A wall listed at STC 50 in a manufacturer’s catalog goes up between two condominiums, and the neighbors still hear each other’s television. Upstairs, an owner swaps carpet for hardwood, and the unit below starts counting footsteps. Both complaints lead back to two ratings, STC and IIC. Each test measures something specific and leaves something out, and California’s building code relies on both.
Two kinds of sound, two ratings
Sound reaches the next unit in two ways. Voices, television and music travel through the air, set the wall or floor vibrating, and radiate again on the far side. Footsteps, a dropped pan or a chair dragged across the floor strike the structure directly. The first is airborne sound, rated by the Sound Transmission Class (STC). The second is impact sound, rated by the Impact Insulation Class (IIC). For both STC and IIC, a higher number means better isolation.
STC starts with a laboratory test under ASTM E90. The assembly fills an opening between two test rooms, and its transmission loss is measured in one-third octave bands from 125 to 4,000 Hz. That is sixteen bands. ASTM E413 shifts a reference contour against the measured curve until the shortfalls below it total no more than 32 dB, with no band more than 8 dB short. The rating is the contour’s value at 500 Hz.
IIC uses a different source. ASTM E492 places a standard tapping machine on the floor of the upper test room. The sound level in the room below is measured in one-third octave bands from 100 to 3,150 Hz, and ASTM E989 turns those levels into the single-number IIC.
Neither rating looks below 100 Hz, and an ASHRAE Journal column on acoustics fundamentals notes that STC does not measure HVAC rumble or bass music. Absorption inside a room is a separate question. Absorptive material changes how a space sounds, as the guide to the acoustics of a sanctuary shows, and does little for the STC of the wall it hangs on.
What the tapping machine does not hear
The tapping machine produces a steady series of uniform impacts at a uniform rate. ASTM E492 itself spells out the limits. The machine does not simulate the weight of a person walking, so the creaks and booms of real footsteps on a light floor are not reflected in the rating. The standard calls the link between tapping-machine results and how people judge real floors uncertain.
That matters most on light wood-frame floors. Such a floor can post a respectable IIC against the hammer blows and still thump under bare feet. The field method, ASTM E1007, repeats the warning for buildings: the creak or boom of footfalls on a limber floor is not adequately evaluated.
A product sheet for an underlayment may quote a Delta IIC, the improvement the product adds. That figure comes from ASTM E2179, a laboratory test of floor coverings on a concrete floor. ASTM E2179 itself warns against using its results to estimate impact levels on floors of only one or two lightweight layers, such as plywood or oriented strand board. On a wood-frame floor with a gypsum ceiling below, the benefit has to be judged for that assembly.
When a condominium association writes a flooring rule, the useful question is: “Which assembly was tested, with which ceiling below?”
Laboratory rating, field rating
A test laboratory is built so that the specimen is the only significant path between the two rooms. A building is not. ASTM E336 describes the indirect paths, called flanking: through the structure around the partition, through common air ducts, and through doors to a shared corridor. Field results therefore carry their own names.
| Sound | Laboratory test and rating | Field test and ratings | California Building Code, Section 1206 |
|---|---|---|---|
| Airborne | ASTM E90, rated by E413: STC | ASTM E336: NIC, NNIC, ASTC | STC 50 tested in the lab, or NNIC 45 tested in the field |
| Impact | ASTM E492, rated by E989: IIC | ASTM E1007: AIIC, ISR, NISR | IIC 50 tested in the lab, or NISR 45 tested in the field |
ASTM E1007 states that good agreement between laboratory and field tests on similar floors is not expected. The code’s field minimums sit five points below the laboratory ones.
Where a rated wall leaks
The California Building Code names the usual suspects itself. Penetrations or openings for piping, electrical devices, recessed cabinets, bathtubs, soffits, and heating, ventilating or exhaust ducts must be sealed, lined, insulated or otherwise treated to maintain the rating.
Two units side by side above a third, vertical section. Schematic, not to scale.
- Airborne sound through the wall: what STC rates in the laboratory
- A footstep strikes the floor and reaches the unit below: what IIC rates
- Flanking: sound travels along a continuous subfloor under the wall
- Cross-talk through a duct that serves both units
- Outlet boxes set back to back in one stud cavity
- Recessed cabinet: one side of the wall loses its gypsum layers
- Floor-ceiling assembly: subfloor, joists with insulation, gypsum ceiling
- Two outlet boxes set back to back in the same stud cavity leave a short path of thin metal and air through the wall. Offsetting them into different stud bays and sealing them keeps the wall closer to its tested condition.
- A medicine cabinet recessed into a party wall removes the gypsum layers on one side over its whole area.
- One supply duct serving bedrooms on both sides of a demising wall links the rooms through the duct. The same duct can also carry background noise from HVAC systems into both rooms.
- Entrance doors are exempt from the rating, but the code requires them to be tight fitting to the frame and sill. A gap under a corridor door lets sound bypass a well-built corridor wall.
- A continuous subfloor, or a ceiling cavity that runs over the top of a partition, carries sound around the wall instead of through it.
What California requires
Section 1206 of the California Building Code applies to walls, partitions and floor-ceiling assemblies between dwelling units and sleeping units, or between those units and public or service areas.
- Airborne sound: STC 50 when tested under ASTM E90, or NNIC 45 if field tested under ASTM E336.
- Impact sound: IIC 50 under ASTM E492, or NISR 45 if field tested under ASTM E1007. Floors over garages, mechanical rooms, storage and other nonhabitable spaces are exempt.
- Instead of a test, the rating may be established by engineering analysis that compares the assembly with tested ones. A registered design professional has to perform it.
The same section limits noise from outside. Interior noise from exterior sources may not exceed 45 dB in any habitable room, as Ldn or CNEL, whichever the local general plan uses. On a site near a freeway, that limit shapes the windows and exterior walls. For those, ASTM E1332 adds the Outdoor-Indoor Transmission Class (OITC), weighted toward the low-frequency sound of traffic and aircraft. How it relates to environmental review is explained in the guide to the CEQA noise study.
These figures are minimums. An owner who wants more than the code minimum has to write a higher target into the design brief, before the wall and floor types are chosen.
Reading STC and IIC test reports before you specify
- Laboratory or field? STC and IIC ratings come from the laboratory; NNIC, ASTC, NISR and AIIC come from a building. Older reports may write FSTC for a field result.
- Which assembly exactly? Stud size and spacing, layers of gypsum board, insulation, resilient channels, slab thickness and the ceiling below all belong in the description.
- Rating or improvement? A Delta IIC describes a floor covering on a concrete reference floor, not the rating of your floor.
- Is low-frequency sound part of the problem? For a home theater, a gym or a mechanical room, ask for the band data behind the single number.
STC and IIC measurements were among the architectural acoustics services the firm listed. The other numbers that come up on a project, from RT60 to CNEL, are sorted out in the overview of architectural acoustics basics.
Questions clients ask
Is STC 50 enough between two apartments?
It is the minimum the California Building Code sets for laboratory-rated walls and floors between dwelling units, with NNIC 45 as the field alternative. Whether residents find it enough depends on what happens next door and on the details built around the wall. Bass from a home theater is exactly what the rating leaves out.
Our field test came in below the lab rating. Was the wall built wrong?
Not necessarily. Field ratings include flanking paths that the laboratory suppresses, and the code already sets its field minimums five points lower. A large gap is still worth investigating. Unsealed outlet boxes, recessed cabinets, shared ducts and gaps at the floor or ceiling line are the first places to look.
Will acoustic panels on our side raise the wall’s STC?
Very little. Absorptive panels reduce reflections and reverberation inside the room, which can make it feel calmer. Stopping sound from passing through depends on other things: the mass of the wall, separating its two sides so they do not share studs, and airtight construction. A panel adds almost none of these.
A neighbor installed hardwood floors. What can be done?
Start with what is under the finish floor and what the ceiling below is made of. An underlayment rated by Delta IIC was tested on concrete, so its benefit on a wood-frame floor has to be checked for that assembly. A field test under ASTM E1007 gives NISR or AIIC values measured on the real floor, which gives both parties the same number to discuss.
Standards and sources
- ASTM E413-22, Classification for Rating Sound Insulation: STC, NIC, NNIC and ASTC.
- ASTM E989-21, single-number metrics for impact noise: IIC, AIIC, ISR, NISR and Delta IIC.
- ASTM E90-23, laboratory measurement of airborne sound transmission loss.
- ASTM E492-25, laboratory measurement of impact sound transmission with the tapping machine.
- ASTM E336-25a, airborne sound attenuation between rooms in buildings, including flanking paths.
- ASTM E1007-25, field measurement of tapping machine impact sound transmission.
- ASTM E1332-22, Classification for Rating Outdoor-Indoor Sound Attenuation: OITC.
- ASTM E2179-25, effectiveness of floor coverings on concrete floors (Delta IIC).
- California Building Code, 2025 edition, Chapter 12, Section 1206 Sound transmission.
- National Research Council Canada, CBD-236, Introduction to Building Acoustics: test frequency ranges for STC and IIC.
- ASHRAE Journal, Engineer’s Notebook, “Learning a Vocabulary for Acoustics Fundamentals.”