My clients frequently ask this simple question about soundproofing and STC rating. But the answer is not simple, and needs some explanation.
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 is about how much soundproofing you need to be happy with noise privacy in your apartment, the answer is more involved. Soundproofing is never perfect. People have different expectations for acoustic privacy and tolerance for unwanted noise. There is no single rated STC value that will satisfy everyone.
Speech and music as noise
Sound Transmission Class (STC) and Apparent Sound Transmission Class (ASTC) are designed to rate a building assembly’s ability to attenuate speech sounds. However, the STC rating does not describe the perceived attenuation of the assembly of music sounds, particularly low frequency beat in music. Heavy concrete building assemblies are much better at attenuating low frequency sounds than light weight wood frame assemblies. So, two walls, one concrete and one wood frame, may have similar STC rating. Concrete wall will be more effective in attenuating low frequency music sounds.
The table below illustrates subjective effect of transmission loss inserted by a wall assembly of certain sound transmission class (STC) .
How is this sound attenuation perceived
Speech can be clearly heard through this wall
Speech can be heard through this wall, but not easily understood
Loud or amplified speech audible, loud music audible, bass notes particularly strong
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.
Speech and loud music inaudible, except for very strong bass notes. This level should be the minimum required for high-end construction.
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 try to answer acoustical questions based on verifiable test data. In this case I will summarize information noted in book Sound Insulation in Buildings  quoting a socio-acoustic research study done in Norway.
Table below gives percentage of surveyed apartment building occupants that are annoyed to various degrees by noise from neighbours in dwellings with different levels of soundproofing by STC rating.
Rated STC of partitions in apartment
Percentage of people moderately, very, or extremely annoyed
Percentage of people slightly annoyed
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 building, a condominium, or other shared type accommodation must decide for himself or herself how much noise from neighbours is tolerable. If your noise situation is not tolerable to you, seek advice from an acoustical engineer about what can be done to reduce your noise level.
..... or PROVIDE YOUR FEEDBACK BELOW