You have lived above or below another apartment at some point in your life. If you were below, you know the sound: the rhythmic thud of footsteps crossing the room, the bass-heavy impact of a child jumping off the couch, the scrape of a chair being dragged across a hard floor, the sharp crack of something dropped on a kitchen tile. You could not hear your neighbour's conversation — the walls were fine — but the floor told you everything about every step they took. That floor had a poor IIC rating.
IIC — Impact Insulation Class — is the number that tells you how well a floor-ceiling assembly blocks the transmission of impact-generated noise from the floor above to the room below. It is completely separate from STC (which covers airborne sound such as speech and music), and it is measured using a completely different test method. In multi-storey residential and commercial buildings, IIC is often the acoustic metric that has the largest effect on occupant satisfaction — and the one most frequently misunderstood or underspecified.
The Definition: What IIC Actually Measures
Impact Insulation Class (IIC) is a single-number rating that describes how much a floor-ceiling assembly reduces the transmission of impact-generated sound from the upper floor to the room below, measured across a standardised frequency range.
IIC is defined by ASTM E989: Classification for Determination of Impact Insulation Class, and the underlying test method is ASTM E492: Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies. The European equivalent standard is ISO 717-2, which produces a similar rating called the Weighted Normalised Impact Sound Pressure Level (Ln,w) — note that for this metric, lower is better, the inverse of IIC.
How the Test Works: The Tapping Machine
The test uses a device called a standard tapping machine — a motorised hammer that drops five steel-tipped hammers onto the floor surface at a rate of 10 impacts per second from a height of 40 mm. The tapping machine generates a standardised impact load that approximates (roughly) the energy of a heel-strike footstep.
While the tapping machine operates on the upper floor, a microphone in the room below measures the sound pressure level at 16 one-third octave band frequencies from 100 Hz to 3150 Hz. These 16 measurements are compared to a standardised IIC contour (defined in ASTM E989). The contour is shifted vertically until the total unfavourable deviation is as large as possible without exceeding 32 dB. The IIC rating is the value of the contour at 500 Hz in its final shifted position.
A higher IIC number means better performance — more of the impact energy is being absorbed or decoupled before it reaches the room below.
What the Numbers Mean in Real Life
| IIC Rating | What You Hear Below |
|---|---|
| 25 | Footsteps are clearly audible and sound like they are in the same room. Every step is distinct. |
| 30 | Footsteps are clearly audible. Running and jumping create loud, startling impacts. |
| 35 | Normal footsteps are noticeable. Heavy footsteps and furniture impacts are clearly audible. |
| 40 | Normal footsteps are audible but less intrusive. Heavy impacts are still clearly transmitted. |
| 45 | Normal footsteps are faintly audible. Heavy impacts are noticeable. Most occupants find this marginal. |
| 50 | The minimum typically required by code. Normal walking is barely audible; heavy impacts are faint. |
| 55 | Normal walking is inaudible. Only heavy impacts (dropped heavy objects, jumping children) are faintly audible. |
| 60+ | Very high performance. Only extreme impacts are barely perceptible in the room below. |
The distinction between IIC 50 and IIC 55 is significant. Research by the National Research Council of Canada (NRCC) found that occupant complaints about floor noise drop sharply once FIIC (field-measured IIC) exceeds approximately 55. Buildings that just barely meet the IIC 50 code minimum consistently generate the highest complaint rates in occupant satisfaction surveys.
Why Impact Noise Is Harder to Control Than Airborne Noise
Airborne sound — speech, music, television — travels through the air and is blocked by dense, decoupled assemblies. Impact sound is different. It is generated at the floor surface and immediately enters the building structure as a structural vibration. The vibration then propagates through the slab, the joists, the beams, and radiates back into the air in the room below as sound.
This means that the strategies for blocking impact noise are fundamentally different from the strategies for blocking airborne noise:
Isolation at the source (soft floor finish, resilient underlay, floating floor) interrupts the vibration before it enters the structure. This is the most effective approach because it addresses the problem where it originates.
Decoupling at the ceiling (resilient channel mounting, independent ceiling suspension, visco-elastic hangers) reduces the transmission from the slab to the ceiling in the room below. This helps but is less effective than source isolation because the vibration has already entered the structure.
Mass (heavier slabs, additional concrete toppings) reduces the amplitude of vibration for a given impact, but the relationship is less efficient than for airborne sound. Doubling the slab mass adds only about 5-6 dB of impact isolation — much less effective than doubling it would be for STC.
Cavity absorption (insulation between a floating floor and the structural slab, or between an independent ceiling and the slab) reduces the resonance within the air cavity, contributing modestly to IIC improvement.
The most effective floor assemblies combine source isolation (floating floor on resilient mounts, or a thick carpet and pad system) with decoupled ceiling construction below. The two approaches address different parts of the transmission path and work synergistically.
A Worked Example: A Concrete Apartment Floor
Consider a common mid-rise residential building with a 200 mm reinforced concrete slab. This is the structural floor and the starting point for the acoustic analysis.
Bare 200 mm concrete slab: Using ASTM E492 tapping machine data:
- Impact level at 100 Hz: approximately 78 dB
- Impact level at 500 Hz: approximately 67 dB
- Impact level at 2000 Hz: approximately 55 dB
- IIC: approximately 28
Option 1 — Add carpet and 6 mm foam underlay: Carpet with a thick pad (total system approximately 15 mm) dramatically reduces the impact energy at source by providing a resilient contact surface.
- IIC improvement: approximately +22 to +26 points
- New IIC: approximately 50-54
Option 2 — Add a floating screed on resilient mounts (acoustic mat system): A 50 mm floating concrete screed separated from the structural slab by a continuous resilient mat (e.g., a 10 mm recycled rubber acoustic underlayment):
- IIC improvement: approximately +24 to +30 points over the bare slab
- New IIC: approximately 52-58
Option 3 — Floating screed plus independent suspended ceiling below: Add a 100 mm independent ceiling system suspended from the structural slab on resilient hangers, with the ceiling surface separated from all walls by a 10 mm resilient isolator:
- Additional IIC improvement from the ceiling: approximately +5 to +8 points
- New IIC: approximately 60-65
The Tapping Machine Problem: What IIC Does Not Tell You
The standard tapping machine test has a well-known limitation: it does not accurately represent all types of impact noise that occur in real buildings. Specifically:
The tapping machine underestimates the transmission of low-frequency bass impacts. A steel-tipped hammer dropping 40 mm generates a sharp, high-frequency impact — similar to a hard heel strike on a hard floor. It does not simulate the soft, body-weight impact of a person walking in socks or the booming low-frequency shock of a large object dropped from height.
Research has shown that assemblies with good IIC ratings can still transmit annoying footstep noise — particularly the low-frequency "thud" component around 30-80 Hz — that the standard test frequency range (100-3150 Hz) simply does not capture. This is why Nordic countries developed supplementary impact noise criteria that include measurements down to 50 Hz or lower, and why ISO 717-2 has introduced an adaptation term (CI) that adjusts for low-frequency impact noise.
The practical implication: if the floor above will be occupied by people who walk heavily, families with children, or spaces with heavy furniture that will be moved, specify IIC performance above the code minimum (55 or higher) and consider independent testing with supplementary low-frequency criteria rather than relying solely on the standard tapping machine IIC number.
Building Code Requirements for IIC
International Building Code (IBC) 2021, Section 1207: Requires FIIC 50 (field-measured) for floor-ceiling assemblies between dwelling units and between a dwelling unit and a public area.
International Residential Code (IRC): Requires IIC 50 for new construction. Many local jurisdictions adopt the IRC with amendments that increase this to IIC 55 for new multifamily construction.
WELL v2 Feature 75 (Acoustics): Specifies IIC 50 minimum for floor-ceiling assemblies in residential buildings seeking WELL certification, with a recommendation of IIC 55 for higher performance.
HUD Minimum Property Standards: The US Department of Housing and Urban Development requires IIC 45 (FIIC 45) for federally assisted housing — a lower threshold than the IBC, which reflects the age of the HUD standard.
LEED v4 Acoustic Performance: Awards a credit for floor-ceiling assemblies that exceed IBC minimums by a specified margin, incentivising higher performance than the code floor.
Common Mistakes When Specifying IIC
Relying on carpet to meet IIC requirements: Many buildings are specified and built with bare concrete slabs, relying on tenants to install carpet to achieve code-compliant IIC. This approach fails the moment a tenant installs hard flooring — a trend that has increased dramatically as hardwood, tile, and luxury vinyl plank flooring have become the dominant residential preferences. Structural acoustic treatment that does not depend on the floor finish is the only reliable approach.
Treating IIC and STC as interchangeable: A wall assembly that performs well for airborne STC does not necessarily perform well for structural impact transmission. The two mechanisms are different; the two ratings test different things; and the two solutions involve different materials and details. A project specification should address both independently.
Ignoring the path from floor slab to wall to ceiling below: Structural vibration from an impact does not only travel straight down through the slab. It also travels laterally through the slab, up walls, and into the floor-ceiling assembly of adjacent spaces. This lateral flanking can undermine an otherwise excellent floor system, particularly in lightweight framed construction where floor joists connect directly to wall studs.
Specifying IIC without specifying FIIC tolerance: Because field performance is consistently lower than laboratory IIC, the specification should either require a laboratory IIC that is 5-8 points above the minimum FIIC requirement, or explicitly require post-construction FIIC testing to verify compliance.
How AcousPlan Helps You Get IIC Right
AcousPlan's Sound Insulation Calculator includes IIC modelling tools for floor-ceiling assemblies in multi-storey buildings. For each assembly configuration, you can:
- Review octave-band impact sound level data from the tapping machine test, not just the summarised IIC number, to identify whether low-frequency performance is a concern for your application
- Model different floor finish combinations — bare slab, floating screed, resilient underlay, carpet — and compare the resulting IIC ratings to understand which investment delivers the greatest performance return
- Check compliance automatically against IBC 2021 Section 1207, WELL v2 Feature 75, and other applicable standards
- Estimate FIIC from laboratory IIC, accounting for flanking transmission, so your specification is realistic about likely post-construction performance
- Generate side-by-side comparisons of assembly options, including cost and thickness data, to support value engineering discussions with clients and contractors
Ready to model your floor assemblies? Try the Sound Insulation Calculator at /insulation — input your floor configuration, compare assembly options, and get IIC ratings with compliance checking instantly.