Quick answer
Sound transmission class (STC) is a single-number rating of how well a wall, floor-ceiling, door or window blocks airborne sound such as voices, TV and music, measured in a laboratory under ASTM E90 and calculated under ASTM E413.
Higher is quieter. A typical interior wall in a home rates about STC 30 to 35, where loud speech is still understood through it.
Quick facts
The key facts about Sound Transmission Class (STC), with sources:
Key takeaways
- STC is a lab rating for airborne noise; a higher number blocks more.
- Most interior walls in homes land around STC 30 to 35, enough to hear loud talk.
- Each step of about 5 STC points changes what you hear, from words to murmurs to near silence.
- Gaps, outlets and ducts can undo a high-rated wall, so sealing matters as much as the assembly.
- STC does not rate footsteps; impact noise through floors has its own rating, IIC.
STC in plain English
STC answers one question: how much of the talking in the next room gets through this wall? The higher the number, the more sound the wall stops. A wall at STC 25 lets normal speech through clearly. A wall at STC 55 stops shouting, according to a classic National Bureau of Standards guide 1.
The number comes from a lab. ASTM E90 is the test. It measures the sound transmission loss of walls, floor-ceilings, doors, windows and other dividers between two test rooms 2. Sound transmission loss is how many decibels quieter the sound is on the far side.
The lab measures that loss at many pitches. The NBS guide says the rating uses 16 frequency bands from 125 to 4,000 Hz, the range that covers most speech 1.
ASTM E413 then fits a standard curve to those values and reads off one number. When the test is E90, that number is called the sound transmission class 3.
STC was built around speech, radio and TV. ASTM notes the ratings track how people judge those sounds through a wall in a general way 3. It is weaker at deep bass, which comes up later on this page.
How a wall stops sound
A wall blocks airborne sound in a few ways at once. Each feature that raises STC works on one of them.
- Mass. Heavier layers are harder for sound to shake. That is why two layers of drywall beat one 1.
- Separation. When the two faces of a wall are not tied together, sound has to cross an air gap. Staggered studs, double studs and resilient channels all decouple the faces 1.
- Absorption in the cavity. Insulation in the stud space soaks up sound that bounces inside the wall.
- Airtightness. Sound goes through any hole. The NBS guide names back-to-back outlets, medicine cabinets, pipe holes and cracks at the floor and ceiling as common leaks 1.
- Depth. A deeper cavity helps. Research at the National Research Council of Canada found gypsum board mass and cavity depth were the most important factors in walls with separated faces 7.
- More massExtra or thicker drywall
- DecouplingStaggered or double studs, resilient channels
- Cavity absorptionInsulation in the stud space
- SealingCaulk gaps, treat outlets and pipes
- Cavity depthWider separation between faces
STC and its relatives: which rating are you looking at?
| Rating | What it measures | Where it comes from |
|---|---|---|
| STC | Airborne sound through one wall, floor or door in a lab | ASTM E90 data rated under E413 23 |
| ASTC | Apparent STC of a built wall, flanking paths included | ASTM E336 field test 34 |
| NIC and NNIC | Noise isolation between two real rooms | ASTM E336 field test 3 |
| IIC | Impact sound, such as footsteps, through a floor-ceiling | ASTM E492, tapping machine test 5 |
| Masonry STC | Calculated rating for concrete or clay masonry | TMS 0302 or an E90 test 5 |
Lab number vs your house
ASTM E336 explains that sound between two rooms also travels through flanking paths around the wall, such as floors, ducts and corridor doors 4. A built wall almost always tests lower than its lab STC.
Example: what you hear at each STC level
These pairs come from Table 4-1 of the National Bureau of Standards guide “Quieting” (Handbook 119, 1976). Each row lists the privacy the rating gives and a wall that reached it 1. Treat the wall builds as examples from that era, not a modern product list.
| STC | What you hear | Example wall in the NBS table |
|---|---|---|
| 25 | Normal speech easily understood | 1/4 inch wood panels on each side of 2x4 studs |
| 30 | Normal speech audible but not intelligible | 3/8 inch gypsum board on one side of 2x4 studs |
| 35 | Loud speech audible and fairly understandable | 5/8 inch gypsum board on both sides of 2x4 studs |
| 40 | Loud speech audible but not intelligible | Two layers of 5/8 inch gypsum board on both sides of 2x4 studs |
| 45 | Loud speech barely audible | Staggered 2x3 studs with two layers of 5/8 inch gypsum board per side |
| 50 | Shouting barely audible | 2x4 studs with resilient channels and 5/8 inch gypsum board each side |
| 55 | Shouting not audible | Metal studs, 3 inch glass fiber blanket, two layers of 5/8 inch board each side |
Read across a row and the pattern is clear. The jump from 35 to 50 came from more mass, then from separating the two faces. That is also what remodelers do today when a bedroom shares a wall with a family room or a media room.
Where STC shows up in a remodel
On product data sheets. Doors, windows and wall assemblies often list an STC from an E90 lab test. Ask for the lab report if a number matters to you.
On plans for shared walls. Townhouses and duplexes may need a rated wall between units if your town adopted the IRC sound appendix. The plans should name the tested assembly 5.
In an ADU or basement suite. A rental unit under or behind a house is where neighbors hear each other. Our guides to accessory dwelling units and basement finishing cover planning those spaces.
On a soundproofing quote. A good quote names the assembly: board layers, stud type, any resilient channel, insulation and sealing details. A quote that only says “soundproof drywall” leaves too much unsaid.
What a higher STC does and does not do
What it helps
Limits of the STC number
STC is a lab rating of one element, tested so that the specimen is the main sound path 2. Your house is not a lab. Sound also goes under doors, through return ducts and along floor framing. That is why field tests under ASTM E336 report different ratings, such as apparent STC 4.
STC also covers a set frequency range. The NBS guide notes that low-frequency truck noise passes through walls much more easily than high-pitched sounds 1. Home theaters with heavy bass can be audible through walls with a respectable STC.
Last, a single hole can sink a good wall. The NBS guide warns that back-to-back outlets, pipe openings and settlement cracks let noise through, and that sealing them can improve an existing wall 1.
What drives the cost of a higher STC
We do not publish prices for soundproofing. The cost depends on how much of the wall you rebuild. Sealing gaps and outlets on an existing wall is the smallest job.
Adding a second layer of drywall means new trim, moved outlet boxes and repainting. Decoupling the faces with resilient channels or new studs means opening the wall.
Materials add up with each layer: extra gypsum board, acoustic sealant, insulation and putty pads for boxes. Doors are a hidden driver. A hollow-core door can be the weak point in an otherwise quiet wall, so a solid door with seals often belongs in the same budget.
These assemblies last as long as the wall, provided cracks are re-caulked and new holes are sealed. Use the whole-home budget tool to plan a wider project, and read how remodeling costs are built before you compare bids.
How a remodel raises a wall’s STC
Interior wall changes usually need a permit when they involve framing, electrical boxes or a rated wall between units. A contractor handles those parts.
- Decide the target. Compare the hearing levels in the NBS table with what you want to stop 1.
- Find the leaks first: outlets, switch boxes, pipe holes, the gap under the door and cracks at the floor and ceiling 1.
- Choose an assembly with a tested rating, such as added mass, resilient channels or staggered studs.
- Fill the cavity with insulation if the wall is open.
- Seal every edge and penetration with resilient caulk, as the NBS guide advises 1.
- For walls between dwelling units, build exactly to the tested assembly on the plans and keep penetrations treated, as the appendix requires 5.
- Listen after the work is done. If noise still gets through, look for flanking paths such as shared ducts.
Keeping a quiet wall quiet
- Re-caulk cracks at the floor and ceiling when settlement opens them 1.
- Seal new holes for TV mounts, cables or pipes the day you make them.
- Keep door seals and sweeps in good shape; doors leak more sound than walls.
- Avoid back-to-back outlets in later electrical work on a shared wall.
- Keep shelving and cabinets from being screwed through to the far face of a decoupled wall, which can bridge it.
Signs a wall is underperforming, and when to call a pro
- You can follow a conversation through a wall at normal volume, a sign of a low rating 1.
- Noise is loudest at outlets or the door, which points to leaks rather than the wall itself.
- Bass carries but voices do not, a low-frequency problem STC does not capture.
- Sound comes from the ceiling or floor, which may be flanking or impact noise.
Wiring and rated walls
Moving or sealing outlet boxes means working near live wiring; turn off the circuit and use a licensed electrician for any box or wiring change. Walls between dwelling units may also be fire-rated, so do not cut or replace them without a permit and the approved assembly.
An acoustical consultant can run a field test under ASTM E336 to find the weak path 4. Any change that opens a wall near wiring should go to a licensed contractor.
Codes and standards for STC
The IRC sound transmission appendix. The International Residential Code carries an optional appendix on sound transmission, with sections AK101 to AK104. It applies only if your jurisdiction’s adopting ordinance references it 5.
Where it does, walls and floor-ceilings between dwelling units, including townhouse units, must reach STC 45 when tested under ASTM E90.
Penetrations for pipes, electrical devices, recessed cabinets, tubs, soffits and ducts must be sealed or treated to keep that rating, and unit entry doors off a shared space must fit tight 56.
Impact noise. The same appendix asks for IIC 45 under ASTM E492 for floor-ceilings between units 5.
How one state reads it. In 2021, the North Carolina Department of Insurance ruled on its 2018 residential code. It confirmed the section requires an STC 45 rating but does not require any particular type of insulation to get there 6.
Standards. ASTM E90 is the lab test, ASTM E413 assigns the STC, and ASTM E336 covers field measurements in buildings 234. Your local building department says which code edition and appendices apply.
STC vs related ratings and terms
| Term | Measures | Main difference from STC |
|---|---|---|
| STC | Airborne sound through one element in a lab 2 | — |
| IIC | Impact sound through a floor-ceiling 5 | Footsteps rather than voices |
| ASTC | Airborne sound in the finished building 3 | Includes flanking paths |
| Window U-factor | Heat flow through a window | Heat, not sound; a window can be good at one and poor at the other |
| Drywall layers | The mass in most STC assemblies | A material, not a rating |
Common STC myths
- Myth Insulation alone soundproofs a wall.
- Reality Insulation helps, but the NBS examples show mass and separated faces drive the big gains from STC 35 to 50 1.
- Myth A high-STC wall stops all noise.
- Reality Sound can flank around it through floors, ducts and doors, which is why field ratings run lower 4.
- Myth STC covers footsteps.
- Reality Footsteps are impact noise, rated separately as IIC 5.
- Myth Every house must meet STC 45.
- Reality The IRC appendix covers walls between dwelling units, and only where a jurisdiction adopts it 5.
- Myth Small gaps do not matter.
- Reality The NBS guide lists outlets, pipe holes and cracks as common leaks that sealing can fix 1.
How local adoption changes the rules
Sound rules are local in a way energy rules are not. The IRC sound appendix is optional, so it applies only where the adopting ordinance names it 5.
Some places tie it to townhouses; North Carolina’s 2018 code, for example, points townhouse sound transmission to its Appendix K 6.
A detached house has no STC requirement in the model code. Ask your building department whether the appendix is adopted before you design a shared wall in a duplex, townhouse or ADU.
When STC should drive your plans
STC matters most where people sleep or work next to noise: a bedroom beside a media room, a home office, a primary suite in a whole-home renovation, or any wall between units. In those cases, choose a tested assembly and plan the sealing details up front.
It matters less for a powder room off a hall, where sealing the door gap may be enough. Ask bidders to name the assembly and expected rating in writing, then compare the answers with the quote comparison tool.
- If You only want to blur voices
- Then Aim near STC 40 and seal outlets and door gaps
- If You want loud talk barely audible
- Then Plan an assembly near STC 45 to 50 with separated faces
- If The wall separates two dwelling units
- Then Follow the adopted code and the tested assembly on the plans
- If The complaint is footsteps
- Then Look at floor IIC, not wall STC
Questions about Sound Transmission Class (STC)
What is a good STC rating for a home wall?
A good target depends on the room. Most interior walls in homes rate about STC 30 to 35, where loud speech is still understood.
The NBS guide shows loud speech becomes unintelligible around STC 40 and barely audible around 45.
For bedrooms and offices next to noisy rooms, many owners aim for the 45 to 50 range.
What does STC 50 sound like?
At STC 50, shouting on the other side is barely audible, according to the National Bureau of Standards table. Normal talk would not come through.
The NBS example wall at that level used 2x4 studs with resilient channels and 5/8 inch gypsum board on each side. Real results drop if the wall has gaps or flanking paths.
How is STC measured?
STC is measured in a lab under ASTM E90. A wall sample is built between two rooms.
A loud sound plays in one room, and the drop in sound level on the other side is measured across 16 frequency bands.
ASTM E413 then fits a standard curve to those results and gives one number, the sound transmission class.
What is the difference between STC and IIC?
STC rates airborne sound, such as voices and TV, passing through a wall or floor. IIC, impact insulation class, rates impact sound, such as footsteps, through a floor-ceiling. The IRC sound appendix sets 45 for both between dwelling units, where adopted. A floor can score well on one and poorly on the other.
Does insulation increase STC?
Insulation in the stud cavity usually helps by absorbing sound inside the wall, but it is not the biggest factor. Research at the National Research Council of Canada found gypsum board mass and cavity depth mattered most in walls with separated faces. Sealing leaks and decoupling the two faces often give larger gains.
Is there a code requirement for STC in houses?
Only between dwelling units, and only where adopted. The IRC’s optional sound appendix requires STC 45 for walls and floor-ceilings between units, including townhouses, tested under ASTM E90.
It applies only if your jurisdiction’s adopting ordinance names it. Walls inside a single-family home have no model-code STC requirement, so any target there is your own choice.
Why does my wall sound worse than its STC rating?
Because the rating is from a lab, and your wall is in a real house. Sound flanks around it through floors, ceilings, ducts and doors, and it leaks through outlets and cracks. ASTM E336 field tests capture this and report an apparent STC, which is usually lower than the lab number.
What STC rating do windows and doors have?
Windows and doors often rate lower than walls, and they leak at their edges. The NBS guide says a light-frame house shell with typical windows and doors can drop to STC 20 overall. Check the product’s lab-tested STC if outside noise is a problem, and pay attention to seals and weatherstripping.
Can I raise the STC of an existing wall without tearing it out?
Often by a few points. Sealing outlets, pipe holes and cracks at the floor and ceiling is the first step the NBS guide recommends. Adding a layer of gypsum board adds mass. Larger gains usually need resilient channels or new framing, which means opening the wall and often moving outlet boxes.
Sources
- National Bureau of Standards (NIST), Handbook 119: Quieting, A Practical Guide to Noise Control (Berendt and Corliss, 1976), retrieved .
- ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss (scope), retrieved .
- ASTM E413-22, Classification for Rating Sound Insulation (scope), retrieved .
- ASTM E336-23, Measurement of Airborne Sound Attenuation between Rooms in Buildings (scope), retrieved .
- Seattle Residential Code 2012, Appendix K Sound Transmission (IRC model text, sections AK101–AK104), retrieved .
- North Carolina Office of State Fire Marshal, Formal Interpretation on 2018 NCRC Appendix K Section AK102 (October 18, 2021), retrieved .
- Warnock and Quirt, National Research Council of Canada, Sound Transmission Through Gypsum Board Walls (Canadian Acoustics), retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Rating definitions come from the published scopes of ASTM E90, E413 and E336.
The hearing levels and example walls come from Table 4-1 of NBS Handbook 119, quoted as a 1976 source.
Code text comes from the Seattle 2012 copy of the IRC sound appendix and North Carolina’s 2021 formal interpretation of its 2018 code; we could not load the ICC 2021 text, so edition-specific wording should be checked locally.
The NRC Canada study supports the factors named. No prices are given.
Suggest a correction. We fix errors and say what changed.