What is IIC?
Impact Insulation Class (IIC) measures how well a floor-ceiling assembly blocks impact sounds like footsteps, dropped objects, and furniture movement. Unlike STC which measures airborne sound (voices, music), IIC specifically addresses structure-borne vibrations that travel through the building.
Higher IIC = better isolation. IIC 50 is what IBC 2021 §1206.3 requires between dwelling units. The figures above it are design targets rather than requirements — 55 and above is commonly specified for quieter multifamily work, and 60 and above where footfall must be close to inaudible — and we could not attribute either to a published standard.
Only the first row is a code requirement. The other two are the targets this tool ships with; we could not attribute them to a published standard, so they are labelled as targets rather than dressed up as requirements.
🎯 What the tapping machine actually does
Impact Insulation Class does not come from a person walking. It comes from a standardised tapping machine: five steel-faced hammers, each striking the floor twice a second for a combined ten impacts per second, run in four positions while sound pressure level is measured in the room below. That is the test described in ASTM E492 for laboratories and ASTM E1007 for finished buildings. The resulting normalised impact sound pressure levels are turned into one number by ASTM E989. All three are paywalled and we have not read them; the description above is from the National Research Council of Canada's own account of how it ran the tests.
The machine is a repeatable source, not a realistic one. NRC ran additional impact measurements with experimental, non-standard devices during the same programme, explicitly to gather information that might be used to improve the existing standardized tapping machine test or to develop new test procedures. Steel hammers are heavier on the high frequencies and lighter on the low thud of a heel than a real footfall, which is why an assembly can rate well and still be complained about.
🔬 How E989 turns sixteen numbers into one
Unlike transmission loss, impact levels are bad when they are high, so the contour fit runs the other way. The reference contour is pushed down onto the measured spectrum until it can go no lower without breaking two rules: the shortfalls where the measurement pokes above the contour must not total more than 32 dB, and no single band may exceed the contour by more than 8 dB. IIC is then 110 minus the contour value at 500 Hz.
Read as offsets from the 500 Hz value, the contour is flat from 100 to 315 Hz, falls 1 dB per one-third octave to 1000 Hz, then falls 3 dB per one-third octave to 3150 Hz. It permits more low-frequency energy than high, and it tightens sharply above 1 kHz — which is exactly where a soft floor covering does its work, and part of why floor coverings move the rating so far.
🧱 What the measured floors show
The assemblies in this calculator are specimens from the National Research Council of Canada report IRC-IR-811, which publishes the full one-third octave impact spectrum and the rating for each floor. Three comparisons in that data are worth more than any rule of thumb.
| Floor | STC | IIC |
|---|---|---|
| Bare concrete on steel deck, 75–150 mm, 272 kg/m² | 51 | 21 |
| 15 mm OSB on 2×10 joists, gypsum ceiling screwed direct, empty cavity | 33 | 28 |
| Same joist floor with resilient channels and 152 mm glass fibre | 52 | 46 |
| That floor with carpet on a 9 mm underpad | 53 | 67 |
Concrete is an excellent airborne barrier and an appalling impact one: STC 51 against IIC 21. Mass does very little against a hammer coupled straight into the structure. Decoupling the ceiling on resilient channels and filling the cavity took the joist floor from IIC 28 to 46 — and then carpet on a 9 mm underpad, laid over that same construction, took it to IIC 67 while its airborne rating moved by a single point. Impact and airborne isolation are separate problems with separate solutions, and the cheapest twenty points of impact isolation are usually the ones you walk on.
The same report measured how repeatable an impact test is, two different ways. It ran a full set of tests on one undisturbed specimen nine times across 13 days: of the eight tapping-machine runs, four returned IIC 45 and four returned IIC 46, and of the nine airborne runs, eight returned STC 50 and one STC 51. Separately it built its reference floor eight times over about a year from new materials each time, which gave a mean of IIC 46 and STC 52 — and NRC notes that this rebuild spread is the larger of the two, as you would expect once materials and workmanship are allowed to vary.
So a floor that is not touched at all still moves a point between tests, and a floor rebuilt to the same drawing moves more. NRC's guidance is that a change of only one point should be regarded as not significant unless an examination of the one-third octave band data shows significant changes. One point, on its own, is noise.
📊 What the code asks for, and what it will not accept
IBC 2021 §1206.3 requires floor-ceiling assemblies separating dwelling units and sleeping units from each other, and from public and service areas, to achieve an impact insulation class of not less than 50 when tested to ASTM E492. Where the assembly is field-tested instead, the section accepts a Normalized Impact Sound Rating of not less than 45, measured to ASTM E1007. §1206.2 sets the matching airborne requirement, covered by the STC calculator.
This calculator does not produce a compliance result. It applies the E989 classification to impact spectra, which is useful for comparing assemblies and for reading a test report you have been handed. The code asks for a tested assembly, and a submittal needs the test report. A number from this page is not the assembly's rating.
One further trap: most laboratory IIC ratings are measured on the bare structural floor. A rating quoted for a floor is not a rating for the floor with your finish on it, and a hard finish laid over an assembly rated with carpet will not perform as rated.
❓ Common questions
What is a good IIC rating?
IIC 50 is the figure that matters, because IBC 2021 section 1206.3 requires an impact insulation class of not less than 50 between dwelling units and sleeping units when the assembly is laboratory tested to ASTM E492, or a Normalized Impact Sound Rating of 45 when it is field tested to ASTM E1007. In practice 50 is a minimum rather than a comfortable result, and complaints about footfall in buildings that meet it are common — partly because the rating comes from a steel-hammer tapping machine rather than from a person walking.
How is IIC calculated?
A tapping machine is run on the floor and the normalised impact sound pressure level is measured in the room below in the sixteen one-third octave bands from 100 Hz to 3150 Hz. The standard reference contour is then lowered onto that spectrum until either the amounts by which the measurement exceeds the contour add up to 32 dB, or one band exceeds it by 8 dB, whichever comes first. The IIC is 110 minus the contour value at 500 Hz. The measurement is ASTM E492 in a laboratory or E1007 in the field, and the classification is ASTM E989; without measured impact data there is no way to calculate an IIC from a floor description alone.
What is the difference between IIC and STC?
STC rates airborne sound — voices, television, music — passing through a partition, and is measured with loudspeakers. IIC rates structure-borne impact sound — footsteps, dropped objects, furniture — and is measured with a tapping machine standing on the floor. They are independent, and a floor can be excellent at one and terrible at the other. In the National Research Council of Canada test series a bare concrete deck measured STC 51 and IIC 21, while a carpeted wood joist floor measured STC 53 and IIC 67.
Is a higher IIC rating better?
Yes. A higher IIC means less impact sound reaches the room below, and each point is one decibel of the contour fit. Two warnings, though. The rating is not linear in annoyance, and it is weighted toward frequencies above 1 kHz, so a floor can gain several points while the low thud that people actually complain about is unchanged. And the rating carries real measurement scatter: when the National Research Council of Canada ran the tapping machine eight times on one floor it had not touched between tests, four runs returned IIC 45 and four returned IIC 46 — and NRC treats a one-point difference as insignificant unless the one-third octave data shows a real change.
What are the STC and IIC ratings for floor underlayment?
An underlayment does not have an IIC of its own, because IIC belongs to a complete floor-ceiling assembly. What it has is an improvement figure, delta IIC, measured over a bare reference concrete slab under ASTM E2179 — and that improvement does not simply add to whatever assembly you like, because it was measured over concrete rather than over your floor. The honest way to specify one is to find a tested assembly that includes the covering, the structure and the ceiling you are actually building. In the National Research Council of Canada series, carpet on a 9 mm underpad took a resiliently suspended joist floor from IIC 46 to 67 while its airborne rating moved one point.
Impact spectra, IIC and STC ratings, the tapping-machine description and the rebuild and re-test repeatability figures on this page come from the National Research Council of Canada: IRC-IR-811, Detailed report for consortium on fire resistance and sound insulation of floors: sound transmission and impact insulation data in 1/3 octave bands and its companion Summary Report for Consortium on Fire Resistance and Sound Insulation of Floors. The code requirement is IBC 2021 §1206.3. ASTM E492, ASTM E989, ASTM E1007 and ASTM E2179 are named to identify what governs each measurement and classification; all are paywalled and we have not read them.