My clients frequently ask this simple question about soundproofing and STC rating. But the answer is not simple, and needs some explanation.
More...
Building Code requirement for soundproofing
If the title question refers to requirements stated in building code, the answer is quite simple. As per National Building Code 2015, sound transmission loss between residencies, that is soundproofing, must be at least ASTC (Apparent Sound Transmission Class) 47. And Ontario Building Code has slightly different requirement, minimum STC (Sound Transmission Class) 50.
If the title question asks how much soundproofing is needed to achieve satisfactory noise privacy in an apartment, the answer is not simple. Soundproofing is never perfect, and people have different expectations for acoustic privacy and varying tolerance levels for unwanted noise. There is no single STC rating that can satisfy everyone.
Speech and music as noise
The Sound Transmission Class (STC) and Apparent Sound Transmission Class (ASTC) ratings are designed to evaluate a building assembly’s ability to reduce speech sounds. However, the STC rating does not accurately reflect how well an assembly attenuates music, particularly low-frequency components such as bass beats. Heavy concrete assemblies are much more effective at blocking low-frequency sounds than lightweight wood-frame constructions. Consequently, two walls — one made of concrete and the other of wood framing — may have similar STC ratings, yet the concrete wall will perform significantly better in reducing low-frequency music noise.
The table below illustrates subjective effect of transmission loss inserted by a wall assembly of certain sound transmission class (STC) [1].
STC rating | How is this sound attenuation perceived |
|---|---|
30 | Speech can be clearly heard through this wall |
40 | Speech can be heard through this wall, but not easily understood |
45 | Loud or amplified speech audible, loud music audible, bass notes particularly strong |
50 | Loud or amplified speech faintly audible, loud music barely audible, but bass notes quite noticeable. This is the minimum STC required by National Building Code for separation between dwellings. |
60 | Speech and loud music inaudible, except for very strong bass notes. This level should be the minimum required for high-end construction. |
70 | Blocks almost all sounds that can be normally expected in a dwelling. This level is the minimum required for special applications, such as a recording studio. Requires specialized wall assembly design. |
Occupants satisfaction with soundproofing
I always aim to answer acoustical questions based on verifiable test data. In this case, I will summarize information presented in the book Sound Insulation in Buildings [2], which cites a socio-acoustic research study conducted in Norway.
The table below shows the percentage of apartment building occupants who reported being annoyed to varying degrees by noise from neighbours, across dwellings with different levels of soundproofing as measured by STC rating.
Rated STC of partitions in apartment | Percentage of people moderately, very, or extremely annoyed | Percentage of people slightly annoyed |
|---|---|---|
45 | 34% | 64% |
50 | 16% | 43% |
60 | 7% | 21% |
70 | 2% | 10% |
From the table you can see that 16% (one in 6) people are at least moderately annoyed and 43% are slightly annoyed by noise from neighbours, when the partition wall has rated STC 50, the minimum required by Ontario Building Code. Even high rated STC value 60 does not guarantee that everyone is happy with the level of privacy it provides in their apartment.
Each occupant of an apartment, condominium, or other shared housing must determine for themselves how much noise from neighbours is acceptable. If the noise level in your situation is intolerable, consult an acoustical engineer to explore possible ways to reduce it.
