GA Corner
Selecting GA-600 Assemblies: A Practical Guide to STC Targets and Sound Control Tradeoffs
A field-focused overview of how STC ratings are organized in GA-600 and how contractors can filter assemblies by fire rating, acoustic target and constructability to reduce risk and meet performance expectations.
Question: Can you explain STC ratings and how to select the right GA-600 assembly for a project I am working on?
Answer: Absolutely! In practice, several related questions tend to come up as you evaluate assemblies and performance tradeoffs. Let’s start with the basics.
What is STC, and what does it actually tell me?
Sound transmission class is a laboratory rating that measures how well a wall, floor-ceiling, or roof-ceiling assembly reduces airborne sound such as speech, television, or general background noise. A higher STC value generally indicates better sound reduction.
However, STC does not represent low-frequency bass performance well, nor does it account for field workmanship, penetrations, or flanking paths, all of which can significantly affect real-world results.
Where do I find STC-rated assemblies in the GA-600 Fire Resistance and Sound Control Design Manual?
In GA-600, STC-rated assemblies are found within the system listings, not in a standalone “STC section.” Refer to Section IV: Fire Resistance and Sound Control Systems, where wall partition, floor-ceiling, and roof-ceiling assemblies are organized by construction type. Within each listed assembly, the Sound Transmission Class (STC) value is shown alongside the fire-resistance rating and other performance data. Systems are grouped in ranges according to their STC. The higher ranges are shown first. All sound tests referenced were conducted according to the requirements of ASTM E90. The STC ratings in the Manual are grouped as follows: 40–44, 45–49, 50–54, etc.
To locate them efficiently, start with the Table of Contents or the Index to Systems by STC Rating, which directs you to the system categories. Then, review the individual assembly tables for the STC values. If you’re using the digital version of GA-600, a quick search for “STC” can help identify assemblies that meet a specific acoustic target.How do I pick the “right” STC target for my project?
Start with the owner’s performance expectations (speech privacy vs. general noise control), then confirm any code or project requirements. Multifamily, hospitality, healthcare, and education projects often have higher acoustic expectations and may warrant higher STC targets.
What’s the fastest way to narrow it down to the right GA-600 system?
A simple four-step filter works well:
- Assembly type: partition vs. floor-ceiling vs. shaft or enclosure.
- Constraints: thickness, stud depth, structural requirements, and fire rating (if applicable).
- Target STC: identify GA-600 systems that meet or exceed the target.
- Constructability: select the simplest system that meets the target (fewer “moving parts” typically means fewer field failures).
Which assembly upgrades typically increase STC in GA-600-type systems?
When listed and tested as part of the system, common STC-enhancing strategies include:
- Decoupling one face (e.g., resilient channels) to reduce mechanical sound paths
- Adding Insulation in the cavity (type and density matter)
- Increasing mass with additional gypsum layers
- Using deeper studs or greater separation, with limitations and tradeoffs to consider
If I “improve” the wall in the field, will STC automatically improve?
Not necessarily. Modifications can change performance, and only the listed configurations should be treated as the rated system. Some changes may help improve performance (i.e., adding layers of gypsum board, resilient channel, deeper studs or sound isolation clips) while others can reduce it (i.e., stiffer framing, tighter framing spacing, or substitutions).
How do I coordinate STC with fire-resistance requirements?
Treat this as a dual-constraint selection process:
- First, filter assemblies by the required fire-resistive rating (if applicable).
- Then, within that subset, select the system meeting the STC target.
Use only assemblies that are tested and listed with the required ratings in GA-600 (or other acceptable listings).
Do I need STC for floors as well?
For floor-ceiling assemblies, two performance metrics typically apply:
- STC for airborne sound (voices and music)
- IIC (Impact Insulation Class) for impact sound such as footsteps
Both should be considered when evaluating occupant comfort.
Should I use STC or something else for exterior walls?
For exterior or façade assemblies exposed to traffic or aircraft noise, OITC (Outdoor-Indoor Transmission Class) is often more appropriate than STC because it better accounts for lower-frequency sound. If exterior noise is a design driver, this should be identified early in the project.
Have more technical questions? The GA’s Technical Services Department provides free technical assistance for design, construction, and code enforcement professionals. Call 301-277-8681, email info@gypsum.org, or submit your question online.
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