Supporting Reference

Control of Sound Transmission through Gypsum Board Walls for the ExAC

Construction Technology Update No. 1, written by A.C.C. Warnock and J.D. Quirt and published by the National Research Council of Canada in January 1997, distills results from tests on more than 250 gypsum-board wall assemblies. Examitect's ExAC study plan lists it as a supplementary reference for Section 3, Assemblies and Detailing.

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Control of Sound Transmission through Gypsum Board Walls overview

The NRC update at a glance

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 (six pages, including the title page)
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); not a paywalled standard

Why this update matters for the ExAC

Sound transmission between dwelling units is an architectural design decision, not a code-compliance afterthought. The 1990 edition of the National Building Code raised the minimum Sound Transmission Class (STC) between units from 45 to 50, and that change drove the research program this bulletin summarizes. Once you understand how Warnock and Quirt arrived at the principles, the NBC 9.11 numbers stop feeling arbitrary.

The major finding is direct: if the two faces of a wall are rigidly connected, sound-absorbing insulation in the cavity provides almost no benefit. Once the faces are isolated from each other through staggered studs, double studs, or resilient channels, mass and cavity depth become your primary STC levers.

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

How to study it for the ExAC

  • Read the Summary of Findings first. It lists the five design moves that raise STC and is the most exam-ready paragraph in the bulletin.
  • Learn the seven wall families and which provide isolation on their own: staggered studs, double studs, and non-load-bearing steel studs all isolate without resilient channels.
  • Internalize the isolation rule: if both faces are rigidly connected to the same studs, cavity insulation is nearly useless.
  • Memorize the rough numbers: doubling mass adds about ten STC points; doubling cavity depth adds ten with insulation, four without; 400 to 600 mm spacing adds one or two.
  • Note the two common errors: internal layers inside a double-stud cavity reduce STC, and resilient channels between two layers of gypsum board also reduce STC. Both appear as wrong-answer distractors.
  • Pair the reading with NBC 2020 Part 9.11. The Update predates the current code but its principles are baked into the NBC's STC tables.

ExAC sections this update supports

  1. Section 2

    Background for NBC 2020 Part 9.11. The STC tables you apply under Section 2 questions trace back to the NRC research summarized in this bulletin.

  2. Section 3

    Listed as a supplementary reference for category 8.3, Assemblies and Detailing. This is the main exam use case for the bulletin.

  3. Section 4

    Indirect use. Helps with construction-review judgment when site conditions (missed resilient channels, compressed insulation) threaten an assembly's STC rating.

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 from a wall section is half the work on an exam question.

Wall system Stud and cavity Isolation strategy
Single wood studs 38 by 89 mm studs, 89 mm cavity, 400 or 600 mm on centre None. Both faces fasten to the same studs. Structure-borne transmission dominates and sound-absorbing insulation gives only a small benefit.
Single wood studs with resilient channels 38 by 89 mm studs, 102 mm effective cavity (with channel offset), 400 or 600 mm on centre Resilient channels on one face decouple the gypsum board from the studs and allow cavity insulation to contribute meaningfully.
Staggered wood studs Two rows of 38 by 89 mm studs on a common 38 by 140 mm plate, 140 mm cavity Each face fastens to 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, reaching the highest STC values in the staggered-stud family.
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 of any wood-stud configuration.
Non-load-bearing steel studs 25-gauge 64 mm or 92 mm steel studs, 64 mm or 92 mm cavity (equal to the stud depth), 400 or 600 mm on centre The studs themselves are flexible enough to reduce sound transmission. Performance is roughly equivalent to a wood-stud wall with resilient channels on one face.
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 decouple the faces and 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 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. Higher is better; STC 50 is the NBC minimum between dwelling units.
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 surfaces 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. The deepest cavities (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, giving built-in isolation.
Non-load-bearing steel studsLight-gauge (25-gauge) steel studs flexible enough to reduce sound transmission through the studs themselves. Roughly equivalent to a wood-stud wall with resilient channels on one face.
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 a properly 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 with an empty cavity.
1x1, 1x2, 2x2 layupsShorthand for layers of gypsum board on each face. 1x1 means one layer each side; 1x2 (asymmetrical) means one layer on one side and two on the other; 2x2 means two layers each side.
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 the two faces.
Sound-absorbing materialGlass fibre, rock fibre, or cellulose-fibre insulation in the wall cavity. Only effective when the faces are isolated. Type of material matters far less than installation quality and coverage.
Internal layerAn extra layer of gypsum board placed inside a double-stud cavity. Reduces STC because it rigidly connects to the studs and shrinks the effective cavity depth.
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 rating.

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 five design moves that raise STC. Read it first, then let the body of the document explain each one. The summary alone answers a large fraction of party-wall exam questions.

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 and shows up in scenarios where an architect specifies expensive insulation and gets almost nothing for it.

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 points. Cavity doubles with insulation, STC up about ten points. Cavity doubles without insulation, STC up about four points. Spacing goes from 400 to 600 mm, STC up one or two points. These small numbers carry many 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 connects rigidly to both stud rows and shrinks the effective cavity depth. Exam-question authors use this one regularly.

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 assembly. Placing them between layers reduces STC rather than raising it.

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; the two documents reinforce each other on stud configurations and cavity strategies.

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 field test between two units. The wall is single 2x6 studs with two layers of gypsum board on 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 performance. 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. Of four assemblies with the same total gypsum-board mass, which one 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 of the party wall?
  • A field sound test underperforms the lab STC of the same wall by eight points. 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 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 is forHow this update relates
Control of Sound Transmission through Gypsum Board Walls (NRC) Principles and tested assembly families for STC in gypsum-board partitions in multi-family construction. The supplementary reference on this page. The vocabulary anchor for party-wall acoustics on the ExAC.
Guide for Sound Insulation in Wood Frame Construction NRC guide on STC and Impact Insulation Class (IIC) for wood-frame walls and floors. Companion Section 3 supplementary. Adds impact insulation and floor-ceiling assemblies that this update does not cover.
Fire Resistance of Gypsum Board Wall Assemblies NRC 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 9.11 National 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 across all building types. 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 principles. For the physics behind isolation, mass law, coincidence, and resilient mounting. Chapters 9 and 10 are the deep dive if you want the theory.

How Examitect reinforces gypsum-wall acoustics

Reading the bulletin gets you most of the way there. 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 rather than hunting through the whole bulletin.

You also get full-length mock exams that mirror ExAC pacing and free study notes covering every section. Try a few sample questions first to see how the NRC principles show up in exam format, then check pricing when you want access to the full question bank.

FAQ

NRC sound transmission 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 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 that account for flanking transmission paths.

The NRC research 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 choice, 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 and multi-family construction.

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.

Yardsticks

Hanscomb Yardsticks for Costing. Canadian elemental cost data by building type and region, for Section 1 Cost Management.

Supporting references

The extended reading list of supplementary references that show up across Examitect's ExAC study plan.