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Soundproofing Project Reasonable Expectations and STC Rating Definitions

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"SOUNDPROOFING" - Expectations / STC Rating Definitions / Green Glue STC Rated Walls

The dictionary definition of "Soundproofing" is (v) the action of making (a room or building) resistant to the passage of sound.  The key part of that definition which is commonly misunderstood is that soundproofing is making a room resistant to sound, not making the room devoid of all sound transmission.

When performing a soundproofing project, a contractor must explain to their customer what the end results will likely be.  If this is not properly explained, the customer will most likely be disappointed.  The customer will be unhappy with your work even though the results meet or exceed your expectations.

EXPLANATION OF STC RATINGS:

We have written many blogs and essays explaining how STC ratings are determined and also about the pros and cons of the STC rating system altogether.  This post will only explain what the STC Rating means in terms of sound transmission.

STC RATING
REASONABLE EXPECTATION
STC 28-30
Low speech audible, most sentences completely understood
STC 30-35
Many phrases and some sentences understood without straining to hear
STC 42-45
Individual words and occasional phrases clearly heard
STC 47-50
Loud speech audible, music easily heard
STC 52-55
Loud speech audible if straining to listen, music can be heard and may still be disturbing
STC 57-60
Loud speech essentially inaudible, music can be heard faintly but bass may still be disturbing
STC 65+
Most sound cannot be heard


In the table listed above, you can see that the higher the STC Rating, the better the structure is for blocking sound.  A standard wall which consists of a 2x4 wood stud wall with no insulation and just 1/2" drywall on both sides will be in that lowest STC range of around STC 35.  Two people can basically have a full conversation through this wall with little difficulty understanding full sentences.

By using 5/8" drywall (more mass) instead of 1/2", adding sound absorbing insulation, and sealing up all joints with acoustical sealant, we can increase the STC rating up to STC 41.  At this level we will still be able to easily hear phrases and some sentences without having to strain to hear them.

If we were to add just one layer of Green Glue Noiseproofing Compound with an additional layer of 5/8" drywall to ONE side of the wall pictured above, the STC Rating will greatly increase up to STC 52 (addition of 11 points!).  This wall will greatly reduce the amount of talking through the wall and only loud talking will be audible if you strain to listen.  One downside would be that loud music can still be a nuisance.

If you added a layer of Green Glue Noiseproofing Compound between an additional layer of 5/8" drywall to BOTH sides of the wall, the STC Rating increases up to STC 56.  This bump up to the higher STC 56 will make loud speech inaudible and music will be heard faintly although bass sounds from music can still be disruptive.

To increase a wall's STC Rating up to STC 66, the addition of Green Glue Whisper Clips and Hat Channel in  conjunction with a layer of Green Glue Noiseproofing Compound between layers of 5/8" drywall.  At STC 66 most noise cannot be heard through the partition although some very loud bass may still be audible.

The examples listed above are for walls using Green Glue Noiseproofing Compound.  Each wall must be properly sealed with acoustical sealant.  STC Ratings are based on lab tests so actual results may vary but tested STC ratings give you a good comparison between designs and material options.

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