Sound transmission class (STC) and weighted sound reduction index (Rw) are important scales for controlling sound.


Sound is one of those mediums that can sometimes irritate us.

Of course, we do not see it, but sometimes, a dire need arises to control it.

These can often be carried out in two major ways: diffusion or isolation.


By diffusion, controlling this phenomenon is simply done within the space you are occupying.

It usually minimizes reflectivity, eliminates the hard surfaces, or provides absorption. (Can sometimes be a combination of two or all three of them).


Isolation involves keeping sounds out of space by utilizing acoustic barriers. For instance, a Moderco moveable wall.


Two specific test procedures have been put in place to determine further how effective the materials and products used in diffusion and isolation can be and create a technique by which the comparisons of these products can be made.


These are the sound transmission class (STC) and weighted sound reduction index (Rw).

Although the testing and reporting of these two are similar, they are also different in several ways.


In this article, you will be reading about the difference between the two and other important aspects of these phenomena you need to know.


Decibels And Frequency

These are two critical elements that need to be considered when trying to determine the ratings of the quality of sound absorption of any specific material.


Decibels (dB) indicate how loud a sound can be. The higher the decibels, the louder the volume and the taller the wave. Sound transmission class (STC) helps to calculate this better.


Frequency, on the other hand, is measured in hertz (HZ). It refers to the pitch and is determined by how fast sound waves follow each other.


For instance, bass music at a club depicts Loud, low-frequency sounds and would indicate that those are tall waves that slowly follow after each other.


On the other hand, a whistle can be seen as a sound with high frequency, and a depiction of the creation of quick waves that follow each other in rapid succession.


Sounds with low frequency typically travel farther and usually last longer than sounds with high frequency.


Sound Absorption

Absorption is an effective method of insulating sound. It reduces the frequency and wave sizes of sound.

When looking out for the materials that can block sound, one must consider two major rating systems.


1. Sound Transmission Class (STC)

Sound transmission class (STC) is the standard rating in the United States. When using this rating, the number is considered.


This implies that the higher the number, the better the material's ability to reduce the sound by causing a reduction in both the decibels and the frequency.


For instance, a standard material, let's say glass, has an STC of about 20s, while the STC of an average wall is about 30s.


Nonetheless, effective sound insulation is considered to begin in about the 50s.


STC Band | Source

It is also important to know that the testing standards of the STC are updated from time to time.


This means that a sound transmission class rating applicable twenty years ago will not be the same as that in recent times.


The major disadvantage of the rating of the STC is that it is not fully capable of covering low frequencies.


A material with a high STC number, not necessarily, might not be capable of blocking sounds, let's say, reverberating construction, rumbling traffic, or whisperings of people talking in the office space next door.


2. Weighted Sound Reduction Index (Rw)

The weighted sound reduction index is an alternative to the STC, and it is used by most people in other parts of the world.


It is the standard sound rating scale for International Organization for Standardization (ISO) and a portion of the ISO 140. Weighted sound reduction index is similar to the sound transmission class because they undergo similar methods of testing.


The major difference is that the weighted sound reduction index covers a wider frequency range than that of the STC. As a result, it is simply impossible to compare the rating of a weighted sound reduction index to the rating of the STC.


The values of the STC are usually higher by about three to four decibels compared to that of the sound insulation in weighted sound reduction index. (49 dB STC = 44 dB Rw).


Some experts, however, prefer weighted sound reduction index because it is in sync with the decibel scale.


So, a weighted sound reduction index 50 rating means that the material is expected to cause a reduction in the unwanted noise by the same 50 decibels.



It's difficult to decide the best rating method between the sound transmission class (STC) and the weighted sound reduction index.


While the STC method is mainly employed in the United States, the weighted sound reduction index is utilized in other world areas.


STC is often quoted for acoustic solutions; this, however, is a result of the concentration of companies that deal with sound and sound systems (in the United States) online.


It is not a result of the STC being superior to the weighted sound reduction index.


In some areas, determining the sound transmission class is challenging to comprehend at face value.


On the other hand, the weighted sound reduction index is in complete sync with the decibel scale.


This means that if you can determine the decibels of the unwanted sound, it will be easy to understand the level of weighted sound reduction index required to reduce the sound in an area.


International standards in charge of dictating the weighted sound reduction index also make it easier to comply with green credentials and global compliance.