One of the most difficult challenges many architects face is to provide a satisfactory wall assembly without taking up too much space. There are a number of considerations, including thermal, structural, and aesthetic, among others. However, one of the most difficult attributes to add to a wall is Increasing the STC (Sound Transmission Class) in a thin, single-stud wall.

As has been widely documented, the primary ways to improve the STC of a wall is to 1) Add Mass, and 2) Add Flexibility (Decoupling). Adding mass helps to build a thicker, denser barrier to reflect sound back towards its source, while flexibility helps to break the rigid connection between wall elements, through which sound can pass easily. Let us examine each of these independently:

Key Elements to Increase STC of a Wall:

1. Add Mass: A standard rule of thumb is that a wall’s STC improves by 6 dB every time its mass is doubled. This is why CMU walls perform so much better than single-stud gypsum partitions. NOTE: This should be considered a low-STC approximation of a law, since it is true for very thin barriers, but drops off rapidly above STC 40.
2. Add Flexibility: This may be done in a number of ways, including adding resilient mounts (not resilient channels, due to their inherently difficult installation), staggering the studs, or adding limp-mass membranes.

It is extremely counter-intuitive to many contractors and design professionals that a 20 gauge stud wall has a LOWER STC than a 25 gauge stud wall, despite being stronger. Conventional wisdom says that being stronger, it should reflect more, and after all, isn’t that more mass? While there is a minor difference in mass, it is allocated only at the stud locations; meanwhile, sound can travel easily through the high-strength stud “connectors” in the wall (like an electrical short-circuit), allowing conversations to be easily heard in adjacent rooms. For the same reason, a wall with studs at 24″ oc has a higher STC than a wall with studs at 16″ oc. That leads us to an old soundproofing proverb:

Proverb in Soundproofing – A stronger wall doesn’t block more sound, a stiffer wall transmits more sound. 

Most Common Mistakes in Soundproofing – Undermining STC Ratings:

1. Adding Drywall: Layering on additional panels of drywall is an extremely inefficient way to improve the STC of a wall. Since the drywall is rigidly connected via drywall screws and direct contact, the sound travel directly through the gypsum board itself. On wood stud walls, each layer of drywall only adds 1-2 points. On metal stud walls, this is 1-3 points.
2. Adding Mineral Wool: Fiberglass batting is acceptable for the vast majority of wall types. Mineral wool does have some low-frequency performance benefits, mainly for mechanical rooms, but should only be used when specified by an acoustic consultant.
3. Using thicker gauge studs: As mentioned above, after ten years of reviewing wall assemblies, this is probably one of the most curious oversights that often arises. Contractors have a tendency to use thicker studs in an effort to “make the walls stronger”, and thus block more sound. While not obvious, this does the exact opposite – heavy gauge studs make walls block less sound! Use 25 gauge studs to improve the STC of your wall. It is cheaper than 16-gauge or 20-gauge, and adds 4-7 STC points to the wall.
   

   

Top Ways to Increase Wall STC:

1. Higher Gauge: By moving to lighter gauge stud, such as 25-gauge, the flexibility of the wall greatly improves. This greatly increases the STC of the wall, without adding much cost. On “average”, a 25-gauge wall will block 6-7 dB more than the same wall with 20-gauge studs.
2. Space Out Studs: Move studs from 16″ o.c. to 24″ o.c., where possible. Like lighter gauge studs, this increases the flexibility and STC of the wall.
3. Increase Depth of Studs: 6″ 25-gauge studs may offer 3-4 STC point improvement over the same 3-5/8″ stud. However, because of the stiffness of wood studs, moving from 2×4 to 2×6 offers little to no improvement.
4. Add Soundproofing Membranes: If you need to achieve an STC of 50 or higher, consider using a soundproofing membrane. These products offer decoupling by separating the drywall from studs, and also significant mass increases.

How to Determine STC of a Wall:

There are a number of ways to determine the STC of a wall. These range from modeling STCs with industry software, to using lab test data and attempting to extrapolate or interpolate data based on heuristics. Often times the best option is to combine two or more approaches, and list an STC range on your wall schedule or analysis to underscore the unknown STC range of a partition.

1. Use an STC calculator that predicts ratings based on standard rules of thumb and underlying lab test data. An STC calculator can show you that 25 gauge studs perform better than 20 gauge, which are better than wood stud walls. Remember: to achieve the highest STC, a strong wall that is also flexible will provide the best performance.
2. Use an existing repository of STC lab test data, also available on third party sites.
3. Create Sound Rating models using a third party software, such as INSUL. See a video on how INSUL is used to estimate STC ratings below.

Why is STC 50 important?

Ultimately, the rating of STC 50 and higher is critical because it meets the International Building Code for Multi-Family properties. At an STC 50, loud voices may be faintly heard, and overall, good speech privacy is achieved between adjacent apartments and dwellings.

STC 50 is only required on demising and corridor walls, where a dwelling unit attaches to another dwelling unit, or to a public space (corridor, building exterior, etc.). In the majority of cases, there is not STC requirement for partitions within a single dwelling unit.

Summary: Increasing STC Ratings

STC ratings are difficult to understand, and once understood, achieve for a number of reasons. There are a wide range of materials on the market place claiming to improve STC ratings, from mineral wool substitutes to vinyl barriers designed to add wide-range attenuation.

Use common rules of thumb, including moving to lighter gauge studs and spacing studs further apart. Then consider adding soundproof membranes and other soundproofing materials to improve your wall STC to 50, 55, or beyond.