Estimate how much a wall, door, or window actually blocks — honestly.

How STC actually works

Sound Transmission Class is a single-number rating defined by ASTM E413. It compresses 16 one-third-octave-band transmission-loss measurements (between 125 Hz and 4 kHz) into a single “STC X” value by fitting a standardized reference contour to the measured data.

The contour is shaped like a flattened L — it rises 15 dB per decade between 125 and 400 Hz, stays flat from 400 to 1250 Hz, then rises another 5 dB up to 4 kHz. The STC value of an assembly is the transmission-loss number at 500 Hz on the best-fit contour, subject to two constraints: no single band may fall more than 8 dB below the contour, and the total deficit summed across all bands cannot exceed 32 dB.

In plain English: STC is a weighted average that punishes assemblies with one bad band. A wall that’s great everywhere except 250 Hz gets dragged down. A wall that’s equally mediocre across all bands gets rewarded.

The bass-leakage problem — STC’s biggest blind spot

STC measurement starts at 125 Hz. Anything below that is invisible to the rating. This is the single most important thing to understand about every STC number you’ll ever read on a product page.

Most real-world soundproofing complaints come from sources below 125 Hz:

• Footfall from the apartment upstairs — the impact transient contains huge low-frequency content (40–100 Hz) that STC simply doesn’t see.
• HVAC rumble, refrigerator compressors, traffic — energy concentrated around 30–100 Hz, completely outside STC’s measurement range.
• Music bass through a shared wall — the kick drum and bass guitar fundamentals are at 50–120 Hz. A wall rated STC 55 can still let bass through clearly.

This is why two walls with the same STC rating can have wildly different perceived performance. An STC 50 wall with a deep cavity and proper decoupling sounds quieter than an STC 55 wall built from thick mass alone — even though the rating disagrees.

For low-frequency assessment, look at OITC (Outdoor-Indoor Transmission Class, defined down to 80 Hz) or, better, the raw transmission-loss values at 63 Hz and 100 Hz. Most manufacturers don’t publish these, which tells you everything about why STC remains the marketing default.

The flanking-path problem — what gets measured vs. what you experience

Every STC rating is measured in a lab, where the assembly is the only sound path between two rooms. In your home, sound also travels through:

• The floor and ceiling. A common joist or floor slab carries vibration around the wall, regardless of how well-built the wall is.
• Electrical outlets and switch boxes. A back-to-back outlet across a stud bay is the equivalent of a small hole through the wall. STC drops 5–10 points just from this.
• HVAC ducts and recessed lights. Air paths are sound paths. Speakers in an adjacent room can be heard through a return-air grille even when the wall between is STC 55.
• The door. A solid wall at STC 52 with a hollow-core door (STC 17) effectively performs at ~STC 25. The weak link dominates.

Real-world acoustic isolation is set by the worst link in the path. A great wall doesn’t fix a bad door or a leaky outlet box.

Mass law, decoupling, and the diminishing-returns curve

Sound transmission through a single layer follows the mass law: doubling the mass-per-unit-area of a partition adds about 6 dB of transmission loss. That’s why a 16 mm sheet of drywall (~12 kg/m²) gives you ~STC 30 and a 32 mm double layer gives you ~STC 33. Linear in mass, sublinear in dB.

This means stacking more drywall hits diminishing returns fast. What actually moves the rating is decoupling: separating the two sides of the wall so they vibrate independently. The mechanisms:

• Resilient channels — metal hat-channels with a small flex; about +5 STC over a hard-fastened single-stud wall, if installed correctly (the screw can’t penetrate into the stud, or the decoupling is gone).
• Staggered studs — two rows of studs offset on a wider plate; about +8 STC. No special hardware, just more lumber.
• Double-stud walls — two completely separate stud rows with a gap between them; +12 to +18 STC over single-stud. The gold standard for residential.
• Isolation clips (Genie, Resilmount, etc.) — rubber-mounted clips that hold the drywall away from the studs; +8 STC, easier to install correctly than resilient channels.

The calculator’s presets and modifiers are built from these mechanisms. A “double-stud wall + batt insulation” preset is STC 58 because that’s what NRC measured in a lab; adding a layer of drywall to it gives +3 because mass stacking is diminishing. Adding resilient channels to a wall that already has staggered studs gives roughly nothing because the decoupling work is already done.

What this calculator can’t tell you

• The exact STC of your specific wall. Lab measurements have ±2 STC point variation between labs measuring identical assemblies. Real-world construction adds another ±3–5 from workmanship.
• How much bass leaks through. STC doesn’t see below 125 Hz. Use OITC or per-band transmission-loss data if low frequency matters.
• The flanking-path floor. Your wall could be theoretically STC 60 but the floor joists, HVAC ducts, or outlet boxes might cap the actual room-to-room isolation at STC 40.
• Whether mass-loaded vinyl is worth it. MLV adds ~5 STC in this calculator (consistent with manufacturer data), but in practice the benefit depends heavily on whether it’s decoupled or sandwiched. Cheap MLV layered between drywall sheets often reduces performance vs. spending that money on a second stud row.

References & sources

Every assembly STC value and modifier delta in this calculator is sourced from one of the publications below.

If you have an NRC, USG, or independent lab test report for an assembly we don’t list (or a value that disagrees with ours), send it in. The presets and deltas update when readers provide evidence.