STC Rating

Measuring the effectiveness of soundproofing materials: The STC rating and transmission loss curves.

Sound Transmission Class (STC)

A commonly used measure of soundproofing effectiveness is the sound transmission class, or STC. It is a single number that attempts to characterize the overall ability of a material to reduce the transmission of noise. Any reputable product that is marketed as a good soundproofing material will include the STC in its product literature. (In Europe, a similar measurement called the weighted sound reduction index is normally used.)

Unfortunately, the actual soundproofing effectiveness of a material or system can't be represented by a single number. One reason is that the degree to which a material can reduce noise depends on the frequency of the noise. For example, a material that is highly effective at blocking voices may be poor at blocking low-frequency music. Not only is the STC unable to convey differences of soundproofing effectiveness at various frequencies, it completely ignores frequencies outside its defined range of 125–4000 Hz. Yet many of the noises that commonly annoy people are below 125 Hz — such as noise from traffic, home theater speakers, or construction equipment.

If your main concern is the transmission of sounds of people talking, this chart gives you an idea of the effect you can expect if you are on the other side of a partition with a given STC rating:

STCSpeech-blocking Effect
20-25Quiet speech is audible
25-30Ordinary speech is audible and intelligible
30-35Loud speech is audible and intelligible
35-40Loud speech is heard but is rarely intelligible
40-50Loud speech can be heard, faintly
50-60 Loud sounds can barely be heard

Although STC has its limitations and is somewhat obsolete, it is likely to continue being widely used, since it is a building standard issued by the American Society of Testing and Measurement (ASTM)* and has been incorporated into many specifications and regulations.

Transmission Loss (TL)

For a better comparison of the soundproofing characteristics of two products, go beyond their STC ratings and look at their transmission loss (TL) data, which shows the noise attenuation as measured at different frequencies. Then, knowing the type of noise you are most interested in reducing and its approximate frequency range, you will be able to tell how effective the materials can be for your particular noise problem. Before purchasing a soundproofing product, always ask to see its transmission loss curve.

Note: The noise reduction you will get from a material once it is installed will probably be something less than that achieved in product testing. Laboratory testing measures the soundproofing ability of a material in isolation, but in a real-world installation there are many other materials involved, and other pathways that might transmit the noise.

Other Noise Reduction Ratings

STC and transmission loss curves measure the reduction of airborne noise, such as voices or music. Another type of noise problem is impact noise, such as footfalls or things dropping or scraping on the floor. The rating used for soundproofing effectiveness against this kind of noise is impact insulation class (IIC). IIC is measured for an entire floor-and-ceiling assembly, not for individual materials, so it is not very helpful when you are selecting materials for a flooring project. It is commonly used as a standard for builders, in specifying and rating finished construction projects.

There is another measurement you might run across, the noise reduction coefficient (NRC). This is a measure of sound absorption, and is used for products that are designed to improve the acoustics of a room or sound studio. It measures how well a product can reduce sound bouncing around or echoing within a room, not how well it will block sound from entering or leaving the room. For soundproofing purposes you should ignore the NRC.

* ASTM Standard E413, "Classification for Rating Sound Insulation," ASTM International, West Conshohocken, PA, DOI: 10.1520/E0413-04.

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