Sound Transmission Class (STC) Ratings and Code Requirements

A sound transmission class rating is a single number that tells you how many decibels of airborne noise a wall, floor, or ceiling blocks, with higher numbers meaning more sound stopped. The International Building Code requires a minimum rating of 50 for any partition separating one dwelling unit from another, and most ordinary single-stud walls do not meet that number without specific acoustic upgrades.

What the Rating Actually Measures

STC covers airborne sound: voices, television audio, music, a barking dog. The number tells you roughly how many decibels are lost as that sound hits the partition and continues through to the other side. A wall rated STC 50 cuts the sound energy reaching the next room by about 50 decibels.

Two things the rating does not cover are worth flagging, because assuming otherwise leads to specifying the wrong assembly. Footsteps overhead, dragged chairs, and dropped objects are impact noise, which travels through the structure itself and is measured separately by Impact Insulation Class (IIC). Low-frequency exterior noise from traffic, trains, and aircraft falls under a different rating called Outdoor-Indoor Transmission Class (OITC), which tests down to 80 Hz where STC stops at 125 Hz. A window with a strong STC can still let highway rumble through.

The rating itself comes from a laboratory test under ASTM E90, which measures how much sound a partition stops across 16 one-third octave bands from 125 Hz to 4,000 Hz.1ASTM International. ASTM E90 – Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements Those measurements are then reduced to a single STC number under ASTM E413.2ASTM International. ASTM E413-22 – Classification for Rating Sound Insulation

What Rating the Code Requires

Section 1206.2 of the 2021 International Building Code requires walls, partitions, and floor-ceiling assemblies that separate dwelling units or sleeping units from one another, or from public and service areas, to achieve an STC of at least 50 in laboratory testing per ASTM E90.3ICC. IBC 2021 Chapter 12 Interior Environment The same rule applies to floor-ceiling assemblies, which must also hit IIC 50 for impact noise.

Field verification uses a different number. Real buildings never match lab conditions, so the code accepts a Normalized Noise Isolation Class (NNIC) of 45 or higher when a partition is tested on site under ASTM E336.4ASTM International. ASTM E336-17 – Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings The five-point allowance between the lab threshold and the field threshold reflects what the construction process actually delivers.

These requirements apply to new multi-family residential construction: apartments, condominiums, hotels, dormitories. The code also requires that penetrations through a rated assembly, for piping, electrical devices, recessed cabinets, bathtubs, soffits, or HVAC ducts, be sealed, lined, insulated, or otherwise treated so they do not degrade the rating.3ICC. IBC 2021 Chapter 12 Interior Environment Entrance doors are exempt from the STC number but must fit tightly against frame and sill, which matters because a loose or hollow-core entry door undercuts an otherwise compliant wall.

When comparing ratings, confirm they come from the same standard. A lab STC under ASTM E90 is not directly comparable to a field NNIC or Apparent Sound Transmission Class (ASTC) value, and treating them as interchangeable can produce an assembly that looks compliant on paper and fails on site.

What Different Assemblies Actually Score

Reaching STC 50 depends on four variables: mass, number of drywall layers, whether the two faces of the wall are decoupled from one another, and whether the cavity is insulated. Common residential wall assemblies land in these ranges:

  • Single wood stud, one layer of drywall each side, no insulation: STC 33 to 35. This is the builder-grade baseline and falls well short of code for a party wall.
  • Single wood stud, one layer each side, batt insulation in the cavity: STC 34 to 39. Insulation helps but does not close the gap.
  • Single wood stud, double layers of 5/8-inch drywall both sides, batt insulation: STC 43 to 45. Closer, still under code.
  • Staggered-stud wall, one layer each side, batt insulation: STC 46 to 47. Staggering partially decouples the two faces.
  • Double-stud wall, one layer each side, batt insulation, air gap between frames: STC 56 to 59. Full decoupling comfortably clears code.

Resilient channels offer a middle path. These thin metal strips mount horizontally across the studs, and the drywall screws into the channel instead of into the framing. The decoupling typically adds about five STC points to an otherwise identical wall. Driving a screw through the channel into the stud behind it short-circuits the system and erases the entire benefit, which is why installation oversight matters on this detail.

Doors and Windows

Doors and windows are almost always the weakest acoustic link in any room they sit in. A standard hollow-core interior door rates about STC 20 to 25. A solid-core door brings that up to roughly STC 30 to 35, better but still well below the surrounding wall. Specialized acoustic doors with perimeter gaskets and drop seals can reach STC 45 or higher at a significant cost premium.

Standard single-pane glass runs about STC 26 to 28. Double-pane insulated units improve that to roughly STC 35 to 40 depending on air gap and glass thickness. Laminated glass, which bonds a polymer interlayer between two sheets, can match double-pane performance in a single lite, and combining laminated glass with a double-pane configuration puts the rating in the STC 45 to 50 range. For buildings near airports or highways, that combination is often the minimum needed, and OITC is the more useful specification than STC for those facades.5ASTM International. ASTM E1332-22 – Standard Classification for Rating Outdoor-Indoor Sound Attenuation

Why Real Walls Underperform Their Rating

A wall that tests at STC 55 in a laboratory can perform closer to STC 40 once it is installed in a real building. Sound finds alternative routes around the rated partition, and those indirect routes, called flanking paths, are the single most common reason acoustic assemblies disappoint occupants.

The usual culprits are predictable:

  • Back-to-back electrical boxes in the same stud cavity create a near-direct hole from one unit to the next. Wrapping each box with an acoustical putty pad restores most of the lost performance.
  • HVAC supply and return ducts that serve adjacent units carry sound through the shared duct system, bypassing the wall entirely.
  • Where a partition meets a continuous floor slab or subfloor, vibrations travel under the wall and radiate into the next room.
  • Suspended ceilings that run uninterrupted over a partition let sound cross through the plenum above.
  • Unsealed perimeters at the top or bottom of a wall, unfilled gaps around pipes, and missing caulk at drywall edges all create direct air paths.

Acoustical sealant at wall perimeters, putty pads on electrical boxes, and insulated duct liners cost very little during construction and are expensive or impossible to retrofit later. The IBC’s rule requiring penetrations through rated assemblies to be sealed or treated exists specifically because of these failure points.3ICC. IBC 2021 Chapter 12 Interior Environment If a building official suspects a partition underperforms, the builder may be asked to hire an acoustic consultant for on-site testing, and a failed field test means remediation after the finishes are already in place.

What the Numbers Sound Like

Manufacturer sheets print STC ratings as if they were guarantees. The number represents a best-case laboratory result, and the installed assembly will usually test lower, sometimes by five to ten points, depending on workmanship and flanking control. The following rough translations help turn ratings into what an occupant actually experiences:

  • STC 25: normal conversation is clearly audible and easily understood through the wall.
  • STC 35: loud speech is audible but hard to make out. This is roughly where a basic insulated single-stud wall lands.
  • STC 45: loud speech is faintly audible but not intelligible. You know someone is talking, you cannot follow the words.
  • STC 50: loud speech is essentially inaudible. This is the IBC minimum for party walls and provides acceptable day-to-day privacy for most residents.
  • STC 60 and above: even amplified music and shouting are barely perceptible. These assemblies typically rely on double-stud framing or concrete and show up in recording studios, high-end condominiums, and healthcare settings.

If a product listing shows only a lab STC and the project needs verified field performance, plan on acoustic testing during construction. Catching a flanking problem before the finishes go in costs a fraction of what it costs to fix one after occupants have moved in.