Acoustics

6 variables that affect STC ratings

May 19, 2023

Understanding the basic concepts of acoustical wall designs

Noisy neighbors exist everywhere, from the apartment next door to the conference room across the hall and the neighboring exam room at the doctor’s office. Reducing sound transmission from one room to another and from outside to inside is beneficial to occupant health and productivity.

However, choosing the right acoustical wall design to balance code, material and client requirements isn’t always straightforward. Altering just one variable in the assembly can change its overall Sound Transmission Class (STC) rating, but each variable differs in its effectiveness. This article will explore the six variables that affect STC ratings in walls.

What is an STC rating?

Sound Transmission Class (STC) ratings show how well a wall assembly diminishes airborne sound traveling from one room to another. Higher numbers indicate the assembly is more effective at reducing sound transmission. These ratings, however, apply only to the aggregate wall assemblies and not their individual components. The International Building Code designates an STC rating of 50 as the minimum allowable design rating for unit-to-unit multifamily construction, but sometimes a higher rating is desired.

How is sound transmitted?

Sound is energy transmitted by vibration through a medium. It travels fastest through solids, followed by liquids, and is slowest through air. Think back to elementary school. The sound that comes from tapping a pencil on a desk seems much louder when you put your ear to the desk, because the sound waves reaching your ear are traveling through the desk itself rather than through the air.

STC values come from testing wall assemblies according to the ASTM E90 Standard Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. Here’s how it works:

Testers mount the wall assembly in a frame between two rooms: a source chamber and a receiving chamber.
Testers play a sound in the source chamber.
A microphone in the receiving chamber measures the decibel level of the sound. The difference in the decibel level on either side of the wall is the transmission loss.
The test measures 16 frequencies between 125 Hz and 4000 Hz, the range to which human ears are sensitive.
Testers plot the values on a graph to form a curve. Where that curve intersects the STC reference curve at 500 Hz is the STC number assigned to the assembly.

Pro tip: The ASTM E90 test report includes results for each individual frequency tested, so ask for the full report if a specific frequency is of interest.

What can be changed in a wall assembly to affect its STC rating or value?

A basic wall assembly consists of wood or metal studs, insulation, fasteners (screws or nails) and gypsum board. The size and spacing of the studs, the type, thickness and amount of gypsum board or the thickness of the insulation can all be changed to affect the wall assembly’s STC rating. Other tools include resilient channels and resilient isolation clips used to decouple the two sides of a wall assembly.

There are six variables that can be changed to affect an acoustical wall assembly’s STC rating:

Mass
Cavity absorption
Cavity depth
Stiffness
Decoupling
Damping

1. Mass

A common way to add mass is to use thicker gypsum board and/or add more layers – 5/8” gypsum board will transmit less sound than ½”. However, there are diminishing returns when it comes to adding mass.

Remember that a heavy, solid door will transmit less sound than a light, hollow-core door. Thermal, energy-efficient windows can also improve the STC of an exterior wall partition.

2. Cavity Absorption

Adding more insulation can have a profound effect on a wall assembly’s STC rating because it adds sound-absorbing material to the assembly. In the examples below (figures 6 and 7 in The SoundBook), going from an uninsulated wall to an insulated wall boosts the STC rating 8 points.

Fiberglass and mineral wool insulation contribute the biggest gains. Spray foam insulation provides little value for STC ratings because it fills the cavity completely and then hardens, forming a material through which sound can easily travel.

3. Cavity Depth

Using deeper studs creates space for more insulation to be added, which can boost the assembly’s STC rating.

The two steel-frame wall assemblies below illustrate this point. On the right, 6” steel studs provide space for additional insulation in the wall, adding four points to the assembly’s STC rating.

4. Stiffness

The wall assembly’s stiffness will affect the STC rating in a big way. Controlling stiffness is often the most challenging aspect of selecting an acoustical wall assembly.

How to mitigate stiffness in a wall assembly

Alter stud spacing: 24" o.c. studs will have a higher STC rating than 16" o.c. framing.
Choose metal studs: Metal studs are inherently more flexible than wood studs.
Use thinner gauge studs: A thinner stud gauge provides better STC performance than a thicker stud gauge. Smaller width studs are less stiff than larger width studs.

These approaches can be used to achieve the desired STC rating and comply with other factors such as height limitations from a stud manufacturer.

What are EQ studs and how do equivalent studs affect a wall assembly’s STC rating?

EQ studs have a thinner metal thickness than their equivalent standard-gauge counterparts but have the same yield strength. Consider these comparisons:

Standard 25-gauge vs. 25-gauge EQ studs: A standard 25-gauge stud is 18 mils thick, while a 25-gauge EQ stud is only 15 mils thick.
Standard 20-gauge vs. 20-gauge EQ studs: A standard 20-gauge stud is 30 mils thick, while a 20-gauge EQ stud is 18-20 mils thick.
20-gauge EQ studs: 20-gauge EQ studs have replaced standard 25-gauge studs as the most commonly used stud in non-loadbearing, interior steel stud partitions. They are stiffer despite being a similar thickness to standard 25-gauge studs.

Be aware that EQ studs may be submitted regardless of whether the chosen wall assembly includes them. The SoundBook includes wall assemblies with standard thickness and EQ studs.

5. Decoupling

Sound moves more easily through solids than air. By decoupling or separating different parts of the wall assembly, air gaps help to slow down sound waves.

Chase walls are one method of decoupling. They have two rows of studs that separate the two sides of the wall assembly. Not only are the two sides of the wall separated, but chase walls also can have two layers of insulation to absorb sound waves. One drawback of chase walls is they take up more floor space. In the example below (figures 8 and 137 in The SoundBook) see the chase wall provides a 14 point increase in STC rating from a standard wall assembly.

If floor space is at a premium, adding a resilient channel or resilient isolation clips provide another decoupling option. A resilient channel is a light gauge horizontal framing member used to attach the gypsum board to the studs. Resilient channels come in two shapes: RC-1 has one leg that attaches to the stud, and the profile of RC-2 has two legs that attach to the stud. Both types of resilient channels separate the board from the stud to reduce the amount of solid material through which sound can travel. In the comparison below, the resilient channel boosts the STC rating by 5 points.

Resilient isolation clips have a rubber disc on the back that does the decoupling. They are used with a rigid furring channel to attach the gypsum board to the studs and are more effective than resilient channels at boosting STC rating, as seen in the example below comparing the use of a resilient channel on the left and resilient isolation clips on the right (figures 16 and 20 in The SoundBook).

6. Damping

Damping is the ability to dissipate vibrational energy produced by sound waves. Gold Bond^® SoundBreak XP gypsum boards consist of a layer of viscoelastic polymer in between two layers of gypsum board, which work together to provide sound damping. The viscoelastic polymer dissipates the sound waves in the form of heat. And because the viscoelastic layer is added during the manufacturing process — and not on the work site — it’s evenly applied for consistent sound damping throughout the board.

Sound-damping gypsum board handles, installs and finishes just like conventional gypsum board. Replacing just one layer of SoundBreak XP could boost the STC rating by 11 points, as seen in the example below.

Keep in mind that the wall assembly is what’s tested — not individual products — so adding SoundBreak to different assemblies will have different effects on their respective STC ratings. Additionally, remember that adding SoundBreak or resilient isolation clips to a chase wall will have negligible effects, since the wall’s sides are already decoupled.

With over 300 tested acoustical assemblies, The SoundBook saves time in the design phase. Download your copy today.