STC Rating Explained: What Homeowners Actually Need to Know

STC stands for Sound Transmission Class. It’s a single number that tells you how many decibels of sound a wall, door, floor, or ceiling blocks. Higher number = better sound blocking.

That’s the simple version. Here’s everything else you need to know — without the acoustic engineering jargon.

What the Numbers Mean in Real Life

STC ratings are tested in laboratories under controlled conditions. But what do the numbers feel like in your home?

STC	What you’ll experience	Common examples
20–25	Can hear normal conversation clearly	Hollow-core interior door, cubicle partition
25–30	Loud speech heard and understood	Single-pane window, thin apartment wall
30–35	Loud speech heard but not clearly understood	Standard interior wall (single drywall on studs)
35–40	Loud speech is a murmur	Better-built interior wall, solid-core door with sealing
40–45	Loud speech barely audible	Double drywall with insulation, good apartment party wall
45–50	Loud sounds faintly heard	Well-built party wall, basic home theater
50–55	Most sounds not audible	Professional office, good hotel room
55–60	Excellent isolation	Recording studio, high-end hotel
60+	Near-complete isolation	Broadcast studio, auditorium

The “STC 40 Rule”

STC 40 is the informal threshold for “you can’t understand your neighbor’s conversation.” This is the target for:

Apartment party walls (shared walls between units)
Bedroom walls in a family home
Office walls for private conversations
Meeting rooms

Most building codes require STC 45–50 for party walls in new multi-family construction. Older buildings often fall short at STC 30–38.

How STC Is Measured

Understanding the test helps you understand the limitations.

The laboratory method (ASTM E90)

Build the wall in an opening between two specially designed test chambers
Play controlled sound in one chamber across 16 frequency bands (125 Hz to 4000 Hz)
Measure the sound level on both sides
Calculate the transmission loss at each frequency
Fit a standard contour curve to the data following specific rules
The STC rating is where the contour sits at 500 Hz

Key detail: The STC contour fitting has rules that limit how much any single frequency can deviate. No single frequency band can be more than 8 dB below the contour, and the total deficiency across all bands can’t exceed 32 dB.

The field method (ASTM E336)

Same principle, but tested in the actual building instead of a lab. Field results are typically 3–7 points lower than lab results because:

Real walls have outlets, gaps, and imperfect construction
Sound flanks around the wall through the floor, ceiling, and side walls
The rooms aren’t acoustically perfect test chambers

Field STC (FSTC) is what your actual wall achieves. Lab STC is what the wall could achieve under perfect conditions. When shopping for products, you’ll usually see lab STC. Expect 3–7 points less in reality.

STC Ratings of Common Barriers

Doors

Door type	STC
Hollow-core interior door (no sealing)	20–22
Hollow-core with weatherstripping	23–26
Solid-core door (no sealing)	27–30
Solid-core door with full sealing	30–35
Solid-core with MLV + sealing	33–38
Acoustic-rated door (STC 40)	38–42 field
Acoustic-rated door (STC 50)	47–52 field

Walls

Wall type	STC
Single drywall on wood studs (no insulation)	30–33
Single drywall on wood studs (insulated)	33–36
Double drywall on wood studs (insulated)	38–42
Double drywall with Green Glue (insulated)	43–49
Resilient channels + drywall (insulated)	42–47
Staggered stud wall (insulated)	47–52
Double stud wall with air gap (insulated)	55–63
8” concrete block	50–52
8” concrete block with drywall + insulation	55–58

Windows

Window type	STC
Single-pane (3mm glass)	26–28
Double-pane (standard IGU)	28–32
Double-pane with laminated glass	32–38
Double-pane with large air gap (100mm+)	35–40
Triple-pane with laminated glass	38–44

Floors/Ceilings

Assembly	STC
Wood joist floor, drywall ceiling below	32–38
Above + insulation in cavity	38–42
Above + resilient channels on ceiling	42–50
Concrete slab (150mm)	50–55
Concrete slab with floating floor above	55–62

STC vs. OITC: Which Rating Matters?

You might encounter two different sound rating systems. Here’s the difference:

STC (Sound Transmission Class)

Focuses on indoor-to-indoor sound
Weighted toward speech frequencies (250–4000 Hz)
Better for evaluating: interior walls, doors, party walls, office partitions
Most commonly used rating in the US

OITC (Outdoor-Indoor Transmission Class)

Focuses on outdoor-to-indoor sound
Weighted toward lower frequencies (80–4000 Hz)
Better for evaluating: exterior walls, windows, roofs
Accounts for traffic noise, aircraft, trains — which are bass-heavy

Which to use?

Neighbor’s conversation or TV → STC
Street traffic or aircraft → OITC
Neighbor’s subwoofer → Neither is great, but OITC is closer because it weights low frequencies more

STC vs. Rw

If you’re outside the US, you’ll encounter Rw (Weighted Sound Reduction Index) instead of STC. They measure similar things but use different standards:

	STC (US)	Rw (International)
Standard	ASTM E413	ISO 717-1
Frequency range	125–4000 Hz	100–3150 Hz
Contour rules	8 dB single-band limit + 32 dB total	32 dB total only
Adaptation terms	None	C and Ctr for different noise spectra

In practice: For most barriers, STC ≈ Rw within 0–3 points. They’re not identical, but close enough for practical decision-making. Don’t directly compare an STC number to an Rw number for precision work — but for “is this wall good enough?” they’re interchangeable.

What STC Rating Do You Need?

For your home

Room	Minimum STC	Recommended STC
Bedroom to hallway	33	40+
Bedroom to bedroom	40	45+
Home office	40	45+
Home theater	50	55+
Music practice room	50	55+
Bathroom to bedroom	40	45+

For apartments and condos

Barrier	Code minimum (typical)	Comfortable	Luxury
Party wall (between units)	45–50	50–55	55+
Floor/ceiling (between units)	45–50	50–55	55+

For offices

Space	Recommended STC
Open office → private office	40–45
Office to office	45–50
Conference room	50+
Executive office	50+
Medical/legal (confidential)	55+

The Diminishing Returns Problem

Each STC point costs more than the last. Going from STC 30 to 40 might cost $500. Going from STC 50 to 55 might cost $5,000. Going from STC 55 to 60 might cost $15,000.

The practical takeaway: For most residential situations, spending to get from STC 30 to STC 45 is excellent value. Getting from STC 45 to STC 55 is expensive but worthwhile for specific rooms (home theater, music room). Above STC 55, you’re in professional studio territory — the cost and complexity escalate dramatically.

How to Find Out Your Current STC

You have three options:

Look up your construction type — If you know your wall construction (single drywall, insulated, etc.), use the tables above for an estimate.
App-based testing — Use QuietScore to get a Sound Isolation Score comparable to STC. Fast, cheap, good enough for decision-making.
Professional testing — Hire an acoustic consultant for a certified field STC rating. Required for code compliance or legal disputes. See our testing guide for details.

Common STC Misconceptions

“STC 50 is twice as good as STC 25.” No. STC is logarithmic. Each 10-point increase represents roughly a perceived halving of loudness. STC 40 sounds about half as loud as STC 30 through the same wall.

“I need the highest STC possible.” You need enough STC. Overbuilding wastes money. An STC 55 bedroom wall is pointless if the door is STC 30.

“The STC rating on the product label is what I’ll get.” Lab STC, not field STC. Expect 3–7 points less in your actual installation due to real-world conditions.

“Higher STC means less bass noise.” Not necessarily. STC focuses on speech frequencies (250–4000 Hz). A wall with STC 45 might still let bass through. For low-frequency problems, check the transmission loss at 125 Hz specifically, or look at OITC ratings.

Next Steps

Soundproofing 101 — The fundamentals
How to Test Soundproofing at Home — Measure your barriers
How to Improve Your Soundproofing — Solutions for every budget
Sound Frequency Guide — Understand which frequencies matter for your situation