any finish is approximately IIC 28–32 — far below the code minimum of IIC 50. The thinner the slab, the lower both ratings.
A 150 mm solid concrete slab is the standard minimum thickness recommended for multi-residential acoustic performance. It typically achieves STC 50–54 — just meeting or narrowly exceeding the airborne code minimum. However, the bare IIC remains approximately IIC 28–35, meaning impact sound treatment is always required regardless of slab thickness to comply with building regulations.
A 200 mm solid slab provides a comfortable airborne margin at STC 55–58, providing headroom above the code minimum. The bare IIC remains stubbornly low at approximately IIC 30–36 because additional mass does not significantly improve impact isolation — resilience is needed, not mass. Increasing concrete thickness beyond 150 mm has diminishing returns for impact sound.
Hollow-core precast slabs are widely used in multi-residential construction for their structural efficiency, but their acoustic performance is significantly worse than solid slabs of the same depth. Due to the internal voids, a 200 mm hollow-core slab may achieve only STC 45–48 for airborne sound and IIC 25–30 for impact — both typically below code minimums and requiring more aggressive treatment than solid slabs.
No matter how thick the concrete slab, a bare concrete surface without resilient treatment will fail the IIC 50 minimum required by building codes in Australia (NCC), the UK (Approved Document E), the US (IBC), and most developed nations. IIC ratings of 28–36 are typical for bare slabs, regardless of thickness. The reason is physical: impact noise is mechanical energy injected directly into the rigid structure. Concrete's mass does not resist this energy — only resilience (the ability to absorb and decouple mechanical energy) can improve impact isolation. Treatment of the floor surface or the ceiling below is always required.
Concrete is one of the best naturally occurring building materials for airborne sound insulation. The key physical property is surface mass density (kg/m²). The heavier the floor per unit area, the more difficult it is for airborne sound pressure waves to set it vibrating, and the more sound energy is reflected and absorbed rather than transmitted. This relationship is expressed by the mass law of sound transmission.
While the mass of a concrete slab provides strong airborne sound insulation in theory, several real-world factors commonly reduce the achieved field performance below laboratory predictions in 2026:
As a general design guideline, a solid reinforced concrete slab of 150 mm or greater thickness will typically provide sufficient airborne sound insulation (STC 50+) to meet building regulations in Australia (NCC), the UK (Approved Document E), and most European jurisdictions — provided penetrations are properly sealed and flanking is controlled. Slabs thinner than 150 mm, or hollow-core slabs, generally require supplementary treatment (additional mass layers or an acoustic ceiling) to achieve airborne code compliance.
Impact sound insulation is the most significant acoustic challenge for concrete floors in multi-residential, hotel, and commercial mixed-use construction in 2026. Unlike airborne sound — where adding mass to concrete provides natural protection — impact noise cannot be controlled by mass alone. The fundamental principle of impact sound control is resilience and decoupling: introducing a compliant, energy-absorbing layer between the impact source and the rigid concrete structure.
When a foot strikes a concrete floor, the mechanical energy is instantly and efficiently conducted through the rigid monolithic slab. Concrete has a very high stiffness (Young's modulus ≈ 30 GPa) and high acoustic wave speed (≈3,500–4,000 m/s), meaning vibration energy travels rapidly and with little natural attenuation through the structure. This is the opposite of what is needed for impact isolation — which requires the structural path to be interrupted or the mechanical energy to be absorbed before it enters the slab.
The resilient layer (orange) is the critical acoustic element — it must be continuous, uncompressed, and completely decoupled from walls and structure.
The floor finish installed on top of the concrete (or floating screed) has a dramatic effect on the measured IIC rating of the assembly. This is one of the most important practical considerations for designers and building owners in 2026, particularly as hard floor finishes (LVT, polished concrete, ceramic tile) have become increasingly popular in residential and hospitality interiors.
| Floor Finish | IIC Effect | Typical IIC on Bare 150 mm Slab | Notes |
|---|---|---|---|
| Polished concrete (no finish) | None (baseline) | IIC 28–35 | Fails all building codes |
| Ceramic / porcelain tile | −2 to −5 IIC | IIC 25–32 | Makes impact worse than bare slab |
| LVT / vinyl plank (no underlay) | ±0 | IIC 25–35 | Effectively no acoustic benefit |
| Engineered timber (no underlay) | +2 to +5 | IIC 30–38 | Still far below code |
| LVT + acoustic underlay (ΔIIC 20+) | +18 to +25 | IIC 48–56 | May meet code with heavy slab |
| Carpet + felt underlay | +30 to +42 | IIC 65–75 | Excellent — easily meets code |
| Floating timber floor + resilient mat | +20 to +30 | IIC 50–60 | Good — meets code when designed correctly |
When a concrete floor fails to meet acoustic performance requirements — whether due to insufficient slab thickness for airborne sound or universally inadequate IIC for impact sound — a range of treatment options are available. The best solution depends on the available floor height, the required performance level, the floor finish, and whether access to the ceiling below is possible in 2026.
The most common and code-compliant solution. A continuous acoustic mat or mineral wool board is laid over the concrete slab, then a 65–75 mm sand-cement screed is poured on top. The screed must be fully floating — not touching the walls (perimeter isolation strip required) and not connected to the slab by any rigid element. This system (e.g., UK Robust Detail E-FC-4 to E-FC-6) typically achieves IIC 50–58 and maintains STC 50+. Depth penalty: 75–90 mm.
A resilient acoustic mat is laid over the concrete, then 18–22 mm particleboard, plywood, or engineered timber board is installed as a dry floating layer. This system is faster, lighter, and uses less floor depth than a wet screed. It typically achieves IIC 48–55 depending on the underlay's ΔIIC rating. The board layer must be floating with perimeter isolation. Best suited for retrofit projects and residential interiors with hard floor finishes.
Instead of treating the floor surface, a resilient-mounted suspended plasterboard ceiling is installed below the concrete soffit. Wire-hung or clip-isolated systems (e.g., RSIC-WHI clips) can achieve STC and IIC ratings in the 50s to low 60s. This approach is particularly valuable when the floor finish cannot be changed (e.g., existing polished concrete) or when maximum acoustic performance is required. The air gap between ceiling and slab is critical — deeper gaps (minimum 200 mm) perform better.
Carpet with a thick felt or foam underlay applied directly to the concrete slab is the simplest and most cost-effective way to achieve excellent IIC performance. A quality carpet and pad can add ΔIIC 30–42, bringing a bare 150 mm concrete slab from IIC 32 to IIC 65–75 — far exceeding code requirements. The limitation is aesthetic: carpet is not always acceptable for commercial, hospitality, or modern residential applications.
Resilient timber battens — or purpose-made cradle-and-batten systems — are installed on the concrete surface, and a timber or board floating floor is fixed to the battens. No screed is used. This system can achieve the highest IIC performance of any floor-side treatment (up to IIC 60+) when combined with a deep batten void filled with acoustic mineral wool. It is also the deepest option, typically requiring 75–120 mm of floor depth above the structural slab.
Combining a resilient floor treatment (Option 1 or 2) with a resilient suspended ceiling (Option 3) provides the highest achievable acoustic performance for a concrete floor assembly. This dual treatment approach can achieve IIC 60–70+ and STC 60+ — suitable for high-specification residential developments, luxury hotels, recording studios, and home theatres. It is also the most expensive and depth-intensive solution, requiring careful coordination of structural and acoustic design.
Regardless of which floating floor system is used, a continuous perimeter isolation strip must be installed between the floating screed (or board) and all surrounding walls and upstands. If the floating layer makes hard contact with the wall at any point, a rigid acoustic bridge is created that short-circuits the resilient layer and can reduce IIC by 10–20 points. Use a 10 mm closed-cell foam or mineral wool perimeter strip and ensure it runs the full height of the floating layer. This is one of the most common acoustic failures in practice and is also the easiest to prevent at the construction stage in 2026.
The table below summarises typical STC and IIC ratings for common concrete floor assemblies in 2026. Values represent laboratory test results — field values (FIIC, FSTC) are typically 3–10 points lower due to flanking, workmanship, and slab-to-wall connections. Always design to exceed code minimums by at least 5 points to account for field losses.
| Assembly Description | STC (Lab) | IIC (Lab) | Code Status | Notes |
|---|---|---|---|---|
| 100 mm solid slab — bare | 44–46 | 28–32 | Fails Both | Below code for airborne and impact |
| 150 mm solid slab — bare | 50–54 | 30–35 | Fails IIC | Airborne marginal; impact always fails |
| 200 mm solid slab — bare | 55–58 | 32–36 | Fails IIC | Good airborne; impact still fails |
| 200 mm hollowcore slab — bare | 45–48 | 25–30 | Fails Both | Worse than solid due to voids |
| 150 mm slab + carpet + pad | 52–54 | 65–75 | Passes Both | Excellent IIC; easy and cheap |
| 150 mm slab + LVT (no underlay) | 51–54 | 28–33 | Fails IIC | No impact improvement from LVT alone |
| 150 mm slab + acoustic underlay + LVT | 51–54 | 48–56 | Marginal | ΔIIC of underlay critical; verify product |
| 150 mm slab + resilient mat + floating screed + hard floor | 52–56 | 50–58 | Passes Both | Standard compliant system (Robust Detail) |
| 150 mm slab + resilient battens + timber floor | 53–57 | 55–63 | Passes Both | High performance; greater depth |
| 150 mm slab + acoustic mat + screed + resilient ceiling | 58–65 | 60–70 | Exceeds Both | Premium dual treatment; best performance |
| 200 mm slab + concrete topping + resilient ceiling | 60–65 | 58–65 | Exceeds Both | High-mass + decoupled ceiling approach |
Flanking noise is the transmission of sound along indirect structural paths — through walls, beams, columns, and other connected elements — rather than directly through the floor-ceiling assembly being assessed. In concrete frame construction, flanking is a particularly significant issue because the monolithic nature of reinforced concrete creates highly efficient structural connections between floors and walls throughout the building in 2026.
A floor assembly with a laboratory IIC of 60 can achieve a field IIC of only 45–50 if significant flanking exists through connected walls or columns. In multi-residential concrete frame buildings, achieving the NCC, Approved Document E, or IBC field performance requirements demands flanking control alongside floor treatment. Key flanking control measures include: using resilient isolation strips at slab-to-wall joints, installing floating floor and wall linings together (not just the floor), and designing acoustic breaks in structural connections at party wall junctions. Acoustic consultants should assess flanking risk early in the structural design phase.
Most developed nations require minimum acoustic performance standards for floor-ceiling assemblies in multi-residential buildings (apartments, townhouses, and units). The two primary metrics are a minimum STC (airborne) and a minimum IIC (impact). Note that building codes typically specify field performance (FSTC and FIIC, measured in the completed building), which is 3–10 points lower than laboratory values. Always design for lab values of at least STC 55 and IIC 55 to ensure comfortable compliance in the field.
| Country / Code | Min. Airborne (STC / DnT,w) | Min. Impact (IIC / L'nT,w) | Standard | Notes |
|---|---|---|---|---|
| 🇦🇺 Australia — NCC 2026 | FSTC 50 | FIIC 50 | NCC Vol. 1, Spec. F5.5 | Field values; Robust Details equivalent accepted |
| 🇬🇧 United Kingdom | DnT,w + Ctr ≥ 45 dB | L'nT,w ≤ 62 dB | Approved Document E | Field measurement; Robust Details pre-approval |
| 🇺🇸 USA — IBC | STC 50 | IIC 50 | IBC §1207 | Lab values acceptable; some states require field |
| 🇨🇦 Canada — NBCC | FSTC 50 | FIIC 50 | NBCC §9.11 | Field values in occupied buildings |
| 🇳🇿 New Zealand — NZBC | DnTw + Ctr ≥ 55 dB | L'nTw ≤ 55 dB | NZBC Clause G6 | High standard; among the toughest globally |
| 🇸🇬 Singapore — BCA | STC 50 | IIC 50 | BCA Code on Envelope Performance | Applies to all Class 2 occupancy buildings |
| 🇪🇺 EU (general) | DnT,w ≥ 50–55 dB | L'nT,w ≤ 58–63 dB | EN ISO 717 | Varies by country; Germany DIN 4109 is stricter |
The gap between laboratory test values and field measured values in completed buildings is typically 5–10 STC/IIC points, caused by flanking, construction variability, and workmanship. To ensure field compliance with a minimum of STC 50 and IIC 50, design the floor-ceiling assembly to achieve STC 55–58 and IIC 55–58 in the laboratory. This design margin is recommended by acoustic engineers in Australia, the UK, New Zealand, and the US to reliably meet code requirements after construction is completed in 2026.
The Australian Building Codes Board's National Construction Code (NCC) specifies acoustic performance requirements for Class 2 buildings (apartments). Volume 1 Specification F5.5 sets FSTC 50 and FIIC 50 minimums for floor-ceiling assemblies in multi-residential construction in 2026.
View NCC →The UK Building Regulations Approved Document E sets acoustic performance standards for residential buildings including minimum DnT,w + Ctr and L'nT,w requirements for floor-ceiling separating elements. Pre-tested Robust Detail systems (E-FC-4 to E-FC-6) provide a compliance route for concrete floors.
View Doc E →ASTM E90 (laboratory airborne), ASTM E492 (laboratory impact), ASTM E413 (STC calculation), and ASTM E989 (IIC calculation) are the primary standards governing acoustic measurement and rating of floor-ceiling assemblies used in North America and widely referenced globally in 2026.
View ASTM →