Impact sound — the noise generated when objects strike a floor surface — is one of the most persistent sources of residential neighbor complaints in multi-family buildings worldwide. Walking, dropped objects, chair movement, children running, and exercise equipment transmit vibration energy directly into floor-ceiling structures, bypassing the airborne transmission path that STC addresses. Impact Insulation Class (IIC) is the rating system that quantifies a floor-ceiling assembly's resistance to this type of noise transmission.
ASTM E492 (Standard Test Method for Laboratory Measurement of Impact Sound Attenuation of Hard Surface Flooring Systems) defines the laboratory test procedure that produces the IIC rating. Its field counterpart, ASTM E1007, measures Field Impact Insulation Class (FIIC) in actual buildings. Together these two standards form the complete impact insulation test framework used across North America.
This guide walks through every clause of ASTM E492 and explains how the IIC number is derived, why laboratory ratings differ from field performance, what building codes require, and how to specify floor assemblies that will actually meet those requirements in a real building.
The Physics of Impact Noise
Before addressing the test procedure, understanding why impact noise is a fundamentally different problem from airborne noise is important.
When someone walks across a floor, the impact force creates vibration in the floor structure. This vibration propagates as structure-borne sound through the building skeleton — joists, beams, columns, and slabs. The vibrating structure then radiates airborne sound into the room below. The transmission path is: mechanical impact → structural vibration → airborne radiation. This is categorically different from airborne transmission (where a noise source in one room creates airborne sound waves that pass through partitions to another room), and it requires different control strategies.
The primary control strategies for impact sound are:
- Resilient surface layer — carpet, foam underlayment, cork — that absorbs the initial impact energy before it enters the structure
- Floating floor — a floor surface decoupled from the structural slab by a resilient material — that interrupts the vibration transmission path
- Resilient ceiling — a ceiling assembly decoupled from the structure by resilient hangers or resilient channel — that interrupts the final radiation step
- Combination systems — floating floor plus resilient ceiling — for high-performance requirements
ASTM E492: Laboratory Test Procedure
Scope and Standard Tapping Machine (Section 1 and Clause 5.1)
ASTM E492-16 (current edition) specifies the test method for measuring impact sound attenuation in a laboratory transmitting suite. The standard tapping machine, defined in ASTM E1007 (and equivalent to the ISO 10140-5 machine), produces controlled impacts with five hardened steel hammers, each:
- Mass: 500 g (±12 g)
- Fall height: 40 mm (±0.5 mm)
- Impact rate: 10 per second (±0.2 per second)
Test Suite Requirements (Section 6)
The laboratory must consist of an upper room (where the tapping machine operates) and a lower room (where the transmitted impact sound is measured). Requirements include:
- Floor area of test floor: minimum 10 m² (about 108 ft²), minimum dimension 2.7 m
- Room volumes: upper room typically 30–80 m³; lower room 25–80 m³
- Flanking control: the test suite must be constructed with structural breaks at all junctions between the test floor and the surrounding structure, preventing flanking transmission. This is the critical difference from field conditions.
- Background noise: at least 10 dB below the tapping machine levels in the lower room at each frequency band; if the margin is less than 10 dB, a correction must be applied
Tapping Machine Positions (Clause 7.1)
The tapping machine is placed on the floor under test in at least 4 positions, each at least 0.5 m from room boundaries and at least 0.5 m from each other. The positions must be distributed across the floor area to sample different structural zones, particularly for composite or non-uniform floors. For floors with visible surface irregularities, additional positions may be required.
Microphone Positions (Clause 7.2)
In the lower room, a minimum of 4 microphone positions are used. These must be:
- At least 0.5 m from any surface
- At least 1.5 m from the center of the floor under test (projected onto the lower room ceiling)
- At least 0.7 m from each other
Reverberation Time Measurement (Clause 7.3)
The reverberation time T in the lower room must be measured at the same microphone positions used for the impact level measurement. This T value is used to normalize the result to the reference absorption area A₀ = 10 m².
Calculating the Normalized Impact Sound Pressure Level (Clause 8)
The Normalization Formula
The normalized impact sound pressure level Ln at each frequency:
Ln = L + 10 log₁₀(A/A₀)
Where:
- L = measured space-time average sound pressure level in the lower room (dB)
- A = absorption area of the lower room (m²) = 0.161 × V / T
- A₀ = 10 m² (reference absorption area)
Frequency Range
ASTM E492 measurements are performed at third-octave center frequencies: 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1,000, 1,250, 1,600, 2,000, 2,500, and 3,150 Hz. The result is a curve of Ln versus frequency, spanning the range most relevant to impact noise from walking and dropping objects.
Calculating the IIC Rating (ASTM E989)
Reference Curve Method
The IIC (Impact Insulation Class) is computed using ASTM E989 (Standard Classification for Determination of Impact Insulation Class) from the normalized impact sound pressure level Ln curve measured under ASTM E492.
The IIC reference curve has fixed values at each third-octave frequency. The procedure:
- Plot the measured Ln values at each third-octave frequency
- Shift the reference curve vertically (in whole-number dB steps) until it is the highest position such that no single measured value exceeds the reference curve by more than 8 dB, AND the sum of the deficiencies (values above the reference curve) does not exceed 32 dB
- The IIC rating equals 110 minus the reference curve value at 500 Hz after the shift
IIC Reference Curve Values at Key Frequencies
| Frequency (Hz) | Reference Ln (dB) | IIC 50 Ln Target (dB) |
|---|---|---|
| 100 | 36 | 86 |
| 125 | 36 | 86 |
| 160 | 36 | 86 |
| 200 | 36 | 86 |
| 250 | 36 | 86 |
| 315 | 35 | 85 |
| 400 | 34 | 84 |
| 500 | 33 | 83 |
| 630 | 32 | 82 |
| 800 | 31 | 81 |
| 1,000 | 30 | 80 |
| 1,250 | 29 | 79 |
| 1,600 | 28 | 78 |
| 2,000 | 27 | 77 |
| 2,500 | 26 | 76 |
| 3,150 | 25 | 75 |
For IIC 50, the assembly's Ln values at each frequency must not exceed the tabulated values in the right column by more than 8 dB individually, and the total exceedance must not exceed 32 dB.
IIC vs. FIIC: The Laboratory-to-Field Gap
Why Field Performance Is Always Worse
The most important practical fact about IIC ratings is that they are laboratory measurements made in a test suite specifically designed to eliminate flanking. In a real building, flanking transmission adds directly to the transmitted impact noise, reducing the effective insulation by:
| Construction type | Typical IIC-to-FIIC gap |
|---|---|
| Heavy concrete slab, isolated structure | 3–5 dB |
| Lightweight concrete over timber joists | 6–9 dB |
| Open-web steel joist with concrete topping | 8–12 dB |
| Wood-frame platform construction | 8–15 dB |
For timber-framed residential construction — which represents the majority of multi-family housing built in North America — specifying an assembly with IIC 55 to achieve FIIC 50 is a reasonable starting point, but not a guarantee. The actual flanking gap depends on junction details, blocking, and workmanship.
ASTM E1007: Field Impact Insulation Class
ASTM E1007 defines the field measurement procedure for FIIC, using the same tapping machine operated per ASTM E492 but measured in the actual completed building. The procedure is identical except that flanking is included. Field measurements should be conducted after all floor finishes and ceiling finishes are installed (soft furnishings removed for a conservative baseline).
Building Code Requirements
International Building Code (IBC) Section 1207
The IBC, adopted with local amendments across most US jurisdictions, requires:
| Application | Minimum IIC | Reference |
|---|---|---|
| Floor/ceiling between dwelling units | 50 (laboratory) | IBC §1207.2 |
| Floor/ceiling below motel/hotel guest rooms | 50 (laboratory) | IBC §1207.2 |
The IBC specifies the laboratory IIC per ASTM E492. Many jurisdictions add supplementary requirements or accept FIIC equivalents. California Title 24 (Part 2, Section 1207) follows the IBC minimum. Local jurisdictions may impose higher requirements.
New York City
NYC Building Code Section 1207 aligns with the IBC minimum of IIC 50. NYC Administrative Code additionally requires that the owner provide evidence of compliance by commissioning ASTM E1007 field tests on a representative sample of floor-ceiling assemblies upon completion.
California
California Title 24 adds the requirement that field measurements (FIIC) be not less than 45. This addresses the IIC-to-FIIC gap by setting a lower field target that is still achievable after flanking losses.
WELL v2 Feature 74
WELL v2 Feature 74 Precondition L04 (Sound Reduction Between Spaces) requires that multi-occupant residential buildings achieve either FIIC ≥ 50 or IIC ≥ 55 between dwelling units.
Typical Code Requirements by Building Type
| Building type | Typical IIC requirement | Notes |
|---|---|---|
| Multi-family residential (IBC) | IIC ≥ 50 | Between dwelling units |
| Multi-family residential (California) | IIC ≥ 50, FIIC ≥ 45 | Title 24 adds field requirement |
| Hotels/motels | IIC ≥ 50 | IBC §1207 |
| Office buildings | Not typically required by code | Owner specification |
| Schools | Not typically required by code | BB93 (UK) addresses separately |
Material Performance Reference
Common Floor Assembly IIC Ratings
| Assembly description | Approximate IIC |
|---|---|
| 150 mm concrete slab (bare) | 26–30 |
| 150 mm concrete slab + 6 mm vinyl tile | 26–30 |
| 150 mm concrete slab + 3 mm carpet pad + 6 mm carpet | 60–72 |
| 150 mm concrete slab + 40 mm floating concrete topping | 44–48 |
| 150 mm concrete slab + floating topping + carpet | 65–75 |
| 200 mm wood joist (bare OSB subfloor) | 24–28 |
| 200 mm wood joist + resilient channel ceiling + 25 mm GWB | 46–52 |
| 200 mm wood joist + floating floor + resilient ceiling + 25 mm GWB | 54–62 |
| Cross-laminated timber (CLT) 120 mm (bare) | 35–42 |
| CLT 120 mm + floating floor + resilient ceiling | 52–58 |
These are representative values from published test reports. Actual IIC depends on specific product configurations, installation details, and the test laboratory used.
The IIC Upgrade Problem: Why Low-Frequency Impact Noise Persists
IIC ratings weight mid-frequency performance heavily and do not adequately capture low-frequency impact noise — the deep "thud" from heavy footfall, bass beats from exercise, or HVAC vibration. An assembly achieving IIC 55 may still transmit objectionable low-frequency impact energy at 63–100 Hz that falls below the measurement range of ASTM E492.
The OITC (Outdoor-Indoor Transmission Class, ASTM E1332) was designed to address low-frequency transmission in the airborne context, but no equivalent low-frequency-weighted impact rating is in widespread use for floor-ceiling systems. The closest solution is to examine the measured Ln curve at 100 Hz and 125 Hz and ensure these values are acceptably low for the specific application, rather than relying solely on the single-number IIC.
For occupied buildings with footfall complaints that cannot be resolved by floor or ceiling treatment, structural decoupling at the building level (isolation pads under columns or shear walls) may be necessary for very demanding applications such as recording studios over apartments or gymnasiums over offices.
Integration with AcousPlan
AcousPlan's Sound Insulation Calculator includes IIC compliance checking per IBC §1207. Input the floor-ceiling assembly type and surface finishes, and the calculator returns the estimated IIC and FIIC alongside code compliance status. The materials database includes IIC test data for over 200 floor assemblies sourced from manufacturer test reports and published ASTM E492 results.
All calculated IIC and FIIC values are advisory estimates. Field performance depends on as-built construction quality and flanking transmission conditions that cannot be predicted from assembly data alone.