STC Ratings Explained: How Much Soundproofing Do You Actually Need?

STC ratings explained without the spec-sheet jargon. Sound Transmission Class is the single most quoted number in soundproofing, and the most misread. A high STC means very little if it is the wrong number for your room, if your wall is undermined by a hollow door, or if you have mistaken a laboratory rating for what the construction will deliver on site.

This guide breaks down what STC actually measures, the scale you can trust, the rating each kind of space needs, and the traps that catch architects and clients out. It is written for people specifying or buying real acoustic work, not for people writing physics papers.

What STC measures

STC stands for Sound Transmission Class. It is a single integer rating, from around 15 at the bottom to 70+ at the top, that summarises how well a building element blocks airborne sound. Walls, partitions, floors, doors, windows and even ceilings all have STC ratings.

The rating compresses a full frequency response into one number. A wall does not block all frequencies equally. It might cut high-frequency speech by 50 dB and bass by only 30 dB. STC takes the measured transmission loss at 16 standard frequencies between 125 Hz and 4,000 Hz, then fits a reference contour to the curve. The contour position gives the STC number.

The result is a sensible shorthand for speech privacy and general office noise, where most useful information sits in that mid-range band. It is much less useful for low-frequency content: bass from music systems, plant noise, traffic rumble or footfall. We come back to this limitation later when we discuss OITC.

How STC is tested

STC is measured in an acoustic laboratory, not on site. The wall, door or window assembly is built into the test aperture between two reverberant chambers. A loudspeaker generates broadband noise on one side, and microphones measure sound levels in both rooms. The difference at each test frequency is the transmission loss.

The test follows ASTM E90 for the measurement and ASTM E413 for the single-number rating. The chambers are highly controlled. They isolate the test specimen from flanking paths so that the only sound reaching the receiving room must pass through the wall itself.

This is exactly why STC numbers do not match real-world performance. A laboratory result is the best possible case for that construction. Site builds rarely meet it. We unpack the gap between lab and field performance below.

The STC scale with real-world examples

The number is much easier to interpret when you know what people sound like through walls at each rating. The descriptions below match the National Research Council of Canada’s reference scale.

STC rating	What you hear through the wall	Typical use
25	Normal speech is clearly understood	Internal stud wall, no insulation
30	Loud speech is understood; normal speech is audible	Light office partition
35	Loud speech is audible but not clearly understood	Basic office partition
40	Loud speech is faintly audible; normal speech is not understood	General office privacy
45	Loud speech is not audible; music is faintly audible	Meeting rooms, hotel rooms, the start of usable privacy
50	Loud sounds reduced to a low background	Executive offices, premium meeting rooms
55	Most sounds inaudible at normal levels	Boardrooms, confidential spaces
60	Excellent privacy; only very loud bass content audible	Recording booths, broadcast studios
65+	Effectively complete isolation for speech and most music	Music studios, live rooms, cinemas

The widely quoted figure is that STC 45 is where soundproofing starts to feel like soundproofing. Anything below that, and you can still follow a conversation through the wall, even if you cannot hear every word. STC 50 is the sensible target for offices that want any real meeting-room privacy. STC 55 and above is where studios and recording spaces live.

A 55 dB wall with a 20 dB door gives you closer to 20 dB. The envelope is only as strong as its weakest link, and the weakest link is almost never the wall.

What STC rating you actually need

The honest answer is that the right STC depends on three things: what is being kept in or out, how loud the source is, and how quiet the receiving room needs to be. A music studio in a noisy basement and an executive office in a quiet floor plate need very different ratings, even if both want privacy.

Here are the practical targets we work to on commercial projects.

Space	Target STC	Notes
Open-plan office, internal partitions	STC 38 to 42	Mostly about reducing speech intelligibility between zones
Standard meeting room	STC 45 to 50	Enough to keep general conversation private
Boardroom or confidential meeting room	STC 50 to 55	Required where commercially sensitive talks happen
Therapist or consultation room	STC 50 to 55	Driven by privacy law and ethical practice as much as comfort
Hotel room to corridor	STC 50 to 55	Industry expectation for premium properties
Podcast or voiceover booth	STC 55 to 60	Critical when shared building or external traffic is present
Music production room or live room	STC 60 to 70+	Drum kits and amplified instruments need full decoupling
Cinema or screening room	STC 60 to 70+	Low-frequency content is the driver, often pushing OITC use

The pattern is consistent. Offices and meeting rooms live in the STC 40s and low 50s. Studios live in the 55+ band. Anything below STC 40 will deliver only mild privacy benefits and is rarely worth the build cost on a commercial project.

For deeper detail on commercial office targets, see our guide to office acoustics services and our breakdown of how to soundproof a meeting room without disrupting the office.

STC vs Rw: how UK and US ratings differ

STC is the North American standard. In the UK and Europe the equivalent is Rw, the Weighted Sound Reduction Index defined in BS EN ISO 717-1. Both compress a frequency-dependent transmission curve into a single integer. Both use a reference contour fitted to measured data. They are not identical, but the two scales line up closely in the speech range.

For most office and studio constructions, Rw and STC fall within 1 dB of each other. A wall rated STC 50 in a North American datasheet is broadly equivalent to a wall rated Rw 50 in a European datasheet. The reference curves differ slightly at low frequencies, so for bass-heavy applications, you should compare octave-band data rather than headline figures.

In the UK, Building Regulations Approved Document E uses DnT,w + Ctr for the airborne sound insulation target between dwellings. Ctr is a spectrum adaptation term that penalises poor low-frequency performance. This matters because real noise sources, especially traffic and music, contain heavy low-frequency content that the standard Rw or STC number underweights.

Lab STC vs field performance

A laboratory STC test is the upper bound of what a wall can do. Real construction always underperforms the lab, often by a meaningful margin. There is a different field rating, FSTC, plus the European equivalent DnT,w, that captures this difference. Field results typically come in 5 to 10 dB below the laboratory value for the same construction.

The gap exists for four reasons. Flanking paths around the wall let sound travel through the floor, ceiling, structural framing or services penetrations. Workmanship varies on site, with sealant gaps, missed acoustic tape and partial penetrations all chipping away at performance. Edge details such as the head and base of partitions are rarely as tight as the lab specimen. And ventilation, electrical and plumbing services route sound around the assembly unless they are designed and detailed for acoustic control.

When we specify a wall on a Sound Zero project, the target rating is always the field number the client needs, plus a margin to absorb the lab-to-field gap. Specifying STC 50 in the lab when the client needs 50 dB of real-world isolation is a recipe for disappointment.

An STC number on a spec sheet describes a perfect laboratory. The wall you build will perform 5 to 10 dB lower in the real world unless flanking, services and edge details are designed in from day one.

The weakest-link rule

The single biggest mistake we see on commercial projects is paying for a high-STC wall and undermining it with a low-rated door or window. Sound finds the easiest path. If your wall is rated STC 55 and your door is rated STC 25, the room will perform close to STC 25, not 55. The combined rating of an assembly is always driven by its weakest element.

The maths is unforgiving. A 30 dB difference between wall and door means 99.9% of the transmitted sound energy now arrives via the door. The wall might as well not be there. The same logic applies to single-glazed windows, ventilation grilles, conduit penetrations, gaps under skirting and any other path with a lower acoustic rating.

This is why studio doors cost what they cost. A solid-core door with full perimeter seals and a drop seal at the threshold sits in the STC 35 to 40 band. A heavy acoustic door set with a double-door vestibule can reach STC 50 to 55. Both costs are eye-watering compared to a standard internal door. Both are necessary if the wall around them is doing its job.

The same applies to soundproofing more generally. The acoustic envelope is a system, not a component.

STC vs OITC: when STC misleads you

STC is built around speech. It weights frequencies between 125 Hz and 4,000 Hz, with most of the weight in the mid-range. That works well for office privacy. It fails badly for low-frequency sources.

OITC, the Outdoor Indoor Transmission Class, was created to fill that gap. It uses a reference spectrum that extends down to 80 Hz and represents traffic, aircraft and similar broadband outdoor noise. OITC values are always lower than STC for the same construction, often by 5 to 15 points, because most building elements perform worse at low frequencies than at speech frequencies.

When a project involves traffic noise, aircraft overflight, plant rooms, music venues, or anything else with serious low-frequency content, OITC or the European Ctr-adjusted Rw value is the more honest number. Two windows with the same STC rating can have OITC ratings 10 points apart. Specifying on STC alone in those situations is how you end up with a building that meets the paperwork but fails the brief.

How to improve a wall’s STC

Four levers move STC. Used together they take a basic partition from STC 35 into the high 50s and beyond. Used in isolation they have diminishing returns.

• Mass. Heavier walls transmit less sound. Doubling the surface mass of a single-leaf wall adds around 5 to 6 dB. Two layers of plasterboard outperform one. Heavy boards such as Soundbloc, Gyproc Soundbloc or dB35 give a useful boost per millimetre of build-up.
• Cavity depth. A deeper air gap between leaves lets the leaves act more independently and lowers the mass-air-mass resonance frequency, improving low-frequency performance. The jump from a 70 mm stud to a 146 mm stud delivers real gains.
• Decoupling. Resilient bars, staggered studs, double-stud walls or full room-within-a-room construction stop sound vibrations bridging from one leaf to the other. This is the single highest-leverage change for ratings above STC 50.
• Absorption inside the cavity. Mineral wool, glass wool or Rockwool inside the cavity damps the air resonance and adds 5 to 10 dB to most builds. Empty cavities ring like a drum. Insulated cavities do not.

The biggest leaps happen when you combine mass and decoupling. A single-stud, single-board wall topping out around STC 35 to 40 becomes a double-stud, double-board, fully insulated wall sitting at STC 55 to 63. That is the leap from speech audibility to confidential privacy.

STC ratings for common building elements

The figures below are typical laboratory ratings. Field performance is generally 5 to 10 dB lower. The point of the table is to anchor expectations and to make the weakest-link rule visible.

Element	Typical STC	Notes
Single plasterboard on timber studs, no insulation	33	Minimal; speech clearly audible
Single plasterboard on studs, insulated cavity	40 to 45	Basic office partition
Double plasterboard on single studs, insulated	45 to 50	Reasonable for general office use
Double plasterboard on staggered or resilient studs, insulated	50 to 55	Good for meeting rooms
Independent double-stud wall, insulated, double board each side	55 to 63	Approaching studio territory
200 mm blockwork, plastered both sides	48 to 52	Mass-law driven
Room-within-a-room, full decoupling	60 to 70+	Professional studio standard
Standard hollow-core door	15 to 20	The weakest link in most partitions
Acoustic door set, solid core, full seals	30 to 40	Essential for any privacy or studio space
Single glazing, 6 mm	~28	Often the dominant weak point on external walls
Acoustic double glazing, 6-12-10 laminated	36 to 42	Asymmetric panes plus a laminated interlayer
Acoustic secondary glazing, 100 mm+ cavity	40 to 50	The best glazing option short of removing the window

The same logic applies to floors and ceilings. A heavy floor on resilient mounts with an isolated ceiling below it can reach STC 60 to 65. A standard timber joist floor sits around STC 35 to 40. The build-up costs are significant. They are also unavoidable if the goal is real isolation.

If you want a wider view of the trade-off between blocking sound and treating it, our guide to acoustic treatment vs soundproofing explains where each one belongs.

Frequently asked questions

What is a good STC rating for a wall?

STC 45 is the threshold where most people would describe a wall as soundproofed. Loud speech becomes inaudible and music becomes faint. For meeting rooms and offices the practical target is STC 50. For studios and confidential rooms, STC 55 to 60 is the working band. Below STC 40 the privacy benefit is mild.

Is STC 50 enough for a recording studio?

For a vocal or podcast booth in a quiet building, STC 50 to 55 is workable. For a music studio with drums, bass guitars or amplified instruments, STC 60 to 70+ is the realistic target, achieved with decoupled construction, mass and acoustic door sets. Anything less and low-frequency content will bleed into adjacent spaces and back into the live room.

What is the difference between STC and Rw?

STC is the North American single-number rating defined by ASTM E413. Rw is the European equivalent defined by BS EN ISO 717-1. The two scales use slightly different reference contours but track each other within about 1 dB for typical office and studio constructions. For UK projects, Rw is the right specification number. STC ratings from US datasheets can usually be read across without recalculating.

Does increasing STC by 10 mean twice the soundproofing?

Roughly. A 10 dB increase corresponds to a tenfold reduction in sound energy, and is generally perceived as halving the loudness. So going from STC 40 to STC 50 cuts perceived noise by about half. The catch is that doubling subjective performance often requires far more than doubling the cost, because the high-STC band needs decoupled construction.

Why is field STC lower than laboratory STC?

Laboratory tests isolate the wall from flanking paths and enforce perfect workmanship. Real construction has neither. Flanking through floors, ceilings, services and edge gaps reduces measured performance by 5 to 10 dB. The field rating, FSTC, captures this difference. A wall rated STC 55 in the lab typically delivers FSTC 45 to 50 on site unless flanking and services are designed in.

When should I use OITC instead of STC?

Use OITC for any project where low-frequency noise dominates: traffic, aircraft, plant rooms, music venues, cinemas or any source with significant bass content. STC underweights the low frequencies where these sources sit, so two assemblies with the same STC can perform very differently in real conditions. OITC values are 5 to 15 points lower than STC for typical building elements.

Can I improve the STC of a finished wall?

Yes, but the gains are limited unless you accept losing internal width. Adding a second layer of plasterboard with a damping compound between the layers, fitted on resilient bars, adds 5 to 10 dB to a typical existing wall. Building a fully independent inner leaf with its own studs adds 10 to 20 dB but costs around 100 mm of internal dimension on each treated face. Doors, glazing and services have to be upgraded in parallel.

References: ASTM E413 (Classification for Rating Sound Insulation), available via ASTM. BS EN ISO 717-1 (Acoustics, rating of sound insulation in buildings and of building elements).

Related acoustic guidance: acoustic treatment vs soundproofing | commercial soundproofing | podcast studio soundproofing

Standards and trusted references

STC, Rw and field sound-insulation results are related but not interchangeable. These references cover the common rating and measurement routes.

• ASTM E413 – classification for rating sound insulation, including STC
• BS EN ISO 717-1 – rating of airborne sound insulation, including Rw
• BS EN ISO 16283-1 – field measurement of airborne sound insulation
• GOV.UK: Approved Document E – UK Building Regulations guidance for resistance to sound