Short answer: Soundproofing reduces airborne noise by roughly its STC or Rw rating across the speech range. A basic hollow interior wall (about STC 30–35) cuts speech by around 30–35 dB, so loud voices still come through. A good party wall of STC 55 or more makes normal speech inaudible. Bass and air gaps are what usually let noise through.
“Soundproof” is rarely absolute. Every wall, floor, window and door lets some sound through, and the useful question is how many decibels it removes. This guide explains the STC and Rw ratings used to compare building elements, gives typical values for walls, windows and doors, shows what each rating means for hearing a neighbour, and explains how to check the result yourself with a before-and-after measurement.
What STC and Rw Ratings Mean
STC (Sound Transmission Class) is the single-number rating used in the US. It is calculated under ASTM E413 from laboratory measurements of transmission loss in one-third-octave bands from 125 Hz to 4,000 Hz. Rw (weighted sound reduction index) is the international equivalent, defined in ISO 717-1 over 100 Hz to 3,150 Hz. Both fit the measured curve against a reference contour, so the number is roughly the reduction in decibels across the speech range. For the same construction, STC and Rw are usually within a point or two.
Because the decibel scale is logarithmic, every 10 extra points means the sound getting through has a tenth of the energy and sounds about half as loud. Laboratory ratings are best-case figures; field ratings measured in real buildings are typically a few points lower because of flanking paths and workmanship. ISO 717-1 also defines a correction called Ctr, which penalises low-frequency noise such as road traffic, so Rw + Ctr is the better guide when traffic or music bass is the problem.
Typical STC Ratings for Walls, Windows and Doors
| Element | Typical STC or Rw | Notes |
|---|---|---|
| Hollow stud wall, one layer of plasterboard each side | about 30–35 | Standard partition between rooms in a house |
| Same wall with cavity insulation | roughly 35–40 | Insulation adds a few points |
| Double plasterboard, staggered studs or resilient mounts | roughly 45–55 | Decoupling the two sides is what helps most |
| US party wall minimum (IBC) | 50 (45 if field-tested) | Between dwelling units |
| Good party wall or heavy masonry | 55 and above | Normal speech inaudible |
| Single-glazed window | roughly 25–28 | Often the weakest part of a façade |
| Standard double glazing | roughly 28–35 | A narrow air gap limits the gain |
| Laminated acoustic glass or secondary glazing with a wide gap | higher, often high 30s or more | Depends on the glass and the gap |
| Hollow-core interior door | about 20 | Gaps around it make it worse |
| Solid-core door with perimeter and drop seals | roughly 30–35 | Seals matter as much as mass |
The International Building Code (IBC) requires walls and floor-ceilings between dwelling units in the US to reach STC 50, or 45 when tested in the field, and floor-ceilings to reach IIC 50 (45 in the field) for impact noise. Many acousticians treat these as a minimum rather than a comfortable standard.
What Each STC Level Means for Hearing Your Neighbour
| STC | What you hear through the wall (quiet room) |
|---|---|
| 25 | Normal speech understood easily |
| 30 | Loud speech understood fairly well; normal speech heard but not clear |
| 35 | Loud speech audible but not understood |
| 40 | Loud speech heard only as a murmur |
| 45 | Loud speech barely audible; you have to strain to hear it |
| 50 | Loud sounds faintly heard; music bass still audible |
| 55–60+ | Most household sounds inaudible; loud music and bass may still come through |
These are rough guides and assume a quiet listening room. Masking matters: a neighbour talking loudly at about 70 dB through an STC 35 wall arrives at roughly 35 dB, which is clearly audible in a bedroom whose background is under 40 dB. Through an STC 55 wall it arrives at roughly 15 dB, below the background of almost any room. Check how quiet your own room is with the background noise test and compare it with typical room noise levels.
Why Low Frequencies Are Hardest to Block
A single wall obeys the mass law: doubling its mass per square metre adds about 6 dB of transmission loss in theory, and somewhat less in practice. The same law says loss rises by about 6 dB each time the frequency doubles, so a wall that blocks voices well does far less against a 60 Hz bass line or a lorry’s rumble. STC also ignores everything below 125 Hz, so a high STC wall can still pass bass.
The frequency analyzer shows which frequencies are getting through; if the peaks sit below 200 Hz, you have a bass problem (see dB vs Hz). A-weighted readings play bass down, so a C-weighted reading shows it better, as explained in dBA vs dBC. Walls that do well at low frequencies combine mass, decoupling (two leaves that don’t touch), an insulated cavity and airtight seals.
Air Gaps and Flanking Paths
Sound takes the easiest route. A gap under a door, back-to-back electrical outlets, holes around pipes, shared ducts and ceiling voids can each cost several STC points, and a wall is only as good as its weakest part. An STC 50 wall with an unsealed STC 20 door behind it performs close to the door. Flanking is sound bypassing the wall altogether through connected floors, ceilings or side walls. Before adding mass, seal gaps with acoustic sealant, fit door seals and stagger outlets.
Impact Noise and IIC
STC only covers airborne sound such as voices and TV. Footsteps, dropped objects and chairs dragged across a floor are impact noise, rated by IIC (Impact Insulation Class, where higher is better) in the US or LnT,w in ISO ratings (where lower is better). Soft floor coverings help most: carpet on underlay, resilient underlays beneath hard floors and floating floors all cut impact noise far more than they change STC.
How to Measure Before and After with Our Meter
- Play a steady test sound on the noisy side: pink noise from the tone generator through a speaker, at a level you can repeat, around 80 dB near the speaker.
- Measure the source room with our sound decibel meter at three or four positions away from the speaker and average them.
- Measure the receiving room the same way. The reading should be at least 10 dB above the background with the sound off; if it is closer, correct it with the subtract tab of the decibel calculator.
- The difference between the rooms approximates the transmission loss plus room effects: soft furnishings in the receiving room lower its reading, bare rooms raise it. It is not a certified STC, which needs one-third-octave measurements to a standard method.
- Repeat after the work with the same speaker, volume and positions. To capture real neighbour noise over an evening, run the noise monitor before and after.
Calibrate your microphone first. Phone and laptop microphones are good for about 40–100 dB, and very quiet receiving rooms can fall below what they resolve (see online decibel meter accuracy). For a formal complaint, read our apartment noise complaint guide: legal evidence needs a Class 1 or Class 2 meter.
Frequently Asked Questions
How many decibels does soundproofing reduce?
Roughly its STC or Rw rating in the speech range. Upgrading an interior wall from about STC 33 to STC 50 cuts speech by about 17 dB more, which sounds roughly a third as loud.
What STC rating is good?
STC 50 is the US minimum between homes, 55–60 is good and above 60 is excellent. For a bedroom or home office next to a living room, aim for 45 or more; a nursery benefits too (see baby room noise levels).
Does acoustic foam soundproof a room?
No. Foam absorbs echoes inside a room but adds almost no mass, so it barely reduces sound passing through walls. It is useful for recording spaces, as in our podcast room noise guide.
Is STC the same as decibels?
Not exactly. STC is a rating derived from decibel measurements at many frequencies. It approximates the reduction in dB for speech-range sound, not for bass.
Do soundproof curtains work?
Heavy curtains help a few dB at middle and high frequencies, mostly by covering gaps around a window. They don’t replace better glazing or seals.
Addito is an audio analysis and sound measurement writer specializing in decibel testing, noise level analysis, sound frequency measurement, and audio engineering tools for creators, audio professionals, students, and everyday users. He creates practical content focused on sound measurement, hearing safety, microphone testing, and acoustic analysis tools.