What is STC Rating?

Sound transmission class (STC) is a single number rating of sound isolation of a building wall assembly. The higher the STC rating, the better the sound isolation. STC is widely used to rate interior partitions, ceilings/floors, doors, and windows. The STC rating figure represents approximately the decibel reduction in noise that the assembly can provide.

It is especially significant important in commercial construction, where different types of sound isolation are desirable. In general, an STC rating of around 50 or higher is considered to be adequate for most residential and commercial applications, although higher STC ratings may be required for applications where high levels of sound insulation are needed, such as in music studios, recording studios, or cinemas and theaters.

The sound transmission class rating is an ancient standard from 1961 that is not really relevant in 2022 – almost dangerous to use, because it leads to failure in building design and achieving isolation goals. At Aural Exchange, we calculate transmission loss in 1/6 octave bands from 50Hz to 20000Hz while specifying building isolation – Ashutosh Pande, Principal Acoustic Consultant

STC was first introduced in 1961 and has since become the standard single-number metric to describe sound blocking materials. People often mention NRC and STC ratings in similar contexts, but they are completely different rating systems. While the STC rating tells us how much sound is blocked from going through a product, the NRC Rating tells us how much sound is absorbed by a product.

The STC rating is important when you are trying to reduce the amount of sound entering or leaving a room. The NRC rating is important when you are trying to reduce echo and improve the sound quality in a room.

STC	What can be heard at this level
25	Soft speech can be heard and understood
30	Normal speech can be heard and understood
35	Loud speech can be heard and understood
40	Loud speech can be heard, but not understood
45	The threshold at which privacy begins
50	Loud sounds can be heard, but are very faint
60+	At this level, good soundproofing begins. Neighbors generally are not disturbed by very loud speech from inside.

 

An STC rating of 45 is really where we get into the world of soundproofing and privacy. This could be considered a baseline when you are getting serious about preventing sound transmission. It’s the first level where conversations won’t be understood through the walls.

Someone in a quiet room on the other side may be able to hear that a conversation is happening, but they wouldn’t be able to understand it, and it wouldn’t be considered disruptive. In offices, this means your conversations will be private behind closed doors.

The International Building Code requires an STC of 50 for residential construction, which is the point at which noise is reduced to a point that people generally feel like their homes are adequately insulated from noise. It is also the point at which respondents to surveys begin a drastic reduction in noise related complaints. 

With a Sound transmission class rating of 50, speech cannot be heard through the walls, and loud sounds are only faintly audible.

If you are unsure of what rating would work for your project, there are some standards you can go by in helping you to determine your target range. The National Research Council of Canada conducted research on the importance of sound insulation, and found that an effective STC rating of 55 is recommended, and 60+ is ideal, especially in rooms where there is going to be music related noise. These values are sufficient to keep most sounds muffled, but if all you are really trying to do is add privacy to your conversations, an STC rating of 45 should do just fine.

Sound transmission class rating numbers give you a rough idea of how much sound is reduced in decibels (db). There is no exact correlation from STC rating to db, however, since materials block sounds of varying frequencies differently. 

The rating works well for things like speech and the daily incidental sounds associated with living or working in a space, but isn’t incredibly accurate when evaluating music or heavy machinery, since those sounds live in lower frequencies, and will vibrate structures differently.

When you are adding material or changing construction to improve the STC rating of a surface, it’s important to think about the whole picture. When it comes to soundproofing, you are usually defined by your weakest point. A high STC rating wall can be rendered ineffective by a hollow core door, or a single pane window. 

Your windows’ STC rating is important to consider as we just discussed. 

A standard, single pane window, offers an STC rating in the range of 26-28, and you can add a few points with well constructed dual pane windows, but if you want to substantially reduce the amount of sound entering through the windows, you should consider upgrading your windows.

The easiest way to improve your rating is by adding mass to the walls to increase the overall thickness of the walls. Adding insulation or air space inside walls also hinders sound from passing through the walls. For example, two four inch walls separated with a two inch gap would transmit less sound than a solid eight inch wall. 

Filling that gap with STC rated insulation, like Nankarrow GeoWool Panels will increase the STC rating even more. Adding treatments inside the room will help stop noise volumes inside the room from increasing too much, but making meaningful changes to your STC rating generally involves making some structural changes.