STC Rating: How Well a Wall Stops Sound Passing Through

STC (Sound Transmission Class) is a single-number rating of how well a wall, door or glazing stops airborne sound passing from one room to the next. A higher number means better insulation. The international equivalent is Rw. Both are measured in a laboratory, so they describe the element, not the finished room.

By Dhruv Agarwal · · 1 min read

Why it matters on a project

"We can hear the next meeting room" is one of the most common complaints in a new office. It is rarely because the wrong partition was bought. It is usually because the rated partition was installed in a way that let sound go around it.

The ratings at a glance

RatingMeasuresMeasured whereStandard
STCSound blocked by an elementLaboratoryASTM E90, rated under ASTM E413
RwSound blocked by an elementLaboratoryRated under ISO 717-1
Site (field) ratingSound blocked between two real roomsOn sitee.g. ISO 16283-1
NRCSound absorbed by a surfaceLaboratorySeparate rating

What protects the rating on site

  • Partitions taken to the slab, or a ceiling-void barrier above them, so sound cannot travel over the top.
  • Sealed perimeter at head, base and abutments.
  • Sealed services penetrations, with sockets offset rather than back to back.
  • Doors with perimeter and drop seals, because the door is usually the weakest element.
  • Glazed partitions with suitable glass and frames, since a single sheet in a light frame often limits the wall.
  • No continuous duct or open grille linking two rooms without attenuation.

Common mistakes

  • Specifying a lab rating and expecting it on site.
  • Stopping partitions at the false ceiling to save cost or time.
  • Adding acoustic panels to fix privacy, which treats absorption, not insulation.
  • A heavy wall with a standard door, where the door sets the real performance.
  • Cutting the partition for cable trays late and not resealing.

Standards referenced

Laboratory measurement under ASTM E90 and rating under ASTM E413; international rating under ISO 717-1; field measurement under ISO 16283-1. Target ratings and constructions for a specific project must be set by the project's acoustic consultant.

Standards referenced

  • ASTM E90 — Laboratory measurement of airborne sound transmission loss of building partitions and elements (ASTM International)
  • ASTM E413 — Classification for rating sound insulation (ASTM International)
  • ISO 717-1 — Rating of sound insulation in buildings — airborne sound insulation (ISO)
  • ISO 16283-1 — Field measurement of sound insulation — airborne sound insulation (ISO)

Frequently asked

They measure opposite things. STC rates how much sound a partition blocks between rooms. NRC rates how much sound a surface absorbs within a room, which reduces echo. A soft acoustic panel can have a high NRC and do almost nothing for privacy, while a dense wall can have a high STC and leave the room echoey. Most meeting rooms need both.

They are close but not identical. STC is calculated under ASTM E413 from laboratory data measured to ASTM E90; Rw is calculated under ISO 717-1. They use slightly different frequency ranges and rating curves, so the numbers for the same wall are usually similar but should not be treated as interchangeable in a specification.

Because sound takes every path available, not only through the wall. It travels over the partition through a shared ceiling void, under it through a raised floor, around door seals, through back-to-back sockets and along continuous ductwork. The laboratory figure tests the wall alone; the site figure, measured under standards such as ISO 16283-1, includes all those flanking paths.

Gaps. A small unsealed opening can undo much of the benefit of a heavy wall. The common culprits are partitions stopping at the ceiling instead of the slab, unsealed service penetrations, poor perimeter sealing, and doors without proper seals.

Specify the performance of the whole construction, not only the panel: partition, door, glazing, head and base details and services penetrations. Ask whether the stated rating is a laboratory or site figure, and agree how it will be checked. The target level depends on how confidential the room needs to be and on the background noise in the adjacent space, which is a decision for the acoustic consultant and the client.

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