What STC Ratings Really Tell You (and What They Don't)
By Jeffrey E. Babich, INCE Bd. Cert.
Sound Transmission Class (STC) is a number that architects, developers, and building owners use to predict whether a wall or floor will keep noise contained. While STC is a helpful shorthand, it is easy to misinterpret. Many complaints about walls "not working" stem from misunderstanding what the rating guarantees, and not from a faulty product.
What STC measures
STC is a single-number rating that describes how well a wall or floor-ceiling assembly reduces airborne sound, such as voices, television, and music. A higher STC means better noise reduction. The rating is determined through laboratory testing according to ASTM E90, and the results are compared to reference contours in another standard.
Heavier assemblies provide better sound isolation, but good detailing also makes a significant difference. Separating two adjacent rooms so they don't share framing, filling the cavity with insulation, and/or resiliently mounting drywall are all effective ways to improve a wall's performance. A basic wall typically achieves an STC in the low 30s, while a well-built, decoupled wall can reach the mid-50s or higher. A few extra STC points can make a noticeable difference.
The lab rating is not the field rating
The STC listed on a product submittal is a laboratory result, but a finished wall in a building almost always performs worse because sound can find paths around the assembly. Field ratings account for these real-world conditions. Apparent STC (ASTC) or Normalized Noise Isolation Class (NNIC), both measured according to ASTM E336, are commonly used to assess how well a partition reduces noise between spaces. These field ratings usually fall several points below the lab STC, so specify and verify performance based on what you can expect in the finished building.
For walls and floors between dwelling units, the International Building Code requires an STC of 50 in the lab, or 45 in the field. At this level, a neighbor's loud conversation becomes a muffled murmur rather than something you can clearly understand. However, the code only sets a minimum, not an ideal comfort target. High-end condominiums, residences above restaurants, or medical exam rooms often require higher performance. Set targets based on how the space will be used, and not just to meet minimum code requirements.
STC does not account for low-frequency noise, which is the range where noise from subwoofers and rooftop equipment is heard. This means two walls with the same STC rating can perform very differently when low-frequency sound is involved. Impact noise has a similar issue: footsteps on a lightweight wood-frame floor mostly produce low-frequency thumps that fall below the range of typical impact ratings.
A newer standard, ASTM E3207, addresses these low frequencies. In one wood-frame apartment building, we traced footstep complaints to the structure itself, not the floor finish. A topping or underlayment has its limits on a light-frame structure. We'll cover impact isolation ratings in more detail next month. For now, remember that any rating that ignores low frequencies may look fine on paper but fail in practice.
Flanking: how sound gets around floors and ceilings
A good rating only protects against sound traveling directly through the assembly. Sound can also flank around the assembly, and floor-ceiling systems offer many detours: a continuous topping or slab that carries energy into surrounding walls, the perimeter joint where the floor meets partitions, and shared ceiling spaces above the units.
The most overlooked sound paths are penetrations, the holes cut through assemblies for plumbing, ductwork, electrical conduits, outlets, lights, and sprinklers. Each breach compromises the layer designed to block sound, and a single unsealed penetration can undermine an otherwise excellent floor or wall. Sealing every penetration, avoiding back-to-back electrical boxes, and paying close attention to perimeter detailing are key to achieving strong sound isolation.
These are the details we look for before construction begins. On one condominium project, the contractor built a full floor-ceiling assembly as a mockup in a closet-sized space, allowing us to measure airborne and footstep sound between two levels in advance. This process identified weak spots during construction, enabling manageable fixes before costly, inconvenient changes were needed across every unit after completion.
When the number isn't enough
STC is a valuable tool, but it's only one piece of the puzzle. The right target depends on the space, the noise source, the assembly construction, and how the building will be built. Whether you're comparing assemblies, troubleshooting a noise complaint, or deciding how quiet a space should be, we address these questions every week. BABICHacoustics helps clients set realistic acoustic targets and design assemblies that deliver results.