Sound transmission through gypsum walls

Placeholder page for the supporting reference Sound transmission through gypsum walls, part of the Examitect reading list for the ExAC.

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The NRC sound transmission update at a glance

Here's the at-a-glance summary an Intern Architect can scan before opening the PDF.

Full titleControl of Sound Transmission through Gypsum Board Walls
AuthorsA.C.C. Warnock and J.D. Quirt, Acoustics Laboratory, NRC Institute for Research in Construction
PublisherNational Research Council of Canada, Institute for Research in Construction (IRC)
Series and numberConstruction Technology Update No. 1, ISSN 1206-1220
PublishedJanuary 1997
LengthShort bulletin (about six pages of body content)
LanguagesEnglish (the NRC Publications Archive also lists French equivalents for many Construction Technology Updates)
Primary audienceArchitects, engineers, builders, and code authorities working on multi-family residential construction in Canada
ExAC relevanceSection 3 supplementary on Examitect's ExAC study plan under Assemblies and Detailing (8.3). Sits beside the wood-frame sound insulation guide and the fire-resistance gypsum-wall guide.
Where to accessNRC Publications Archive, free download (DOI 10.4224/40002867). The document is in the public technical literature, not a paywalled standard.

Why this update matters for the ExAC

Sound transmission between dwelling units is an architectural design decision, not just a code-compliance afterthought. The 1990 edition of the NBC raised the minimum STC between units from 45 to 50, and that bump drove the entire NRC research program this bulletin summarizes. Once you understand how Warnock and Quirt got to the principles, the NBC 9.11 numbers stop feeling arbitrary.

Examitect's ExAC study plan lists this Update as a Section 3 supplementary reference under Assemblies and Detailing (8.3). It sits next to two related supplementary documents on the same plan: the NRC's Guide for Sound Insulation in Wood Frame Construction and Fire Resistance of Gypsum Board Wall Assemblies. The three together form the Canadian foundation for partition-wall design questions on the exam.

You will not be asked to derive an STC. You will be asked which assembly choice raises the rating, which detail compromises it, and which fix an architect would specify when a project fails a field sound test.

ExAC sections this reference supports

Section 1
Section 2
Section 3
Section 4

What the NRC update is

Control of Sound Transmission through Gypsum Board Walls is a six-page Construction Technology Update from the National Research Council's Institute for Research in Construction. It was written to translate a much longer, industry-supported research project into a single bulletin a designer can read in under an hour. The research consortium included Canada Mortgage and Housing Corporation, the Canadian Sheet Steel Building Institute, the Cellulose Insulation Manufacturers Association of Canada, Forintek, the Gypsum Manufacturers of Canada, Owens Corning Fiberglas Canada, and Roxul.

The project tested over 250 gypsum-board wall assemblies in NRC's acoustic laboratory and measured the Sound Transmission Class (STC) of each. The variables studied were the type, density, and thickness of the gypsum board, symmetrical and asymmetrical layer arrangements, wood and steel stud configurations, resilient channel placement, the type and amount of cavity insulation, and cavity depth. Fire resistance was tested on some of the same assemblies, which is why this document has a companion bulletin, Fire Resistance of Gypsum Board Wall Assemblies.

The research findings were folded into the new STC tables introduced in the 1995 edition of the National Building Code of Canada. Those tables are the direct ancestor of the acoustic provisions you read in NBC 2020 Part 9.11 today.

Inside the update, the seven wall systems tested

The bulletin organizes its tested assemblies into seven basic wall families, each with one-layer and two-layer variants and with or without cavity insulation. Recognizing the family is half the work on an exam question.

Wall systemStud and cavityIsolation strategy
Single wood studs 38 by 89 mm studs, 89 mm cavity, 400 or 600 mm on centre None. Both faces fastened to the same studs, so structure-borne transmission dominates. Sound-absorbing insulation in the cavity gives only a small benefit.
Single wood studs with resilient channels 38 by 89 mm studs, 102 mm cavity (with the channel offset), 400 or 600 mm on centre Resilient channels on one face decouple the gypsum board from the studs and let the cavity insulation contribute.
Staggered wood studs Two rows of 38 by 89 mm studs on a common 38 by 140 mm plate, 140 mm cavity Each face is on a different line of studs. Mechanical separation is built in without resilient channels.
Staggered wood studs with resilient channels Two rows of 38 by 89 mm studs on a 140 mm plate, 153 mm effective cavity Staggered framing plus resilient channels add another decoupling layer for the highest staggered-stud STC values.
Double wood studs Two rows of 38 by 89 mm studs on separate plates set 25 mm apart, 203 mm cavity Full structural separation. The deepest cavities in the test program and the highest STC potential.
Non-load-bearing steel studs 25-gauge 64 mm or 92 mm steel studs, 400 or 600 mm on centre The studs themselves are flexible enough to provide isolation. STC behaves similarly to a wood-stud wall with resilient channels.
Load-bearing steel studs with resilient channels 20-gauge or heavier 92 mm or 152 mm steel studs, channels on one face Heavier steel studs behave like wood studs, so resilient channels are needed to control sound through the studs.

Doubling the gypsum-board mass per unit area raises the STC by about ten points on a properly isolated wall. Doubling the cavity depth raises it about ten points when the cavity is filled with insulation, or about four points when empty. Going from 400 mm to 600 mm stud or channel spacing usually adds one or two points.

Key terms every ExAC candidate should know

The exam reuses this vocabulary without redefining it. Learn the terms early so you spend exam time picking the answer, not parsing the question.

TermWhat it means
Sound Transmission Class (STC)A single-number laboratory rating of how well a wall resists airborne sound transmission, measured per ASTM E90 and E413.
Resilient channelA thin steel strip with a stepped profile, about 25 mm wide. One leg fastens to a stud, the other to the gypsum board. Decouples the two and reduces structure-borne transmission.
Double-stud wallTwo rows of studs on separate plates, typically set 25 mm apart. Provides full mechanical separation between faces. Largest cavity (203 mm) and highest STC potential in the NRC tests.
Staggered-stud wallTwo rows of studs offset on a common, wider plate. Each gypsum-board face is supported by a different line of studs.
Non-load-bearing steel studsLight-gauge steel (25 gauge) studs flexible enough to reduce sound transmission through the studs themselves. Roughly equivalent to a wood-stud wall with resilient channels.
Mass per unit areaWeight of gypsum board in kg/m². STC rises with the logarithm of mass; doubling mass raises STC by about ten points on an isolated assembly.
Cavity depthDistance between the inner faces of the gypsum board. Doubling cavity depth raises STC about ten points (with insulation) or four points (empty cavity).
1x1, 1x2, 2x2 layupsShorthand for layers of gypsum board on each side. 1x1 means one layer each face; 1x2 (asymmetrical) means one layer on one side, two on the other; 2x2 means two layers each face.
Stud spacing400 mm or 600 mm on centre. Wider spacing improves STC by one to two points by reducing the number of rigid connections between faces.
Sound-absorbing materialGlass fibre, rock fibre, or cellulose-fibre insulation in the cavity. Effective only when faces are isolated. Type matters far less than installation quality.
Internal layerAn extra layer of gypsum board or fibreboard placed inside a double-stud cavity. Reduces STC because it rigidly connects to the studs and shrinks the effective cavity.
Flanking transmissionSound that bypasses the separating wall along header and sole plates, ducts, or framing. The reason field performance is typically lower than the lab STC.

How the NRC update compares to other ExAC references

This bulletin is short and focused. The references below sit around it on Examitect's ExAC study plan. Use the table to decide which document answers which kind of question.

ReferenceWhat it's forHow this update relates
Control of Sound Transmission through Gypsum Board Walls (NRC)Principles and tested assembly families for STC in gypsum-board partitions.The supplementary reference on this page. The vocabulary anchor for party-wall acoustics.
Guide for Sound Insulation in Wood Frame ConstructionNRC guide on STC and IIC for wood-frame walls and floors.Companion Section 3 supplementary. Adds impact insulation (IIC) and floor-ceiling assemblies that this update does not cover.
Fire Resistance of Gypsum Board Wall AssembliesNRC bulletin on fire-resistance ratings for the same families of gypsum-board partitions.Tested on many of the same assemblies. Reminds you that an acoustic fix and a fire-resistance fix can interact, sometimes against each other.
NBC 2020 Part 9.11National Building Code sound transmission provisions: minimum STC and apparent STC between dwelling units.Sets the code minimums. The NRC tables in NBC trace back to the research summarized in this Update.
CHING (Building Construction Illustrated)Diagrammatic reference for assemblies and detailing.CHING shows the typical wall section drawings; this NRC update tells you what each version of the wall is worth acoustically.
Architectural Acoustics (Long)University-level technical text on architectural acoustics.If you want the physics behind isolation, mass law, coincidence, and resilient mounting, Long's chapters 9 and 10 are the deep dive.

How to study the NRC update for the ExAC

  • Read the Summary of Findings list first. It captures the six design moves that raise STC and is the most exam-ready paragraph in the document.
  • Learn the seven wall families and which provide isolation by themselves. Single wood studs do not. Staggered studs, double studs, and non-load-bearing steel studs do. Resilient channels add isolation to single-stud assemblies.
  • Internalize the isolation rule. If both faces are rigidly connected, cavity insulation does not help. The exam likes scenarios where an architect specifies expensive insulation in a rigidly connected wall and gets nothing for it.
  • Remember the rough numbers. Doubling mass adds about ten STC points. Doubling cavity depth adds about ten with insulation, four without. Going from 400 mm to 600 mm spacing adds one or two.
  • Note the two common errors. Internal layers reduce STC. Resilient channels placed between two layers of gypsum board reduce STC. Both show up as wrong-answer distractors.
  • Pair the reading with NBC 2020 Part 9.11. The Update predates the current code, but the principles and the code's STC tables share the same NRC research lineage.

ExAC sections this update supports

The table below shows where the bulletin shows up on Examitect's ExAC study plan and how it backs adjacent categories.

ExAC sectionWhere this update appears on Examitect's study plan
Section 1
Design and analysis		Indirect. Useful background for engineering coordination and schematic decisions on multi-unit residential projects, but not listed on Examitect's study plan for any Section 1 category.
Section 2
Codes		Background for NBC 2020 Part 9.11. The STC tables you apply under Section 2 trace back to the NRC research summarized here.
Section 3
Sustainability and final project		Listed as a supplementary reference for category 8.3, Assemblies and Detailing. The main exam use case.
Section 4
Construction and practice		Indirect. Helps with construction-review judgment when site conditions threaten an assembly's STC rating, such as missed resilient channels or compressed insulation.

Tips for Intern Architects reading the update

This is one of the shortest documents on Examitect's ExAC study plan. The reading is quick; the trick is remembering the right details under exam pressure.

Tip 1, start with the Summary of Findings. The bulleted summary near the end of the bulletin lists the six design moves that raise STC. Read it first, then let the body of the document explain each one.

Tip 2, lock in the isolation rule. If both faces of the wall are rigidly connected to the same studs, cavity insulation is wasted money. This single principle answers a surprising number of party-wall questions.

Tip 3, learn the seven wall families by shape, not by name. The architectural drawings in the bulletin are easier to recognize than the labels. On an exam wall section, ask yourself: is this single, staggered, double, or steel? Are channels present? That tells you which family of STC ratings to expect.

Tip 4, memorize the doubling rules. Mass doubles, STC up about ten. Cavity doubles with insulation, STC up about ten. Cavity doubles empty, STC up about four. Spacing 400 to 600, STC up one or two. These small numbers carry exam questions.

Tip 5, beware the internal-layer trap. Adding a layer of gypsum board inside a double-stud cavity sounds like it should help acoustics. It does not. It rigidly connects the layer to both stud rows and shrinks the cavity. Exam-question authors love this one.

Tip 6, never put resilient channels between two layers of gypsum board. The bulletin says this explicitly. Channels go between the studs and the first gypsum layer, not sandwiched inside the wall.

Tip 7, read it next to the wood-frame sound guide. Together with the NRC's Guide for Sound Insulation in Wood Frame Construction, this bulletin covers the airborne side of Canadian party-wall design. Read them back to back and the two will reinforce each other.

Common ExAC scenarios where this update is the answer

The scenarios below show up in different wording across ExAC sittings. If you recognize the pattern, the NRC findings tell you how to think about the answer.

  • A condo project is failing the STC test between two units. The wall is single 2x6 studs with two layers of gypsum each side. What is the most cost-effective change the architect would investigate first?
  • A contractor proposes adding an internal layer of gypsum board inside a double-stud party wall to improve sound. What is the design response?
  • A duplex specifies rock-fibre insulation in a single-stud party wall and the owner asks why it does not perform better than the empty wall next door. How does the architect explain?
  • A multifamily project needs STC 50 between units. Which of four assemblies, with the same total mass, will reach the target?
  • A subtrade asks whether resilient channels can be installed between two layers of gypsum board to save framing depth. What does the architect say?
  • A renovation switches from 400 mm to 600 mm stud spacing for cost reasons. What happens to the STC rating?
  • A field sound test underperforms the lab STC of the same wall. What flanking paths would the architect inspect first along the header and sole plates?

Each scenario maps back to a specific NRC finding: isolation, mass, internal layers, channel placement, spacing, or flanking. Read the bulletin once with these scenarios in mind and the connections cement quickly.

How Examitect reinforces gypsum-wall acoustics

Reading the bulletin gets you most of the way. The other half of the work is recognizing party-wall patterns under timed conditions. Examitect's question bank includes scenario questions on STC, resilient channels, double and staggered studs, cavity insulation, and the NBC 9.11 minimums. Each answer explanation points back to the relevant NRC finding or code clause, so you can re-read just the few pages you need.

You also get full-length mock exams that mirror ExAC pacing and free study notes for every section. Try a few sample questions first, then check pricing when you want the full bank.

NRC sound transmission and ExAC FAQ

Control of Sound Transmission through Gypsum Board Walls is Construction Technology Update No. 1, a short technical bulletin written by A.C.C. Warnock and J.D. Quirt and published by the National Research Council of Canada's Institute for Research in Construction in January 1997. It summarizes results from an industry-supported research project that tested more than 250 gypsum-board wall assemblies to identify the parameters that drive Sound Transmission Class ratings in multi-family construction.

It is a supplementary reference on Examitect's ExAC study plan. The plan lists it under Section 3, Assemblies and Detailing, alongside Fire Resistance of Gypsum Board Wall Assemblies and Guide for Sound Insulation in Wood Frame Construction. The three NRC bulletins together cover the sound and fire performance of gypsum-board partitions tested for Canadian construction.

The major factor controlling sound transmission is the isolation of the gypsum-board layers on each face of the wall. If at least one face is not resiliently supported or the two faces are not isolated from each other, sound-absorbing material in the cavity is rendered ineffective. Once isolation is provided, STC can then be raised by increasing mass, cavity depth, and absorptive insulation.

The 1990 edition of the National Building Code raised the minimum STC between dwelling units from 45 to 50, and the 1995 NBC incorporated new STC tables based on the NRC research that this Update summarizes. NBC 2020 Part 9.11 continues to set minimum sound transmission ratings for separating assemblies in residential buildings, with apparent STC provisions covering flanking transmission.

The NRC preliminary study found that the type of absorptive material has a relatively minor effect on the STC rating. Glass fibre, cellulose fibre, and rock fibre all performed similarly in cavities with properly isolated faces. Installation quality matters more than material brand, and stiff or overfilled cavities can actually reduce performance because sound transmits through the absorbent itself.

It is a short bulletin, so a careful read takes under an hour. Focus on the Summary of Findings list, the role of resilient channels and staggered or double studs, the mass per unit area relationship, and the warning against internal layers and against placing resilient channels between two layers of gypsum board. Pair it with NBC 2020 Part 9.11 and the wood-frame sound insulation guide to round out party-wall coverage.

The ExAC focuses on architectural decisions, not acoustic engineering math. Expect questions that ask which assembly choice raises or lowers the rating, which assemblies meet a code minimum, or which detail an architect would flag during construction review. You will not be asked to derive an STC from first principles.

The Update predates NBC 2020 but the underlying principles, isolation, mass, cavity depth, and absorption, have not changed. The research fed directly into the NBC 1995 STC tables and remains the foundation of how Canadian designers think about gypsum-board partition acoustics. Always pair it with the current NBC Part 9.11 provisions for code-compliant numbers.

Keep exploring

Other ExAC reference books.

Control of Sound Transmission through Gypsum Board Walls is one of many supplementary references on Examitect's ExAC study plan. The core reading list runs through the six primary references below.

CHING

Building Construction Illustrated. The visual reference for assemblies, materials, and detailing across Sections 1 and 3.

CHOP

Canadian Handbook of Practice. The RAIC's practice reference covering most categories in Sections 1, 3, and 4.

NBC 2020

National Building Code of Canada. Part 9.11 carries the sound transmission provisions for housing.

NECB

National Energy Code of Canada for Buildings. Supports the envelope and energy questions in Section 2.

RSMeans

Construction cost data. Used for elemental estimating and cost-management questions in Section 1.

Supporting references

The extended reading list of supplementary references that show up across the ExAC reading list.